CA2187477C - Self align leadframe - Google Patents
Self align leadframe Download PDFInfo
- Publication number
- CA2187477C CA2187477C CA002187477A CA2187477A CA2187477C CA 2187477 C CA2187477 C CA 2187477C CA 002187477 A CA002187477 A CA 002187477A CA 2187477 A CA2187477 A CA 2187477A CA 2187477 C CA2187477 C CA 2187477C
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- Prior art keywords
- carrier
- leadframe
- leads
- fingers
- resilient
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Fee Related
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Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
- H01L21/70—Manufacture or treatment of devices consisting of a plurality of solid state components formed in or on a common substrate or of parts thereof; Manufacture of integrated circuit devices or of parts thereof
- H01L21/71—Manufacture of specific parts of devices defined in group H01L21/70
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L23/00—Details of semiconductor or other solid state devices
- H01L23/48—Arrangements for conducting electric current to or from the solid state body in operation, e.g. leads, terminal arrangements ; Selection of materials therefor
- H01L23/488—Arrangements for conducting electric current to or from the solid state body in operation, e.g. leads, terminal arrangements ; Selection of materials therefor consisting of soldered or bonded constructions
- H01L23/495—Lead-frames or other flat leads
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L23/00—Details of semiconductor or other solid state devices
- H01L23/48—Arrangements for conducting electric current to or from the solid state body in operation, e.g. leads, terminal arrangements ; Selection of materials therefor
- H01L23/50—Arrangements for conducting electric current to or from the solid state body in operation, e.g. leads, terminal arrangements ; Selection of materials therefor for integrated circuit devices, e.g. power bus, number of leads
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L2924/00—Indexing scheme for arrangements or methods for connecting or disconnecting semiconductor or solid-state bodies as covered by H01L24/00
- H01L2924/0001—Technical content checked by a classifier
- H01L2924/0002—Not covered by any one of groups H01L24/00, H01L24/00 and H01L2224/00
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- Engineering & Computer Science (AREA)
- Physics & Mathematics (AREA)
- Condensed Matter Physics & Semiconductors (AREA)
- General Physics & Mathematics (AREA)
- Computer Hardware Design (AREA)
- Microelectronics & Electronic Packaging (AREA)
- Power Engineering (AREA)
- Manufacturing & Machinery (AREA)
- Lead Frames For Integrated Circuits (AREA)
Abstract
The present invention relates to an improved leadframe and method of attaching leads of a leadframe to contract pads on a carrier. The leadframe includes resilient engaging means or fingers for engaging the carrier and for exerting forces on the carrier to thereby support the carrier and accurately position and align the contact pads on the carrier with the leads. This invention overcomes the expense and inaccuracies of known leadframe designs and techniques in attaching leads to carrier contact pads using such things as fixtures for positioning the leadframe and carrier, visual alignment procedures and retaining tabs.
Description
SELF ALIGN LEADFRAME
Field of the Invention This invention generally relates to integrated circuit chip p~çk~gin.~ and in particular 5 relates to the attachment of leadframes to carrier substrates for integrated circuit or semiconductor circuit chip devices. The invention relates to providing a leadframe and method for supporting a carrier and accurately positioning and holding the carrier and ~ligning the carrier contact pads with the leads of the leadframe so that the leads can be properly and accurately bonded to the contact pads.
Back~,round of the Invention Packaging of integrated circuit devices is becoming increasingly more difficult and complex in view of the increased number of input/output connections being required for the carrier-mounted chip devices. Multiple hundreds of connections are not uncomrnon with the 15 accompanying number of leads. One way external connections are provided is to mount a chip on a substrate carrier whereby the contact pads on the chip are aligned with contacts appropriately arranged and typically in a matrix pattern on one side of the carrier. These contacts on the one side of the carrier coll~-lunicate via conductive traces within the carrier to contacts on the other side of the carrier to which are connected the leads of the leadframe.
20 Alternatively, as permitted by some device designs, the contact pads to be connected to the leadframe could be located on the same side of carrier to which the chip is mounted. Thus, in the resultant packaged chip device, external leads are electrically connected as required to the chlp.
A present process used for ~tt~rhing a leadframe to a carrier includes the combination of the use of a brazing fixture and visual alignment of the leads of the leadframe and the pads on the carrier. The fixture consists of two parts and both are usually made of a graphite material but could also be made of other materials including ceramic materials. The first part holds a 5 ceramic substrate or carrier and the second part is placed over the first part and holds the leadframe. This arrangement7 however, is only able to grossly or coarsely position the leads of the leadframe with respect to the pads on the carrier. This is so because of the di~er~llces in coefficients of thermal expansion, amongst the fixtures, the carrier and the leadframe. It has been experienced that there has to be a considerable amount of spatial play built into the various 10 element.e amongst the respective fixtures, the carrier and the leadframe in order to accommodate for these tllerm~l miem~tches. No way is presently known that fixtures can be built to result in precise alignment of the leads on the leadframe and the pads on the carrier because a considerable amount of play amongst the elements is required when connections are accomplished using silver brazing which requires temperatures of 800~C to 1000~C or even at 15 lower temperatures for solder brazing. To accomplish the solder reflow of the pads, the combination of the assembled elements is placed in a furnace. Thus, significant play is required to accommodate the need for the dirrerellces in thermal expansion. With the presently known a~p~lus and process involved, only gross positioning of the various elements is possible and flne positioning results from a visual alignment of the leads and the pads by a human operator.
20 The leads are then bonded to the pads.
Description of the Prior Art There are a variety of arrangements known and described in the prior art for attaching leads to an integrated circuit chip device. These include the following patent documents with 25 the accompanying sllmm~ries:
U.S. Patent No. 4,536,825, entitled "Leadframe Having Severable Fingers for Aligning One or More Electronic Circuit Device Components", which issued August 20, 1985 to Unitrode Corp., describes fixed alignment fingers and positioning tabs on a leadframe for ~ligning a circuit device with respect to a leadframe.
U.S. Patent No. 5,275,897, entitled "Precisely Aligned Leadframe Using Registration Traces and Pads", which issued January 4, 1994 to Hewlett Packard Company, relates to tape automated bonding of leadframes to a substrate and instead of ~ligning the pattern of signal leads of the frame to the signal leads on the substrate, the focus is on ~ligning the registration pads on 10 a substrate and the alignment traces on the tape.
U.S. Patent No. 4,466,183, entitled "Integrated Circuit Packaging Process", which issued August 21, 1984 to National Semiconductor Corp., is directed to an automatic tape assembly process where an integrated circuit device is assembled to leads on a tape which also provides 15 dummy leads for holding the integrated circuit device prior to testing and encapsulation.
U.S. PatentNo. 5,214,846, entitled " Packaging of Semiconductor Chips", which issued June 1, 1993 to Sony Corp., relates to p~ck~ging of semiconductor chips directly on to leadframes with fingers on the leadframe for restricting movement of the chip with respect to 20 the leadframe.
U.S. PatentNo. 4,651,415, entitled"l s~ded Chip Carrier", which issued March 24, 1987 to Diacon, Inc., relates to a leaded chip carrier and deals with mi~m~tch of thermal coefficient of expansion in the packaging process by use of four separate subframe quadrants and the use of tabs to align between an assembly locating plate and a base plate.
U.S. Patent No. 5,299,097, entitled "Electronic Part Mounting Board and Semiconductor Device Using the Same", which issued March 29, 1994 to Ibiden Co. Ltd., pertains to an electronic part mounting board and a semiconductor device which teaches use of four board fixing pins at each corner of leadframe through which press fit pins are inserted to ensure 5 accurate positioning of the leadframe during processing.
U.S. Patent No. 5,278,447, entitled "Semiconductor Device Assembly Carrier", which issued January 11, 1994 to LSI Logic Corp., pertains to semiconductor device assembly carrier for supporting and protecting the device and its leads from damage during handling.
U.S. Patent No. 5,307,929, entitled "Lead Arrangement for Integrated Circuits and Method of Assembly", which issued May 3, 1994 to North American Specialities Corp., relates to lead arr~ngPmPnh for integrated cil~;uil~ and describes fold over leadframe relailling tabs for holding and re~i~ g the substrates against the leads during soldering.
Sl~mm~ry of the Invention It is therefore an object of the invention to provide a new self align leadframe and associated method for attaching leads to a carrier which provide for significant improvements and results over that obtained from the use of the known prior art leadframes and methods 20 previously described.
It is a further object of this invention to provide a new self align leadframe and method for supporting a carrier and attaching leads to contact pads of the carrier without the difficulty in accuracies and expense of using fixtures as is presently done.
According to one aspect of the invention, there is provided an improved leadframe for supporting a carrier for an integrated circuit device and m~int~ining alignment of contact pads on the carrier with leads of the leadframe, which includes a leadframe circumscribing an area and having a plurality of leads on each side of said frame and extending into the circumscribed 5 area, said carrier having contact pads on one side thereof, said carrier being of the same general shape and size as said area circumscribed by said leadframe such that each of said plurality of leads overlays one of said contact pads, the improvement compri~ing resilient carrier positioning means attached to said leadframe and extending into the circumscribed area such that said positioning means eng~ges the carrier by applying forces against the carrier, thereby supporting 10 the carrier and m~ g each ofthe contact pads of the carrier in contact and alignment with a respective lead of the leadframe.
According to another aspect of the invention, there is provided a method for a~çhing leads to contact pads on a carrier for an integrated circuit device wherein said leads are part of 15 a leadframe and said leadframe has resilient means for eng~ging said carrier for supporting the carrier and aligning the contact pads of the carrier with the leads including the steps of mounting the carrier within the leadframe such that said resilient means engages the carrier by exerting forces on the carrier so as to support the carrier and align each with a respective lead contact pad joining each lead to a contact pad, and removing the leadframe and the resilient means to result 20 in a carrier with leads attached to the contact pads thereof.
According to a further aspect of ~e invention, there is provided a leadframe having a frame and a plurality of conductive leads attached to the frame and having lead end portions overlying contact pads on a carrier for an integrated circuit device, such that each end portion 25 of a lead is to be connected to a respective contact pad, and pairs of resilient positioning fingers attached to the frame for eng~ging respective edges of the carrier so as to support and position the carrier such that each contact pad on the carrier is in contact with a respective end portion of a lead.
5 Brief Description of the Drawin~
The foregoing and other objects, aspects and advantages of the invention will be better understood from the following detailed description of the pl~r~lled embodiment of the invention, with reference to the accompanying drawings in which:
Figure 1 is a top view showing a leadframe and a carrier, according to the invention, Figure 2 provides in more detail aspects of Figure 1 showing the locator or positioning fingers, and Figure 3 is a more detailed view from the underside of the carrier showing aspects of Figure 2.
Detailed Description of the Preferred Embodiment The plerell~d embo~iment of the invention includes the addition of a number and in the prefelled embodiment, four, thermal expansion compensator locating legs or fingers each positioned at a corner of the leadframe. These locating legs are resilient and flexible and hold, 20 support and retain the carrier by a spring effect. Thus, it is necessary that the additional legs or fingers be somewhat longer than the distance from the leadframe to the ceramic substrate or ca~rier so that when the carrier is properly positioned within the leadframe, the locating legs of the leadframe are flexed and thus the carrier is supported by these additional fingers or legs and the spring effect resulting ther~rlolll. Preferably the fingers would be positioned uniformly 25 around the frame, possibly one at each corner, so that the substrate is actually supported by the -resilient fingers. It is preferable that the fingers engage the substrate towards the top of each side or corner of the substrate. The ends of the fingers could also be notched to accommodate and engage the corner of the substrate. Thus, when the substrate is properly positioned and supported by the legs, the legs are flexed by the spring effect resulting from the resiliency of the 5 legs. This results in the contacts of the carrier being properly positioned with respect to the leads of the leadframe. The leadframe with the fingers as part thereof could be conveniently stamped in an interference fit fashion with respect to the substrate.
During the brazing activity to attach the leads to the contact pads and the resultant high 10 temperature, the spring effect of the legs may decrease slightly because of the difference in therTnAl coefficient of e~rAn~ion between the material of the leadframe and the carrier but since the differences in expansion will be uniform, the accurate positioning of the contacts of the carrier and the leads is still assured. It has been found that the thermal expansion of the leadframe during h~.Ating is ~ "" and because ofthe dirrerellce of materials of the leadframe 15 and the carrier, the leadframe total expansion is approximately .008" whereas that of the carrier is between .003" and .004" for a 50mm carrier substrate. Although the pitch of the leads, which is the distance between the leads, may change slightly during the brazing process and the l~n~el~lule resulting therefiom, this has been found not to be enough to move the lead off the respective contact pad on the carrier or to result, for example, in a lead contacting or bridging 20 two adjacent pads.
Thus, by use of the present invention since there is no need for use of fixtures and because there is no need for designing built-in play between each fixture and the leadframe or carrier, there is not significant movement of the leads away from the contact pads and thus 25 proper bonding occurs. Any additional step of visually Aligning the leads and the contacts before br~7ing is also not required. Thus, with this disclosed leadframe of the preferred embodiment and its use in ~ ing leads to a carrier, it has been found that the alignment problems resulting from the dirre~ coefficients of thermal expansion and the amount of play that had previously been required between the various elements and the fixtures, have been overcome.
The invention also overcomes the need for providing retaining tabs on the leadframe and the need for folding or bending such tabs as described in some prior art references. Thus the present invention overcomes an additional fabrication step and the size or area of the leadframes does not have to be increased to provide for the alignment and positioning fingers.
Reference is now made to the accompanying drawings, which illustrate a plerel-edembodiment of the invention, in order to provide more detailed aspects of the invention.
With reference to Figure 1, leadframe 1 is a leadframe according to aspects of the 15 invention. As is typical in the integrated circuit industry, the leadframe circumscribes an area in which is positioned a substrate or carrier 2. Lead~ame 1 includes a plurality of leads 3. Each lead at the one end is formed as part of and attached to the leadframe 1 and at the other end, ext~.n~s to and overlays contact pads on the carrier 2 as will be subsequently described in more detail. As is common in the integrated circuit packaging industry, leads 3 extend inwardly from 20 frame 1 uniformly all around the frame, although only a limited number of leads are shown in Figure 1. The invention is not limite~ to a leadframe having any particular number of leads.
Not shown in Figure 1 but as is well known to those in the industry, contact pads on carrier 2 to which the end portions of leads 3 of the leadframe 1 are to be connected, and which 25 are actually on the reverse side of carrier 2 as shown in Figure 1 as will be subsequently described, are electrically connected by conductive traces to contact pad array on the opposite side of the carrier which would be the visible or top side of carrier 2 shown in Figure 1. The integrated circuit device or chip, has a contact array matching the pad array on the upper side of the carrier and this is how the chip is connected or mounted to the carrier. As is known in the 5 art, the chip and leadframe may ~ltçrn~tively be connected and mounted to the same side of the carrier. This detail is not shown in Figure 1 as it is well known in the art and forms no aspect of the subject invention.
Also shown in Figure 1 are resilient carrier positioning means. These could be implemented by the fmgers or locating legs 4, 5, 6 and 7. In Figure 1, in this preferred embo-liment such fingers are shown at each of the four corners of the carrier 2 and leadframe 1.
Typically, leadframe 1 with leads 3 and fingers 4, 5, 6 and 7, could be made by ~lalllpillg processes from a sheet of the same material, etching processes or any other ~propliate 15 ",~ lr~cturing processes. Leadframes are typically supplied by a variety of companies and the process of m~king the leadframes is well known. The material of the leadframe could be any suitable material and has to be such that a spring or tension property results in the locating fingers as will be subsequently described. Material Alloy 42 supplied by Carpenter Technology Corp. has been found to be a~plopl;ate. A plurality of such leadframes could be made in strips 20 so as to facilitate the automation of the attaching of the leads to the carrier.
Leads 3 of leadframe 1 of course must be long enough to extend past the edge of the carrier and all around the carrier as appropliate and for each lead to overlay the contact pad on the carrier when the carrier is properly positioned within leadframe 1. The number of leads for 25 the invention is really irrelevant but certainly the invention achieves a greater degree of , usefulness as the number of leads increases.
Fingers 4, 5, 6 and 7 are somewhat shorter or extend a lesser distance into the area circumscribed by the leadframe when compared with the length of leads 3. As is shown, fingers 4, 5, 6 and 7 only abut or engage the edge or corner of carrier 2. However, fingers 4, 5, 6 and 7 must be long enough to be able to apply positioning or retentive force on carrier 2. This results and is accomplished from opposing pairs of fingers as shown in Figure 1. By opposing pairs of fingers is meant fingers 4 and 6 and fingers 5 and 7 as shown. Thus, before carrier 2 is mounted in leadframe 1, the distance between the ends of the opposing fingers, for example fingers 5 and lo 7, or fingers 4 and 6 in Figure 1, would be slightly less than the measurement of the carrier between diagonal corners. When carrier 2 is positioned within leadframe 1 and in particular between fingers 4 and 6 and 5 and 7, carrier 2 is then held in position by the forces resulting from the spring effect caused by the flexing of the opposing fingers. The pressure on the carrier reslllting from the pairs of opposing fingers also m~int~in~ the carrier and the leadframe properly positioned so that the contact pads on carrier 2 are positioned over leads 3 or the end portions of leads 3. As can be seen, the fingers 4, 5, 6 and 7 and their resulting actions with the carrier substrate, not only abut or engage the carrier but can also be thought of as acting as a clip-on lead frame, or providing a clip-on lead frame function to hold and support the carrier.
When the combination of the leadframe and carrier, as has been described, is positioned in a furnace to reflow the metal on the contact pads in order to form a permanent connection between the pads and the leads, the pair of opposing fingers 4, 6 and 5, 7 compensate for any di~-~nce in therrn~l Pxp~n~ion between the carrier and the leadframe and prevents the leads 3 or end portions of leads 3 from moving off of the contacts on carrier 2. The forces resulting from any change in exp~n~ion of the leadframe as a result of the pairs of fingers, are equal and 2t 87477 opposite. The forces thereby effectively cancel each other out and leads 3 remain properly positioned on the contacts of carrier 2. In a similar way, any lateral movement resulting from temperature mi~m~tch ofthe leadframe and the carrier is compensated for and the end portions of the leads remain properly positioned on the respective contact pads on carrier 2.
After the combination as described above has been placed in a furnace resulting in the reflow of the metal of the contact pads so that the leads are properly positioned and connected to the contacts, once the cormection is cooled and hardened, the leadframe is removed from the carrier by 1~ g the leads 3 at an ~plopl;ate length from the carrier. Frame 1 and the fingers 4, 5, 6 and 7 are then discarded and carrier 2 with the leads 3 appropl;ately attached thereto then remain for further pack~ging, processing or mounting on a circuit board, as the case may be.
Referring now to Figure 2 of the drawings, more details of one of the comer arrangements shown in Figure 1 is illustrated. It can be seen that the thickness of the material of leadframe 15 1 and finger 4 is less than that of carrier 2. A slight buckling or flexing of finger 4 is apparent and this has resulted, as has been previously described, because of the equal and opposite retentive forces exerted by the pair of fingers, for example fingers 4 and 5, on the carrier.
Finger 4 can engage or abut carrier 2 at any height within the thickness of the carrier. However it has been found for best m~tçhin~ to have the finger close to the top edge of carrier 2, as is 20 shown. Finger 4 may also be appropriately shaped or notched, as shown by notch 9 in the end of finger 4, in order to accommodate the shape of the corner of carrier 2, thereby more firmly eng~ing the carrier and minimi7.ing accidental movement or slippage of the end of the finger with respect to the carrier. Figure 2 is representative of any one of the corner arrangements. As has been previously described, the leads 3 shown are only representative of the number of leads 25 which can be accommodated.
21 8747?
Referring now to Figure 3 of the drawings which shows in more detail one of the representative corners of the structure illustrated in Figure 1 and is the underside of carrier 2 or the reverse side of what is shown in Figure 2. Finger 4 is shown abutting against the corner of carrier 2 and effectively supporting the carrier in combination with the other fingers as 5 previously described. Also, as a result of the action of the fingers and in particular Finger 4 as shown, the ends of leads 3 are propclly positioned over contact pads 8 on carrier 2. Thus, when carrier 2 is mounted within the resilient carrier positioning means comprising the various fingers 4, 5, 6 and 7 in the preferred embodiment of the invention, it can be seen that carrier 2 is now properly positioned for attaching the ends of leads 3 onto the contact pads 8. As is also shown 10 in Figures 1 and 2, Figure 3 shows finger 4 as being slightly flexed or bowed. Once this combination is placed into a furnace for reflow of the material of pads 8, any mi~m~tch in lell~ lure coefficient between the leadframe 1 and carrier 2 is compensated for by the fingers 4, 5, 6 and 7 and the ends of leads 3 remain effectively properly positioned on contacts 8 as previously described.
It is apparent that it is not necessary for the resilient carrier positioning means to be positioned at the corners of carrier 2. It may very well be that the positioning means could be located along the edges of the carrier. However, it is preferable that the positioning means be employed as pairs so that each one exerts uniform and opposite force in order to cancel out and 20 prevent any movement of the carrier with respect to the leadframe and between the ends of leads 3 and contact pads 8.
It should be understood that the foregoing description is only illustrative of a preferred embodiment of the invention. Various alternatives and modifications can be devised by those 25 sl~lled in the art without departing from the intended invention. Thus, the present invention is intended to embrace all such alternatives, modifications and variations which fall within the scope of the appended claims.
Field of the Invention This invention generally relates to integrated circuit chip p~çk~gin.~ and in particular 5 relates to the attachment of leadframes to carrier substrates for integrated circuit or semiconductor circuit chip devices. The invention relates to providing a leadframe and method for supporting a carrier and accurately positioning and holding the carrier and ~ligning the carrier contact pads with the leads of the leadframe so that the leads can be properly and accurately bonded to the contact pads.
Back~,round of the Invention Packaging of integrated circuit devices is becoming increasingly more difficult and complex in view of the increased number of input/output connections being required for the carrier-mounted chip devices. Multiple hundreds of connections are not uncomrnon with the 15 accompanying number of leads. One way external connections are provided is to mount a chip on a substrate carrier whereby the contact pads on the chip are aligned with contacts appropriately arranged and typically in a matrix pattern on one side of the carrier. These contacts on the one side of the carrier coll~-lunicate via conductive traces within the carrier to contacts on the other side of the carrier to which are connected the leads of the leadframe.
20 Alternatively, as permitted by some device designs, the contact pads to be connected to the leadframe could be located on the same side of carrier to which the chip is mounted. Thus, in the resultant packaged chip device, external leads are electrically connected as required to the chlp.
A present process used for ~tt~rhing a leadframe to a carrier includes the combination of the use of a brazing fixture and visual alignment of the leads of the leadframe and the pads on the carrier. The fixture consists of two parts and both are usually made of a graphite material but could also be made of other materials including ceramic materials. The first part holds a 5 ceramic substrate or carrier and the second part is placed over the first part and holds the leadframe. This arrangement7 however, is only able to grossly or coarsely position the leads of the leadframe with respect to the pads on the carrier. This is so because of the di~er~llces in coefficients of thermal expansion, amongst the fixtures, the carrier and the leadframe. It has been experienced that there has to be a considerable amount of spatial play built into the various 10 element.e amongst the respective fixtures, the carrier and the leadframe in order to accommodate for these tllerm~l miem~tches. No way is presently known that fixtures can be built to result in precise alignment of the leads on the leadframe and the pads on the carrier because a considerable amount of play amongst the elements is required when connections are accomplished using silver brazing which requires temperatures of 800~C to 1000~C or even at 15 lower temperatures for solder brazing. To accomplish the solder reflow of the pads, the combination of the assembled elements is placed in a furnace. Thus, significant play is required to accommodate the need for the dirrerellces in thermal expansion. With the presently known a~p~lus and process involved, only gross positioning of the various elements is possible and flne positioning results from a visual alignment of the leads and the pads by a human operator.
20 The leads are then bonded to the pads.
Description of the Prior Art There are a variety of arrangements known and described in the prior art for attaching leads to an integrated circuit chip device. These include the following patent documents with 25 the accompanying sllmm~ries:
U.S. Patent No. 4,536,825, entitled "Leadframe Having Severable Fingers for Aligning One or More Electronic Circuit Device Components", which issued August 20, 1985 to Unitrode Corp., describes fixed alignment fingers and positioning tabs on a leadframe for ~ligning a circuit device with respect to a leadframe.
U.S. Patent No. 5,275,897, entitled "Precisely Aligned Leadframe Using Registration Traces and Pads", which issued January 4, 1994 to Hewlett Packard Company, relates to tape automated bonding of leadframes to a substrate and instead of ~ligning the pattern of signal leads of the frame to the signal leads on the substrate, the focus is on ~ligning the registration pads on 10 a substrate and the alignment traces on the tape.
U.S. Patent No. 4,466,183, entitled "Integrated Circuit Packaging Process", which issued August 21, 1984 to National Semiconductor Corp., is directed to an automatic tape assembly process where an integrated circuit device is assembled to leads on a tape which also provides 15 dummy leads for holding the integrated circuit device prior to testing and encapsulation.
U.S. PatentNo. 5,214,846, entitled " Packaging of Semiconductor Chips", which issued June 1, 1993 to Sony Corp., relates to p~ck~ging of semiconductor chips directly on to leadframes with fingers on the leadframe for restricting movement of the chip with respect to 20 the leadframe.
U.S. PatentNo. 4,651,415, entitled"l s~ded Chip Carrier", which issued March 24, 1987 to Diacon, Inc., relates to a leaded chip carrier and deals with mi~m~tch of thermal coefficient of expansion in the packaging process by use of four separate subframe quadrants and the use of tabs to align between an assembly locating plate and a base plate.
U.S. Patent No. 5,299,097, entitled "Electronic Part Mounting Board and Semiconductor Device Using the Same", which issued March 29, 1994 to Ibiden Co. Ltd., pertains to an electronic part mounting board and a semiconductor device which teaches use of four board fixing pins at each corner of leadframe through which press fit pins are inserted to ensure 5 accurate positioning of the leadframe during processing.
U.S. Patent No. 5,278,447, entitled "Semiconductor Device Assembly Carrier", which issued January 11, 1994 to LSI Logic Corp., pertains to semiconductor device assembly carrier for supporting and protecting the device and its leads from damage during handling.
U.S. Patent No. 5,307,929, entitled "Lead Arrangement for Integrated Circuits and Method of Assembly", which issued May 3, 1994 to North American Specialities Corp., relates to lead arr~ngPmPnh for integrated cil~;uil~ and describes fold over leadframe relailling tabs for holding and re~i~ g the substrates against the leads during soldering.
Sl~mm~ry of the Invention It is therefore an object of the invention to provide a new self align leadframe and associated method for attaching leads to a carrier which provide for significant improvements and results over that obtained from the use of the known prior art leadframes and methods 20 previously described.
It is a further object of this invention to provide a new self align leadframe and method for supporting a carrier and attaching leads to contact pads of the carrier without the difficulty in accuracies and expense of using fixtures as is presently done.
According to one aspect of the invention, there is provided an improved leadframe for supporting a carrier for an integrated circuit device and m~int~ining alignment of contact pads on the carrier with leads of the leadframe, which includes a leadframe circumscribing an area and having a plurality of leads on each side of said frame and extending into the circumscribed 5 area, said carrier having contact pads on one side thereof, said carrier being of the same general shape and size as said area circumscribed by said leadframe such that each of said plurality of leads overlays one of said contact pads, the improvement compri~ing resilient carrier positioning means attached to said leadframe and extending into the circumscribed area such that said positioning means eng~ges the carrier by applying forces against the carrier, thereby supporting 10 the carrier and m~ g each ofthe contact pads of the carrier in contact and alignment with a respective lead of the leadframe.
According to another aspect of the invention, there is provided a method for a~çhing leads to contact pads on a carrier for an integrated circuit device wherein said leads are part of 15 a leadframe and said leadframe has resilient means for eng~ging said carrier for supporting the carrier and aligning the contact pads of the carrier with the leads including the steps of mounting the carrier within the leadframe such that said resilient means engages the carrier by exerting forces on the carrier so as to support the carrier and align each with a respective lead contact pad joining each lead to a contact pad, and removing the leadframe and the resilient means to result 20 in a carrier with leads attached to the contact pads thereof.
According to a further aspect of ~e invention, there is provided a leadframe having a frame and a plurality of conductive leads attached to the frame and having lead end portions overlying contact pads on a carrier for an integrated circuit device, such that each end portion 25 of a lead is to be connected to a respective contact pad, and pairs of resilient positioning fingers attached to the frame for eng~ging respective edges of the carrier so as to support and position the carrier such that each contact pad on the carrier is in contact with a respective end portion of a lead.
5 Brief Description of the Drawin~
The foregoing and other objects, aspects and advantages of the invention will be better understood from the following detailed description of the pl~r~lled embodiment of the invention, with reference to the accompanying drawings in which:
Figure 1 is a top view showing a leadframe and a carrier, according to the invention, Figure 2 provides in more detail aspects of Figure 1 showing the locator or positioning fingers, and Figure 3 is a more detailed view from the underside of the carrier showing aspects of Figure 2.
Detailed Description of the Preferred Embodiment The plerell~d embo~iment of the invention includes the addition of a number and in the prefelled embodiment, four, thermal expansion compensator locating legs or fingers each positioned at a corner of the leadframe. These locating legs are resilient and flexible and hold, 20 support and retain the carrier by a spring effect. Thus, it is necessary that the additional legs or fingers be somewhat longer than the distance from the leadframe to the ceramic substrate or ca~rier so that when the carrier is properly positioned within the leadframe, the locating legs of the leadframe are flexed and thus the carrier is supported by these additional fingers or legs and the spring effect resulting ther~rlolll. Preferably the fingers would be positioned uniformly 25 around the frame, possibly one at each corner, so that the substrate is actually supported by the -resilient fingers. It is preferable that the fingers engage the substrate towards the top of each side or corner of the substrate. The ends of the fingers could also be notched to accommodate and engage the corner of the substrate. Thus, when the substrate is properly positioned and supported by the legs, the legs are flexed by the spring effect resulting from the resiliency of the 5 legs. This results in the contacts of the carrier being properly positioned with respect to the leads of the leadframe. The leadframe with the fingers as part thereof could be conveniently stamped in an interference fit fashion with respect to the substrate.
During the brazing activity to attach the leads to the contact pads and the resultant high 10 temperature, the spring effect of the legs may decrease slightly because of the difference in therTnAl coefficient of e~rAn~ion between the material of the leadframe and the carrier but since the differences in expansion will be uniform, the accurate positioning of the contacts of the carrier and the leads is still assured. It has been found that the thermal expansion of the leadframe during h~.Ating is ~ "" and because ofthe dirrerellce of materials of the leadframe 15 and the carrier, the leadframe total expansion is approximately .008" whereas that of the carrier is between .003" and .004" for a 50mm carrier substrate. Although the pitch of the leads, which is the distance between the leads, may change slightly during the brazing process and the l~n~el~lule resulting therefiom, this has been found not to be enough to move the lead off the respective contact pad on the carrier or to result, for example, in a lead contacting or bridging 20 two adjacent pads.
Thus, by use of the present invention since there is no need for use of fixtures and because there is no need for designing built-in play between each fixture and the leadframe or carrier, there is not significant movement of the leads away from the contact pads and thus 25 proper bonding occurs. Any additional step of visually Aligning the leads and the contacts before br~7ing is also not required. Thus, with this disclosed leadframe of the preferred embodiment and its use in ~ ing leads to a carrier, it has been found that the alignment problems resulting from the dirre~ coefficients of thermal expansion and the amount of play that had previously been required between the various elements and the fixtures, have been overcome.
The invention also overcomes the need for providing retaining tabs on the leadframe and the need for folding or bending such tabs as described in some prior art references. Thus the present invention overcomes an additional fabrication step and the size or area of the leadframes does not have to be increased to provide for the alignment and positioning fingers.
Reference is now made to the accompanying drawings, which illustrate a plerel-edembodiment of the invention, in order to provide more detailed aspects of the invention.
With reference to Figure 1, leadframe 1 is a leadframe according to aspects of the 15 invention. As is typical in the integrated circuit industry, the leadframe circumscribes an area in which is positioned a substrate or carrier 2. Lead~ame 1 includes a plurality of leads 3. Each lead at the one end is formed as part of and attached to the leadframe 1 and at the other end, ext~.n~s to and overlays contact pads on the carrier 2 as will be subsequently described in more detail. As is common in the integrated circuit packaging industry, leads 3 extend inwardly from 20 frame 1 uniformly all around the frame, although only a limited number of leads are shown in Figure 1. The invention is not limite~ to a leadframe having any particular number of leads.
Not shown in Figure 1 but as is well known to those in the industry, contact pads on carrier 2 to which the end portions of leads 3 of the leadframe 1 are to be connected, and which 25 are actually on the reverse side of carrier 2 as shown in Figure 1 as will be subsequently described, are electrically connected by conductive traces to contact pad array on the opposite side of the carrier which would be the visible or top side of carrier 2 shown in Figure 1. The integrated circuit device or chip, has a contact array matching the pad array on the upper side of the carrier and this is how the chip is connected or mounted to the carrier. As is known in the 5 art, the chip and leadframe may ~ltçrn~tively be connected and mounted to the same side of the carrier. This detail is not shown in Figure 1 as it is well known in the art and forms no aspect of the subject invention.
Also shown in Figure 1 are resilient carrier positioning means. These could be implemented by the fmgers or locating legs 4, 5, 6 and 7. In Figure 1, in this preferred embo-liment such fingers are shown at each of the four corners of the carrier 2 and leadframe 1.
Typically, leadframe 1 with leads 3 and fingers 4, 5, 6 and 7, could be made by ~lalllpillg processes from a sheet of the same material, etching processes or any other ~propliate 15 ",~ lr~cturing processes. Leadframes are typically supplied by a variety of companies and the process of m~king the leadframes is well known. The material of the leadframe could be any suitable material and has to be such that a spring or tension property results in the locating fingers as will be subsequently described. Material Alloy 42 supplied by Carpenter Technology Corp. has been found to be a~plopl;ate. A plurality of such leadframes could be made in strips 20 so as to facilitate the automation of the attaching of the leads to the carrier.
Leads 3 of leadframe 1 of course must be long enough to extend past the edge of the carrier and all around the carrier as appropliate and for each lead to overlay the contact pad on the carrier when the carrier is properly positioned within leadframe 1. The number of leads for 25 the invention is really irrelevant but certainly the invention achieves a greater degree of , usefulness as the number of leads increases.
Fingers 4, 5, 6 and 7 are somewhat shorter or extend a lesser distance into the area circumscribed by the leadframe when compared with the length of leads 3. As is shown, fingers 4, 5, 6 and 7 only abut or engage the edge or corner of carrier 2. However, fingers 4, 5, 6 and 7 must be long enough to be able to apply positioning or retentive force on carrier 2. This results and is accomplished from opposing pairs of fingers as shown in Figure 1. By opposing pairs of fingers is meant fingers 4 and 6 and fingers 5 and 7 as shown. Thus, before carrier 2 is mounted in leadframe 1, the distance between the ends of the opposing fingers, for example fingers 5 and lo 7, or fingers 4 and 6 in Figure 1, would be slightly less than the measurement of the carrier between diagonal corners. When carrier 2 is positioned within leadframe 1 and in particular between fingers 4 and 6 and 5 and 7, carrier 2 is then held in position by the forces resulting from the spring effect caused by the flexing of the opposing fingers. The pressure on the carrier reslllting from the pairs of opposing fingers also m~int~in~ the carrier and the leadframe properly positioned so that the contact pads on carrier 2 are positioned over leads 3 or the end portions of leads 3. As can be seen, the fingers 4, 5, 6 and 7 and their resulting actions with the carrier substrate, not only abut or engage the carrier but can also be thought of as acting as a clip-on lead frame, or providing a clip-on lead frame function to hold and support the carrier.
When the combination of the leadframe and carrier, as has been described, is positioned in a furnace to reflow the metal on the contact pads in order to form a permanent connection between the pads and the leads, the pair of opposing fingers 4, 6 and 5, 7 compensate for any di~-~nce in therrn~l Pxp~n~ion between the carrier and the leadframe and prevents the leads 3 or end portions of leads 3 from moving off of the contacts on carrier 2. The forces resulting from any change in exp~n~ion of the leadframe as a result of the pairs of fingers, are equal and 2t 87477 opposite. The forces thereby effectively cancel each other out and leads 3 remain properly positioned on the contacts of carrier 2. In a similar way, any lateral movement resulting from temperature mi~m~tch ofthe leadframe and the carrier is compensated for and the end portions of the leads remain properly positioned on the respective contact pads on carrier 2.
After the combination as described above has been placed in a furnace resulting in the reflow of the metal of the contact pads so that the leads are properly positioned and connected to the contacts, once the cormection is cooled and hardened, the leadframe is removed from the carrier by 1~ g the leads 3 at an ~plopl;ate length from the carrier. Frame 1 and the fingers 4, 5, 6 and 7 are then discarded and carrier 2 with the leads 3 appropl;ately attached thereto then remain for further pack~ging, processing or mounting on a circuit board, as the case may be.
Referring now to Figure 2 of the drawings, more details of one of the comer arrangements shown in Figure 1 is illustrated. It can be seen that the thickness of the material of leadframe 15 1 and finger 4 is less than that of carrier 2. A slight buckling or flexing of finger 4 is apparent and this has resulted, as has been previously described, because of the equal and opposite retentive forces exerted by the pair of fingers, for example fingers 4 and 5, on the carrier.
Finger 4 can engage or abut carrier 2 at any height within the thickness of the carrier. However it has been found for best m~tçhin~ to have the finger close to the top edge of carrier 2, as is 20 shown. Finger 4 may also be appropriately shaped or notched, as shown by notch 9 in the end of finger 4, in order to accommodate the shape of the corner of carrier 2, thereby more firmly eng~ing the carrier and minimi7.ing accidental movement or slippage of the end of the finger with respect to the carrier. Figure 2 is representative of any one of the corner arrangements. As has been previously described, the leads 3 shown are only representative of the number of leads 25 which can be accommodated.
21 8747?
Referring now to Figure 3 of the drawings which shows in more detail one of the representative corners of the structure illustrated in Figure 1 and is the underside of carrier 2 or the reverse side of what is shown in Figure 2. Finger 4 is shown abutting against the corner of carrier 2 and effectively supporting the carrier in combination with the other fingers as 5 previously described. Also, as a result of the action of the fingers and in particular Finger 4 as shown, the ends of leads 3 are propclly positioned over contact pads 8 on carrier 2. Thus, when carrier 2 is mounted within the resilient carrier positioning means comprising the various fingers 4, 5, 6 and 7 in the preferred embodiment of the invention, it can be seen that carrier 2 is now properly positioned for attaching the ends of leads 3 onto the contact pads 8. As is also shown 10 in Figures 1 and 2, Figure 3 shows finger 4 as being slightly flexed or bowed. Once this combination is placed into a furnace for reflow of the material of pads 8, any mi~m~tch in lell~ lure coefficient between the leadframe 1 and carrier 2 is compensated for by the fingers 4, 5, 6 and 7 and the ends of leads 3 remain effectively properly positioned on contacts 8 as previously described.
It is apparent that it is not necessary for the resilient carrier positioning means to be positioned at the corners of carrier 2. It may very well be that the positioning means could be located along the edges of the carrier. However, it is preferable that the positioning means be employed as pairs so that each one exerts uniform and opposite force in order to cancel out and 20 prevent any movement of the carrier with respect to the leadframe and between the ends of leads 3 and contact pads 8.
It should be understood that the foregoing description is only illustrative of a preferred embodiment of the invention. Various alternatives and modifications can be devised by those 25 sl~lled in the art without departing from the intended invention. Thus, the present invention is intended to embrace all such alternatives, modifications and variations which fall within the scope of the appended claims.
Claims (22)
1. An improved leadframe for supporting a carrier for an integrated circuit device and maintaining alignment of contact pads on the carrier with leads of the leadframe, which includes:
a leadframe circumscribing an area and having a plurality of leads on each side of said frame and extending into the circumscribed area;
said carrier having contact pads on one side thereof;
said carrier being of the same general shape and size as said area circumscribed by said leadframe such that each of said plurality of leads overlays one of said contact pads;
the improvement comprising:
resilient carrier positioning means attached to said leadframe and extending into the circumscribed area such that said positioning means engages the carrier by applying forces against the carrier thereby supporting the carrier and maintaining each of the contact pads of the carrier in contact and alignment with a respective lead of the leadframe.
a leadframe circumscribing an area and having a plurality of leads on each side of said frame and extending into the circumscribed area;
said carrier having contact pads on one side thereof;
said carrier being of the same general shape and size as said area circumscribed by said leadframe such that each of said plurality of leads overlays one of said contact pads;
the improvement comprising:
resilient carrier positioning means attached to said leadframe and extending into the circumscribed area such that said positioning means engages the carrier by applying forces against the carrier thereby supporting the carrier and maintaining each of the contact pads of the carrier in contact and alignment with a respective lead of the leadframe.
2. An improved leadframe as recited in claim 1, wherein said plurality of leads and said resilient carrier positioning means are attached to inner edges of the leadframe.
3. An improved leadframe as recited in claim 2 wherein said resilient carrier positioning means consists of fingers attached to inner edges of the leadframe and the forces applied on the carrier result from a spring effect of the fingers pushing on the carrier.
4. An improved leadframe as recited in claim 3 wherein said leadframe circumscribes a square area and said fingers are attached to each inner corner of the leadframe.
5. An improved leadframe as recited in claims 3 or 4 wherein said fingers engage the carrier at the corners thereof and the ends of each finger is shaped to accommodate the respective shape of the corner of the carrier.
6. A method for attaching leads to contact pads on a carrier for an integrated circuit device wherein said leads are part of a leadframe and said leadframe has resilient means for engaging said carrier for supporting the carrier and aligning the contact pads of the carrier with the leads including the steps of:
mounting the carrier within the leadframe such that said resilient means engages the carrier by exerting forces on the carrier so as to support the carrier and align each contact pad with a respective lead;
joining each lead to a contact pad, and removing the leadframe and the resilient means to result in a carrier with leads attached to the contact pads thereof.
mounting the carrier within the leadframe such that said resilient means engages the carrier by exerting forces on the carrier so as to support the carrier and align each contact pad with a respective lead;
joining each lead to a contact pad, and removing the leadframe and the resilient means to result in a carrier with leads attached to the contact pads thereof.
7. The method for attaching leads to a carrier as recited in claim 6 wherein said leadframe is rectangular and said leads are along each side thereof and extend inwardly and said resilient means are inwardly extending fingers at opposing corners of the leadframe.
8. The method for attaching leads to a carrier as recited in claim 7 wherein the forces to support the carrier and align the contact pads and the leads are caused by a spring effect of each finger on the carrier resulting from the flexing of each finger.
9. A leadframe comprising a frame and a plurality of conductive leads attached to the frame and having lead end portions overlying contact pads on a carrier for an integrated circuit device, such that each end portion of a lead is to be connected to a respective contact pad, and pairs of resilient positioning fingers attached to the frame for engaging respective edges of the carrier so as to support and position the carrier such that each contact pad on the carrier is in contact with a respective end portion of a lead.
10. The leadframe as recited in claim 9 such that each finger of a pair of fingers engages the carrier on opposite edges by resiliently pushing on the carrier in an opposing manner to each other.
11. A leadframe for supporting a carrier for an integrated circuit device and maintaining alignment of contact pads on the carrier with the leads of the leadframe, comprising:
a leadframe circumscribing a predetermined area commensurate in shape and size as the carrier and having a plurality of leads on each side of said frame and extending into the circumscribed area; and resilient carrier positioning means attached to said leadframe and extending into the circumscribed area such that said resilient carrier positioning means engages the carrier by applying forces against the carrier to support the carrier and maintain each of the contact pads of the carrier in contact and alignment with a respective lead of the leadframe.
a leadframe circumscribing a predetermined area commensurate in shape and size as the carrier and having a plurality of leads on each side of said frame and extending into the circumscribed area; and resilient carrier positioning means attached to said leadframe and extending into the circumscribed area such that said resilient carrier positioning means engages the carrier by applying forces against the carrier to support the carrier and maintain each of the contact pads of the carrier in contact and alignment with a respective lead of the leadframe.
12. The leadframe as recited in claim 11, wherein said plurality of leads and said resilient carrier positioning means are attached to inner edges of the leadframe.
13. The leadframe as recited in claim 12 wherein said resilient carrier positioning means consists of fingers attached to inner edges of the leadframe and the forces applied on the carrier result from a spring effect of the fingers pushing on the carrier.
14. The leadframe as recited in claim 13 wherein said leadframe circumscribes a square area and said fingers are attached to each inner corner of the leadframe.
15. The leadframe as recited in claim 13 wherein said fingers engage the carrier at the corners thereof and the end of each finger is shaped to accommodate the respective shape of the corner of the carrier.
16. A leadframe arrangement comprising:
a carrier for an integrated circuit and having a plurality of contact pads;
a leadframe circumscribing an area commensurate in shape and size as said carrier and having a plurality of leads on each side of said frame and extending into the circumscribed area, said leadframe further having resilient carrier positioning means attached to said leadframe and extending into the circumscribed area such that said resilient carrier positioning means engages said carrier by applying forces against said carrier to support said carrier and maintain each of the contact pads of said carrier in contact and alignment with a respective lead of said leadframe.
a carrier for an integrated circuit and having a plurality of contact pads;
a leadframe circumscribing an area commensurate in shape and size as said carrier and having a plurality of leads on each side of said frame and extending into the circumscribed area, said leadframe further having resilient carrier positioning means attached to said leadframe and extending into the circumscribed area such that said resilient carrier positioning means engages said carrier by applying forces against said carrier to support said carrier and maintain each of the contact pads of said carrier in contact and alignment with a respective lead of said leadframe.
17. The leadframe arrangement as recited in claim 16, wherein said plurality of leads and said resilient carrier positioning means are attached to inner edges of the leadframe.
18. The leadframe arrangement as recited in claim 17 wherein said resilient carrier positioning means consists of fingers attached to inner edges of the leadframe and the forces applied on said carrier result from a spring effect of the fingers pushing on said carrier.
19. The leadframe arrangement as recited in claim 18 wherein said leadframe circumscribes a square area and said fingers are attached to each inner corner of the leadframe.
20. The leadframe arrangement as recited in claim 18 wherein said fingers engage said carrier at the corners thereof and the end of each finger is shaped to accommodate the respective shape of the corner of said carrier.
21. The leadframe as recited in claim 11 or 16 wherein there are at least two opposed resilient carrier positioning means having a distance between them that increases when the carrier is placed in the leadframe such that the opposed resilient carrier positioning means flex to resiliently support the carrier.
22. The leadframe as recited in claim 9 wherein there is at least one pair of opposed resilient positioning fingers having a distance between them that increases when the carrier is placed in the leadframe such that the opposed resilient positioning fingers flex to resiliently support the carrier.
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Priority Applications (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA002187477A CA2187477C (en) | 1996-10-09 | 1996-10-09 | Self align leadframe |
| US09/238,824 US6351883B1 (en) | 1996-10-09 | 1999-01-27 | Self align leadframe having resilient carrier positioning means |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA002187477A CA2187477C (en) | 1996-10-09 | 1996-10-09 | Self align leadframe |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2187477A1 CA2187477A1 (en) | 1998-04-09 |
| CA2187477C true CA2187477C (en) | 2002-07-30 |
Family
ID=4159049
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002187477A Expired - Fee Related CA2187477C (en) | 1996-10-09 | 1996-10-09 | Self align leadframe |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2187477C (en) |
-
1996
- 1996-10-09 CA CA002187477A patent/CA2187477C/en not_active Expired - Fee Related
Also Published As
| Publication number | Publication date |
|---|---|
| CA2187477A1 (en) | 1998-04-09 |
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