US20020009869A1 - Reflow of low melt solder tip C4's - Google Patents
Reflow of low melt solder tip C4's Download PDFInfo
- Publication number
 - US20020009869A1 US20020009869A1 US09/833,949 US83394901A US2002009869A1 US 20020009869 A1 US20020009869 A1 US 20020009869A1 US 83394901 A US83394901 A US 83394901A US 2002009869 A1 US2002009869 A1 US 2002009869A1
 - Authority
 - US
 - United States
 - Prior art keywords
 - solder
 - layer
 - melting temperature
 - lead
 - tin
 - 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.)
 - Abandoned
 
Links
- 229910000679 solder Inorganic materials 0.000 title claims abstract description 114
 - 238000000034 method Methods 0.000 claims abstract description 23
 - 230000008018 melting Effects 0.000 claims abstract description 20
 - 238000002844 melting Methods 0.000 claims abstract description 20
 - 229910052718 tin Inorganic materials 0.000 claims description 17
 - 239000000758 substrate Substances 0.000 claims description 13
 - ATJFFYVFTNAWJD-UHFFFAOYSA-N Tin Chemical compound [Sn] ATJFFYVFTNAWJD-UHFFFAOYSA-N 0.000 claims description 12
 - 229910045601 alloy Inorganic materials 0.000 claims description 11
 - 239000000956 alloy Substances 0.000 claims description 11
 - 229910052738 indium Inorganic materials 0.000 claims 7
 - APFVFJFRJDLVQX-UHFFFAOYSA-N indium atom Chemical compound [In] APFVFJFRJDLVQX-UHFFFAOYSA-N 0.000 claims 7
 - BQCADISMDOOEFD-UHFFFAOYSA-N Silver Chemical compound [Ag] BQCADISMDOOEFD-UHFFFAOYSA-N 0.000 claims 5
 - 229910052709 silver Inorganic materials 0.000 claims 5
 - 239000004332 silver Substances 0.000 claims 5
 - 229910052797 bismuth Inorganic materials 0.000 claims 3
 - JCXGWMGPZLAOME-UHFFFAOYSA-N bismuth atom Chemical compound [Bi] JCXGWMGPZLAOME-UHFFFAOYSA-N 0.000 claims 3
 - 229910052793 cadmium Inorganic materials 0.000 claims 2
 - BDOSMKKIYDKNTQ-UHFFFAOYSA-N cadmium atom Chemical compound [Cd] BDOSMKKIYDKNTQ-UHFFFAOYSA-N 0.000 claims 2
 - 238000010438 heat treatment Methods 0.000 claims 2
 - 230000004888 barrier function Effects 0.000 claims 1
 - 238000009792 diffusion process Methods 0.000 claims 1
 - 239000010410 layer Substances 0.000 description 29
 - 239000010949 copper Substances 0.000 description 14
 - 235000012431 wafers Nutrition 0.000 description 11
 - 230000005496 eutectics Effects 0.000 description 7
 - 239000007788 liquid Substances 0.000 description 7
 - 230000009977 dual effect Effects 0.000 description 6
 - 229910020220 Pb—Sn Inorganic materials 0.000 description 5
 - 238000009736 wetting Methods 0.000 description 5
 - 229910020658 PbSn Inorganic materials 0.000 description 4
 - 101150071746 Pbsn gene Proteins 0.000 description 4
 - 230000015572 biosynthetic process Effects 0.000 description 4
 - 239000011651 chromium Substances 0.000 description 4
 - 229910052802 copper Inorganic materials 0.000 description 4
 - 229910052745 lead Inorganic materials 0.000 description 4
 - 229920002120 photoresistant polymer Polymers 0.000 description 4
 - 229910052804 chromium Inorganic materials 0.000 description 3
 - 238000004070 electrodeposition Methods 0.000 description 3
 - 238000009713 electroplating Methods 0.000 description 3
 - 230000005484 gravity Effects 0.000 description 3
 - 229910052751 metal Inorganic materials 0.000 description 3
 - 239000002184 metal Substances 0.000 description 3
 - 239000004065 semiconductor Substances 0.000 description 3
 - 239000007787 solid Substances 0.000 description 3
 - VYZAMTAEIAYCRO-UHFFFAOYSA-N Chromium Chemical compound [Cr] VYZAMTAEIAYCRO-UHFFFAOYSA-N 0.000 description 2
 - RYGMFSIKBFXOCR-UHFFFAOYSA-N Copper Chemical compound [Cu] RYGMFSIKBFXOCR-UHFFFAOYSA-N 0.000 description 2
 - 229910001080 W alloy Inorganic materials 0.000 description 2
 - 230000001133 acceleration Effects 0.000 description 2
 - 238000005137 deposition process Methods 0.000 description 2
 - 239000002355 dual-layer Substances 0.000 description 2
 - 230000008020 evaporation Effects 0.000 description 2
 - 238000001704 evaporation Methods 0.000 description 2
 - 238000005304 joining Methods 0.000 description 2
 - 239000000463 material Substances 0.000 description 2
 - 239000000155 melt Substances 0.000 description 2
 - 238000004377 microelectronic Methods 0.000 description 2
 - 230000004048 modification Effects 0.000 description 2
 - 238000012986 modification Methods 0.000 description 2
 - 238000007747 plating Methods 0.000 description 2
 - 230000008569 process Effects 0.000 description 2
 - 238000001878 scanning electron micrograph Methods 0.000 description 2
 - 230000009471 action Effects 0.000 description 1
 - 230000015556 catabolic process Effects 0.000 description 1
 - 239000000919 ceramic Substances 0.000 description 1
 - 238000003486 chemical etching Methods 0.000 description 1
 - 238000006243 chemical reaction Methods 0.000 description 1
 - 238000006731 degradation reaction Methods 0.000 description 1
 - 230000000593 degrading effect Effects 0.000 description 1
 - 238000000151 deposition Methods 0.000 description 1
 - 230000008021 deposition Effects 0.000 description 1
 - 238000009826 distribution Methods 0.000 description 1
 - 238000000866 electrolytic etching Methods 0.000 description 1
 - 238000005538 encapsulation Methods 0.000 description 1
 - 238000005516 engineering process Methods 0.000 description 1
 - 238000005530 etching Methods 0.000 description 1
 - 230000002401 inhibitory effect Effects 0.000 description 1
 - 230000003993 interaction Effects 0.000 description 1
 - 229910000765 intermetallic Inorganic materials 0.000 description 1
 - 230000000670 limiting effect Effects 0.000 description 1
 - 230000009021 linear effect Effects 0.000 description 1
 - 238000004519 manufacturing process Methods 0.000 description 1
 - 238000001000 micrograph Methods 0.000 description 1
 - 238000004806 packaging method and process Methods 0.000 description 1
 - 230000036961 partial effect Effects 0.000 description 1
 - 230000002093 peripheral effect Effects 0.000 description 1
 - 230000002829 reductive effect Effects 0.000 description 1
 - 229910052710 silicon Inorganic materials 0.000 description 1
 - 239000010703 silicon Substances 0.000 description 1
 - 230000007480 spreading Effects 0.000 description 1
 - 238000003892 spreading Methods 0.000 description 1
 - 239000000126 substance Substances 0.000 description 1
 - MAKDTFFYCIMFQP-UHFFFAOYSA-N titanium tungsten Chemical compound [Ti].[W] MAKDTFFYCIMFQP-UHFFFAOYSA-N 0.000 description 1
 - 229910052721 tungsten Inorganic materials 0.000 description 1
 - 238000001771 vacuum deposition Methods 0.000 description 1
 
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Definitions
- the present invention relates to the reflow of low temperature solder interconnects, especially in microelectronics fabrication.
 - Controlled Collapse Chip Connection is an advanced interconnect technology for microelectronic chip packaging. C4 is also known as “flip chip,” “solder bump” and “solder balls.”
 - C4 The basic idea of C4 is to connect chips, chip packages or other such units by means of solder bumps partially collapsed between the surfaces of two units.
 - Each unit has a pad pattern which corresponds to a mirror image pattern of the other.
 - the bumps of electrically conductive solder bridge the gap between respective pairs of metal pads on the units being connected.
 - the solder bumps on the pads of the first unit are pressed against the corresponding conductive pads on the second unit, resulting in the partial collapse of the solder bump and formation of an interconnect between respective pads. This allows for the simultaneous formation of all interconnects between the units in a single step, in spite of slight variations in the surfaces of the units being joined.
 - the solder bumps are formed directly on the metal pads of one unit.
 - the pads are electrically isolated from each other and other components by the insulating substrate that surrounds each pad.
 - the substrate may be un-doped silicon or some other material.
 - the bottom of each pad is in contact with a via, forming electrical continuity with the chip circuitry.
 - C4 A major application of C4 is in joining semiconductor integrated circuit chips to chip packages. Integrated circuits are fabricated from semiconductor wafers in an array of repeat patterns, then diced into individual chips in order to minimize the processing cost per chip. Once separated into individual units, the chips are then assembled into packages large enough to handle. C4 bumps are placed on the chips prior to dicing, incorporating the benefits of wafer scale processing.
 - Chip sizes are continually shrinking, while circuit densities and I/O counts continue to increase, in order to enhance performance and reduce costs. These trends place higher demands on interconnects, making traditional bonding methods such as wire bonding and tape automated bonding (TAB) very difficult.
 - TAB tape automated bonding
 - C4 solder bumps serve two functions; first, they act as electrical interconnects and second, they act to form a physical bond between the semiconductor chip and package. This demands a very precise placement of each C4 as well as uniform control of solder volumes.
 - solder bumps are by vacuum deposition.
 - a specially made mask with high precision vias is placed over the wafer for locating the solder bumps.
 - the entire assembly is then placed into a vacuum chamber where solder is evaporated through the mask to form solder bumps on the wafer.
 - This deposition process is non-selective, thereby solder deposits throughout the chamber as well as on the mask.
 - the wafer and mask are heated, therefore careful selection of mask material to match the coefficient of thermal (CTE) expansion of the wafer is needed.
 - CTE coefficient of thermal
 - solder bumps are electrodeposition, also called electrochemical plating or electroplating. This method also uses a mask to form solder bumps only at selected sites, but is vastly different than the evaporation technique.
 - Electrodeposition of solder bumps requires a continuous electrically conductive “seed layer” 14 adhered to the insulating substrate.
 - the seed layer 14 function is to carry current necessary for electroplating the solder.
 - FIG. 1A labeled “prior art,” shows a wafer substrate 10 whose surface is overlaid with a conductive layer 11 of either chromium (Cr) or a titanium tungsten alloy (Ti—W). Metal layer 11 will function as part of the seed layer for electrodepositing solder bumps.
 - a third layer 13 of pure copper is deposited over the entire wafer surface.
 - the Cr or Ti—W, Cr—Cu and Cu layers are of comparable thickness.
 - seed layer 14 is deposited, the wafer is coated with photoresist, patterned and then exposed. The unexposed regions can then be developed or dissolved away to leave behind the cured photoresist as a mask 16 shown in FIG. 1A. Photoresist mask 16 forms the desired pattern of holes or vias across the wafer.
 - the next step is the electrodeposition of solder into the vias of the mask 16 . All vias are filled simultaneously with the desired volume of solder during the deposition process.
 - An electroplated solder bump 18 is shown in FIG. 1A. Once the solder bumps 18 are formed, photoresist mask 16 is removed leaving behind the solder bumps 18 and the continuous seed layer 14 .
 - solder bumps 18 In order to electrically isolate solder bumps 18 , it is necessary to remove the seed layer 14 between solder bumps 18 . This is accomplished by etching away layers 11 - 13 with chemical or electrolytic action, in either case the solder bump 18 protects the layers 11 - 13 under it.
 - FIG. 1B shows the seed layers 11 - 13 removed to leave the solder bumps electrically isolated but mechanically fixed to substrate 10 .
 - U.S. Pat. No. 5,486,282 (which is incorporated herein by reference) discloses an invention related to the selective removal of Cu and phased Cr—Cu by electroetching.
 - FIG. 1C shows solder ball 18 ′, formed by melting or reflowing the solder bump 18 of FIGS. 1 A- 1 B. At this stage the solder ball is ready for joining.
 - Solder alloys used in C4 interconnects generally consist of lead (Pb) and tin (Sn).
 - Pb lead
 - Sn tin
 - MLC multi-layer ceramic
 - DCA direct chip attach
 - a Pb—Sn alloy used for the high temperature application may contain 97% Pb and 3% Sn by weight which melts at 353° C., and for the low temperature application may contain 37% Pb and 63% Sn by weight (eutectic PbSn) which melts at 183° C.
 - solder bump 18 to form solder ball 18 ′ Sn present in the solder reacts with the upper most Cu region of the third layer 13 of Cu, to form an intermetallic (Cu x Sn y ) where x is 6 and y is 5 or where x is 3 and y is 1.
 - This intermetallic layer forms a strong bond between the solder ball 18 ′ and the third layer 13 of Cu.
 - the degree of intermetallic formation is self limiting.
 - the excessive amount of Sn can react with and consume the underlying third layer 13 of Cu, degrading the solder-seed layer interface.
 - One method of forming a low temperature C4 structure is by capping a high temperature C4 bump with low temperature eutectic Pb—Sn solder, such as described in U.S. Ser. No. 08/710,992 filed Sep. 25, 1996 by Berger et al. entitled “Method for Making Interconnect for Low Temperature Chip Attachment” (IBM Docket YO996073) and assigned to assignee herein which is incorporated herein by reference.
 - this method does not address the issue of low temperature solder wicking down around the high temperature C4 structure and attacking seed layer 14 from the side or exposed edge.
 - the present invention relates to a process of forming a low temperature tip C4 ball with minimal attack on the edges of the underlying seed layer by reaction with Sn from the low melting solder tip.
 - the invention provides a method of reflowing a low temperature eutectic tip C4 structure with minimal wicking of the molten low melt solder down the side wall of the high melt solder of the C4 during intermediate reflow prior to assembly.
 - a low melting solder tip C4 bump, reflowed with the molten tip facing down in the direction of the earth's gravitational field or force, has been shown to have significantly less wicking of the molten solder up the side wall of the high melt C4 structure. The reduced wicking prevents the attack on the edges of the underlying seed layers by inhibiting the interaction of Sn along the side of the C4 structure.
 - FIGS. 1 A- 1 C are enlarged, cross-sectional views of C4 solder ball formation by electroplating in accordance with the prior art.
 - FIG. 2A is an enlarged, cross-sectional view of an electrochemically fabricated C4 bump made up of about half high melt and half low melt solder.
 - FIG. 2B is a Scanning Electron Microscope (SEM) micro graph of an electrochemically fabricated C4 bump made up of about half high melt and half low melt solder.
 - FIG. 3A is an enlarged, cross-section view of an electrochemically fabricated C4 ball, reflowed with molten tip facing up with respect to earth during reflow.
 - FIG. 3B is an enlarged view of a portion of FIG. 3A.
 - FIG. 3C is an SEM micro graph of electrochemically fabricated C4 ball, reflowed with low melt solder tip facing up with respect to earth during reflow.
 - FIG. 4A is an enlarged, cross-section view of an electrochemically fabricated C4 ball, ref lowed with the low melt solder tip facing down with respect to earth during reflow.
 - FIG. 4B is an enlarged view of a portion of FIG. 4A showing the forces on molten solder of a solder bump.
 - FIG. 4C is an SEM micro graph of an electrochemically fabricated C4 ball, reflowed with molten tip facing down with respect to earth during reflow.
 - the present invention relates to a method of forming a low temperature tip C4 ball, with minimal degradation of the solder-seed layer interface during the preliminary reflow. More specifically, the method was developed to reflow a dual solder alloy C4 bump 24 shown in FIGS. 2 A- 2 B, where the lower portion 26 of solder bump 24 is made of a high melt solder for example 97% Pb and 3% Sn by Wt., and the upper portion 28 is made of a low melt solder for example eutectic Pb—Sn, 37% Pb and 63% Sn by Wt.
 - This dual alloy C4 bump 24 may be electrochemically fabricated by the process described in the related art, with the modification of plating a dual layer solder structure as opposed to a single alloy structure.
 - solder bumps 24 are reflowed. In the reflow of a dual solder, low melt tip C4 bump 24 , the temperature is raised above the melting temperature of the low melt solder (>183° C. for eutectic Pb—Sn). The high melt solder base 26 remains intact while the low melt tip 18 ′ transforms to the liquid state.
 - the equilibrium shape of the liquid is defined by the minimal total interfacial energy for all phase boundaries present.
 - the stable configuration or lowest energy state results in the spreading of the liquid solder 18 ′ over the surface of the solid (also known as wetting).
 - the combination of the forces due to wetting on liquid solder 18 ′ on sidewall 31 is shown by arrow 36 in FIG. 3B and the force due to gravity or acceleration, linear or centrifugal on liquid solder 18 ′ at sidewall 31 is shown by arrow 38 in FIG. 3B.
 - the present invention addresses the issue of the molten solder wicking in a dual alloy C4 bump during preliminary reflow.
 - the wetting forces shown by arrow 36 ′ in FIG. 4B act to cause the molten solder 42 ′ to wet and wick along the sidewall 31 of C4 structure 40 .
 - the wetting forces shown by arrow 36 ′ are offset by the force of gravity or by acceleration as shown by arrow 38 ′ in FIG. 4B.
 - the weight of the molten droplet can be used to prevent the wicking along the sidewall 31 of high melt solder base 26 of C4 structure 40 .
 - This method of reflowing a dual alloy C4 solder bump, with the low melt tip facing down with respect to earth in the direction of the gravitational force prevents the contact of the high Sn containing molten solder 42 ′ with the seed layers 11 - 13 , as shown in FIGS. 4 A- 4 C.
 - the combination of gravity in one direction and the wetting force or surface tension in the opposite direction prevent the molten low melt solder 42 ′ from flowing down the high melt solder standoff 26 and sidewall 31 .
 - FIGS. 1A through 4C of the drawing like references are used for elements or components corresponding to elements or components of an earlier figure.
 - the reflow method of this invention is also applicable to structures other than dual alloy C4 solder structures 40 , for instance, stud or column solder structures.
 
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- Engineering & Computer Science (AREA)
 - Manufacturing & Machinery (AREA)
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 - Wire Bonding (AREA)
 - Electric Connection Of Electric Components To Printed Circuits (AREA)
 
Abstract
An array of C4 solder bumps and a method for making is described incorporating an array of conductive areas on an electrical device, each conductive area having a layer of ball limited metalurgy at the device surface and two layers of solder having respective melting temperatures to form the C4 structure. The method includes melting the second layer of solder in the down position or towards earth to form a C4 solder ball or bump. The invention overcomes the problem of low temperature solder from wicking over the sidewall surfaces of the high melt solder of the C4 structure and attacking the edges of the underlying seed layers of the ball limited metalurgy. 
  Description
-  The present invention relates to the reflow of low temperature solder interconnects, especially in microelectronics fabrication.
 -  Controlled Collapse Chip Connection (C4) is an advanced interconnect technology for microelectronic chip packaging. C4 is also known as “flip chip,” “solder bump” and “solder balls.”
 -  The basic idea of C4 is to connect chips, chip packages or other such units by means of solder bumps partially collapsed between the surfaces of two units. Each unit has a pad pattern which corresponds to a mirror image pattern of the other. The bumps of electrically conductive solder bridge the gap between respective pairs of metal pads on the units being connected. As the units are brought together, the solder bumps on the pads of the first unit are pressed against the corresponding conductive pads on the second unit, resulting in the partial collapse of the solder bump and formation of an interconnect between respective pads. This allows for the simultaneous formation of all interconnects between the units in a single step, in spite of slight variations in the surfaces of the units being joined.
 -  In C4, the solder bumps are formed directly on the metal pads of one unit. The pads are electrically isolated from each other and other components by the insulating substrate that surrounds each pad. The substrate may be un-doped silicon or some other material. The bottom of each pad is in contact with a via, forming electrical continuity with the chip circuitry.
 -  A major application of C4 is in joining semiconductor integrated circuit chips to chip packages. Integrated circuits are fabricated from semiconductor wafers in an array of repeat patterns, then diced into individual chips in order to minimize the processing cost per chip. Once separated into individual units, the chips are then assembled into packages large enough to handle. C4 bumps are placed on the chips prior to dicing, incorporating the benefits of wafer scale processing.
 -  Chip sizes are continually shrinking, while circuit densities and I/O counts continue to increase, in order to enhance performance and reduce costs. These trends place higher demands on interconnects, making traditional bonding methods such as wire bonding and tape automated bonding (TAB) very difficult. C4 allows for very high density I/O with area array distribution as compared to peripheral contacts in TAB and wire bonding.
 -  C4 solder bumps serve two functions; first, they act as electrical interconnects and second, they act to form a physical bond between the semiconductor chip and package. This demands a very precise placement of each C4 as well as uniform control of solder volumes.
 -  One method of forming solder bumps is by vacuum deposition. A specially made mask with high precision vias is placed over the wafer for locating the solder bumps. The entire assembly is then placed into a vacuum chamber where solder is evaporated through the mask to form solder bumps on the wafer. This deposition process is non-selective, thereby solder deposits throughout the chamber as well as on the mask. During deposition, the wafer and mask are heated, therefore careful selection of mask material to match the coefficient of thermal (CTE) expansion of the wafer is needed. However, for this reason, the evaporation technique has limited extendibility to larger wafers.
 -  An alternative technique for making solder bumps is electrodeposition, also called electrochemical plating or electroplating. This method also uses a mask to form solder bumps only at selected sites, but is vastly different than the evaporation technique.
 -  Electrodeposition of solder bumps requires a continuous electrically conductive “seed layer” 14 adhered to the insulating substrate. The
seed layer 14 function is to carry current necessary for electroplating the solder. FIG. 1A, labeled “prior art,” shows awafer substrate 10 whose surface is overlaid with aconductive layer 11 of either chromium (Cr) or a titanium tungsten alloy (Ti—W).Metal layer 11 will function as part of the seed layer for electrodepositing solder bumps. On top oflayer 11 is deposited a thin “phased”layer 12 of 50% chromium and 50% copper (Cr—Cu). Finally, athird layer 13 of pure copper is deposited over the entire wafer surface. The Cr or Ti—W, Cr—Cu and Cu layers are of comparable thickness. Onceseed layer 14 is deposited, the wafer is coated with photoresist, patterned and then exposed. The unexposed regions can then be developed or dissolved away to leave behind the cured photoresist as amask 16 shown in FIG. 1A.Photoresist mask 16 forms the desired pattern of holes or vias across the wafer. -  The next step is the electrodeposition of solder into the vias of the
mask 16. All vias are filled simultaneously with the desired volume of solder during the deposition process. An electroplatedsolder bump 18 is shown in FIG. 1A. Once thesolder bumps 18 are formed,photoresist mask 16 is removed leaving behind thesolder bumps 18 and thecontinuous seed layer 14. -  In order to electrically isolate
solder bumps 18, it is necessary to remove theseed layer 14 betweensolder bumps 18. This is accomplished by etching away layers 11-13 with chemical or electrolytic action, in either case thesolder bump 18 protects the layers 11-13 under it. FIG. 1B shows the seed layers 11-13 removed to leave the solder bumps electrically isolated but mechanically fixed tosubstrate 10. U.S. Pat. No. 5,486,282 (which is incorporated herein by reference) discloses an invention related to the selective removal of Cu and phased Cr—Cu by electroetching. U.S. Pat. Nos. 5,462,638 and 5,800,726 (which are incorporated herein by reference) disclose inventions related to the removal of a Ti—W alloy layer by chemical etching. FIG. 1C showssolder ball 18′, formed by melting or reflowing thesolder bump 18 of FIGS. 1A-1B. At this stage the solder ball is ready for joining. -  Solder alloys used in C4 interconnects generally consist of lead (Pb) and tin (Sn). One characteristic used to select the solder alloy is the melting temperature. Conventionally chips were joined to multi-layer ceramic (MLC) substrates which could withstand temperatures greater than 350° C. However, there is a growing need to attach chips to organic packages, as well as direct chip attach (DCA) to organic boards such as FR 4 boards, which can generally only withstand temperatures less than 300° C. A Pb—Sn alloy used for the high temperature application may contain 97% Pb and 3% Sn by weight which melts at 353° C., and for the low temperature application may contain 37% Pb and 63% Sn by weight (eutectic PbSn) which melts at 183° C.
 -  During the reflow of
solder bump 18 to formsolder ball 18′, Sn present in the solder reacts with the upper most Cu region of thethird layer 13 of Cu, to form an intermetallic (Cux Sny) where x is 6 and y is 5 or where x is 3 and y is 1. This intermetallic layer forms a strong bond between thesolder ball 18′ and thethird layer 13 of Cu. In the high temperature application, with minimal Sn present (3 Wt. %), the degree of intermetallic formation is self limiting. However, in the low temperature application, with eutectic PbSn solder (63 Wt. % Sn), the excessive amount of Sn can react with and consume the underlyingthird layer 13 of Cu, degrading the solder-seed layer interface. -  One method of forming a low temperature C4 structure is by capping a high temperature C4 bump with low temperature eutectic Pb—Sn solder, such as described in U.S. Ser. No. 08/710,992 filed Sep. 25, 1996 by Berger et al. entitled “Method for Making Interconnect for Low Temperature Chip Attachment” (IBM Docket YO996073) and assigned to assignee herein which is incorporated herein by reference. However, this method does not address the issue of low temperature solder wicking down around the high temperature C4 structure and attacking
seed layer 14 from the side or exposed edge. -  Accordingly, the present invention relates to a process of forming a low temperature tip C4 ball with minimal attack on the edges of the underlying seed layer by reaction with Sn from the low melting solder tip. In particular, the invention provides a method of reflowing a low temperature eutectic tip C4 structure with minimal wicking of the molten low melt solder down the side wall of the high melt solder of the C4 during intermediate reflow prior to assembly. A low melting solder tip C4 bump, reflowed with the molten tip facing down in the direction of the earth's gravitational field or force, has been shown to have significantly less wicking of the molten solder up the side wall of the high melt C4 structure. The reduced wicking prevents the attack on the edges of the underlying seed layers by inhibiting the interaction of Sn along the side of the C4 structure.
 -  FIGS. 1A-1C are enlarged, cross-sectional views of C4 solder ball formation by electroplating in accordance with the prior art.
 -  FIG. 2A is an enlarged, cross-sectional view of an electrochemically fabricated C4 bump made up of about half high melt and half low melt solder.
 -  FIG. 2B is a Scanning Electron Microscope (SEM) micro graph of an electrochemically fabricated C4 bump made up of about half high melt and half low melt solder.
 -  FIG. 3A is an enlarged, cross-section view of an electrochemically fabricated C4 ball, reflowed with molten tip facing up with respect to earth during reflow.
 -  FIG. 3B is an enlarged view of a portion of FIG. 3A.
 -  FIG. 3C is an SEM micro graph of electrochemically fabricated C4 ball, reflowed with low melt solder tip facing up with respect to earth during reflow.
 -  FIG. 4A is an enlarged, cross-section view of an electrochemically fabricated C4 ball, ref lowed with the low melt solder tip facing down with respect to earth during reflow.
 -  FIG. 4B is an enlarged view of a portion of FIG. 4A showing the forces on molten solder of a solder bump.
 -  FIG. 4C is an SEM micro graph of an electrochemically fabricated C4 ball, reflowed with molten tip facing down with respect to earth during reflow.
 -  The present invention relates to a method of forming a low temperature tip C4 ball, with minimal degradation of the solder-seed layer interface during the preliminary reflow. More specifically, the method was developed to reflow a dual solder
alloy C4 bump 24 shown in FIGS. 2A-2B, where thelower portion 26 ofsolder bump 24 is made of a high melt solder for example 97% Pb and 3% Sn by Wt., and theupper portion 28 is made of a low melt solder for example eutectic Pb—Sn, 37% Pb and 63% Sn by Wt. This dualalloy C4 bump 24 may be electrochemically fabricated by the process described in the related art, with the modification of plating a dual layer solder structure as opposed to a single alloy structure. -  In order to form a uniform array of C4 solder balls, solder bumps 24 are reflowed. In the reflow of a dual solder, low melt
tip C4 bump 24, the temperature is raised above the melting temperature of the low melt solder (>183° C. for eutectic Pb—Sn). The highmelt solder base 26 remains intact while thelow melt tip 18′ transforms to the liquid state. -  For a liquid such as molten solder in contact with a solid, the equilibrium shape of the liquid is defined by the minimal total interfacial energy for all phase boundaries present. In the case of molten low
melt PbSn solder 18′ in contact with solid highmelt PbSn solder 26, the stable configuration or lowest energy state results in the spreading of theliquid solder 18′ over the surface of the solid (also known as wetting). The combination of the forces due to wetting onliquid solder 18′ onsidewall 31 is shown byarrow 36 in FIG. 3B and the force due to gravity or acceleration, linear or centrifugal onliquid solder 18′ atsidewall 31 is shown byarrow 38 in FIG. 3B. The forces onliquid solder 18′ shown by 36 and 38 results in the encapsulation of the higharrows melt standoff structure 26 by the moltenlow melt solder 18′, as shown in FIGS. 3A-3C. As thelow melt solder 18′ wets sidewall 31 ofhigh melt base 26, it wicks down thesidewall 31 and comes into contact with the edges of underlying seed layers 11-13. In the case of eutectic Pb—Sn tip C4s, the Sn present reacts with the underlying Cu, to form excessive CuxSny intermetallics where x is equal to 6 and y is equal to 5 and degrades the edge of solder-seed layers 11-13 ofinterface 14. -  The present invention addresses the issue of the molten solder wicking in a dual alloy C4 bump during preliminary reflow. By reflowing the dual
layer C4 structure 40 with the lowmelt solder tip 42 facing down with respect to earth as shown in FIG. 4A, the wetting forces shown byarrow 36′ in FIG. 4B act to cause themolten solder 42′ to wet and wick along thesidewall 31 ofC4 structure 40. The wetting forces shown byarrow 36′ are offset by the force of gravity or by acceleration as shown byarrow 38′ in FIG. 4B. By controlling the volume of themolten solder 42′, the weight of the molten droplet can be used to prevent the wicking along thesidewall 31 of highmelt solder base 26 ofC4 structure 40. This method of reflowing a dual alloy C4 solder bump, with the low melt tip facing down with respect to earth in the direction of the gravitational force, prevents the contact of the high Sn containingmolten solder 42′ with the seed layers 11-13, as shown in FIGS. 4A-4C. The combination of gravity in one direction and the wetting force or surface tension in the opposite direction prevent the moltenlow melt solder 42′ from flowing down the highmelt solder standoff 26 andsidewall 31. -  In FIGS. 1A through 4C of the drawing, like references are used for elements or components corresponding to elements or components of an earlier figure.
 -  The reflow method of this invention is also applicable to structures other than dual alloy
C4 solder structures 40, for instance, stud or column solder structures. -  While there has been described and illustrated a method for reflow of low temperature solder tip C4's having a high melt solder standoff or base, it will be apparent to those skilled in the art that modifications and variations are possible without deviating from the broad scope of the invention which shall be limited solely by the scope of the claims appended hereto.
 
Claims (7)
 1. A method for forming an array of C4 bumps on a first substrate of an electrical device comprising the steps of: 
    forming an array of conductive areas on said first substrate, each conductive area including a ball limited metalurgy to provide adhesion to said first substrate and a diffusion barrier, 
 forming a first layer of solder having a first melting temperature on said conductive areas, forming a second layer of solder having a second melting temperature lower than said first melting temperature on said first layer of solder, 
 applying a first force on said first and second layer of solder away from said ball limited metalurgy, and 
 heating said second layer of solder above said second melting temperature to cause said molten second layer to flow into a shape determined by said first force and a second force generated by said molten second layer of solder to wick to the edges of said first layer of solder, 
 said first force adjusted to prevent said melted second layer from wicking over exposed sidewall surfaces of said ball limited metalurgy. 
  2. An array of C4 solder balls on a substrate of an electrical device comprising: 
    an array of conductive areas on said first substrate, 
 a first layer of solder having a first melting temperature on said respective conductive areas, 
 a second layer of solder having a second melting temperature lower than said first melting temperature on said first layer of solder on said respective conductive areas. 
  3. The array of C4 solder balls according to claim 2  wherein said first solder alloy is selected from the group consisting of 63% tin 37% lead, 60% indium 40% lead, 63% tin 36% lead 1% silver, 70% indium 30% lead, 43% lead 43% tin 14% bismuth 80% indium 15% lead 5% silver, 51% tin 31% lead 18% cadmium, 97% indium 3% silver, 58% bismuth 42% tin, 52% indium 48% tin, 46% bismuth 34% tin 20% lead and 44% indium 42% tin 14% cadmium (by weight). 
     4. The array of C4 solder balls according to claim 2  wherein said second solder alloy is selected from the group consisting of 97% lead 3% tin, 95% lead 5% tin, 92% lead 5% indium 3% silver and 92% lead 5% tin 3% silver (by weight). 
     5. A reflow method for an array of C4 bumps on a first substrate wherein said C4 bumps includes a first layer of solder having a first melting temperature on a second layer of solder having a second melting temperature greater than said first melting temperature on said first substrate comprising the steps of: 
    positioning said array of C4 bumps wherein said first layer of solder is facing down with respect to said second layer of solder, and 
 reflowing said first layer of solder. 
  6. The reflow method according to claim 5 , further including the step of heating said C4 bumps to a temperature 20-40 degrees C higher than said first melting temperature but below said second melting temperature of said second solder. 
     7. The reflow method according to claim 5 , wherein upon the completion said step of reflowing, lowering the temperature of said C4 back to below melting temperature while maintaining the orientation of said first solder layer facing down.
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| US09/833,949 US20020009869A1 (en) | 1999-07-21 | 2001-04-12 | Reflow of low melt solder tip C4's | 
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| US09/359,061 US6258703B1 (en) | 1999-07-21 | 1999-07-21 | Reflow of low melt solder tip C4's | 
| US09/833,949 US20020009869A1 (en) | 1999-07-21 | 2001-04-12 | Reflow of low melt solder tip C4's | 
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| US09/833,949 Abandoned US20020009869A1 (en) | 1999-07-21 | 2001-04-12 | Reflow of low melt solder tip C4's | 
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| KR100319813B1 (en) * | 2000-01-03 | 2002-01-09 | 윤종용 | method of forming solder bumps with reduced UBM undercut | 
| US6732913B2 (en) * | 2001-04-26 | 2004-05-11 | Advanpack Solutions Pte Ltd. | Method for forming a wafer level chip scale package, and package formed thereby | 
| US7252514B2 (en) * | 2004-09-02 | 2007-08-07 | International Business Machines Corporation | High density space transformer and method of fabricating same | 
| DE102005030946B4 (en) * | 2005-06-30 | 2007-09-27 | Infineon Technologies Ag | Semiconductor device with wiring substrate and solder balls as a connecting element and method for producing the semiconductor device | 
| US10971398B2 (en) | 2018-10-26 | 2021-04-06 | International Business Machines Corporation | Cobalt interconnect structure including noble metal layer | 
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| US5130779A (en) * | 1990-06-19 | 1992-07-14 | International Business Machines Corporation | Solder mass having conductive encapsulating arrangement | 
| US5462638A (en) | 1994-06-15 | 1995-10-31 | International Business Machines Corporation | Selective etching of TiW for C4 fabrication | 
| US5486282A (en) | 1994-11-30 | 1996-01-23 | Ibm Corporation | Electroetching process for seed layer removal in electrochemical fabrication of wafers | 
| US5800726A (en) | 1995-07-26 | 1998-09-01 | International Business Machines Corporation | Selective chemical etching in microelectronics fabrication | 
| US5808853A (en) * | 1996-10-31 | 1998-09-15 | International Business Machines Corporation | Capacitor with multi-level interconnection technology | 
| US6083773A (en) * | 1997-09-16 | 2000-07-04 | Micron Technology, Inc. | Methods of forming flip chip bumps and related flip chip bump constructions | 
| US5956606A (en) * | 1997-10-31 | 1999-09-21 | Motorola, Inc. | Method for bumping and packaging semiconductor die | 
| JP4343286B2 (en) * | 1998-07-10 | 2009-10-14 | シチズンホールディングス株式会社 | Manufacturing method of semiconductor device | 
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        1999
        
- 1999-07-21 US US09/359,061 patent/US6258703B1/en not_active Expired - Lifetime
 
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        2001
        
- 2001-04-12 US US09/833,949 patent/US20020009869A1/en not_active Abandoned
 
 
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| US20040084509A1 (en) * | 2002-11-01 | 2004-05-06 | Heinrich Meyer | Method of connecting module layers suitable for the production of microstructure modules and a microstructure module | 
| US7199036B2 (en) | 2003-05-31 | 2007-04-03 | The Hong Kong University Of Science And Technology | Under-bump metallization layers and electroplated solder bumping technology for flip-chip | 
| US20060060639A1 (en) * | 2004-09-21 | 2006-03-23 | Byrne Tiffany A | Doped contact formations | 
| US20070111527A1 (en) * | 2005-10-25 | 2007-05-17 | Thomas Gutt | Method For Producing And Cleaning Surface-Mountable Bases With External Contacts | 
| US7723158B2 (en) * | 2005-10-25 | 2010-05-25 | Infineon Technologies Ag | Method for producing and cleaning surface-mountable bases with external contacts | 
| US20080315356A1 (en) * | 2007-06-20 | 2008-12-25 | Skyworks Solutions, Inc. | Semiconductor die with backside passive device integration | 
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| US12132155B2 (en) * | 2012-03-08 | 2024-10-29 | Micron Technology, Inc. | Etched trenches in bond materials for die singulation, and associated systems and methods | 
| US20140166346A1 (en) * | 2012-12-19 | 2014-06-19 | Ngk Spark Plug Co., Ltd. | Ceramic substrate, and method of manufacturing the same | 
| US9338897B2 (en) * | 2012-12-19 | 2016-05-10 | Ngk Spark Plug Co., Ltd. | Ceramic substrate, and method of manufacturing the same | 
| KR20160094431A (en) * | 2013-12-06 | 2016-08-09 | 에프코스 아게 | Method for packaging a microelectronic device in a hermetically sealed cavity and managing the atmosphere of the cavity with a dedicated hole | 
| US9908773B2 (en) * | 2013-12-06 | 2018-03-06 | Commissariat à l'énergie atomique et aux énergies alternatives | Method for packaging a microelectronic device in a hermetically sealed cavity and managing the atmosphere of the cavity with a dedicated hole | 
| KR102115068B1 (en) * | 2013-12-06 | 2020-05-26 | 티디케이 일렉트로닉스 아게 | Method for packaging a microelectronic device in a hermetically sealed cavity and managing the atmosphere of the cavity with a dedicated hole | 
| US20180033756A1 (en) * | 2014-03-13 | 2018-02-01 | Taiwan Semiconductor Manufacturing Co., Ltd | Method for forming bump structure | 
| WO2017164848A1 (en) * | 2016-03-22 | 2017-09-28 | Intel Corporation | Void reduction in solder joints using off-eutectic solder | 
| US11358859B2 (en) * | 2016-12-30 | 2022-06-14 | Sonion Nederland B.V. | Micro-electromechanical transducer | 
| US11760624B2 (en) | 2016-12-30 | 2023-09-19 | Sonion Nederland B.V. | Micro-electromechanical transducer | 
| WO2021061560A1 (en) * | 2019-09-23 | 2021-04-01 | Cisco Technology, Inc. | Low temperature solder in a photonic device | 
| US11181689B2 (en) | 2019-09-23 | 2021-11-23 | Cisco Technology, Inc. | Low temperature solder in a photonic device | 
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