SE523164C2 - Mounting arrangement for semiconductor elements with solder beads for alignment to receiving surface - Google Patents

Abstract

A mounting arrangement is proposed wherein a semiconductor element (10, 50) containing integrated circuitry has solder bumps (20) for electrically coupling the IC to a PCB (40) also comprises a number of further raised members (30) of larger volume than the solder bumps, at least one of which is electrically insulated from the IC. The raised members are solder masses that when heated will reflow only onto designated locations on the PCB. During mounting, the raised member will pull the semiconductor element into correct alignment with the receiving surface, allowing easy automatic mounting of the chip using flip-chip techniques regardless of the arrangement of the connection bumps. The raised members also alleviate the effects of thermal expansion mismatch between the chip and PCB by absorbing the predominant proportion of the resulting stress. A chip scale package is further proposed wherein a carrier (60) is applied to the upper surface of the chip (50) and the raised members are applied to the underside of the carrier.

Description

25 30 0523 164 2 is even larger than the chip itself. Examples of such CSPs are described in JP-A-9199540 and JP-A-9129679. Such packages also serve as adapters by allowing essentially all circuit configurations to be mounted in standard circuit board arrangements and have the advantage that they take up considerably less space. However, these packages also share the disadvantage of a complex manufacturing process.

To mount IC chips directly on the receiving circuit, fl ip chip techniques can be used. This usually involves soldering solder balls directly to exposed or deposited connection pads on the chip. The chip is then inverted for mounting on the PCB or other receiving surface. An IP chip mounting arrangement is described in US 5,569,960. In this arrangement, solder balls are formed in accordance with the stresses and voltages to which they are likely to be subjected. The reference shows that solder balls, which have a so-called "hand drum shape" with a narrow waist and a larger surface at connection points, withstand better and thus have a longer life. The different solder ball shapes over a chip are obtained either by using the same volume of solder paste and varying the connection pad surfaces or alternatively by using connection pads of the same size and varying the solder volume. The most stress-resistant solder balls thus have a smaller volume than other solder balls.

A further arrangement described in WO99 / 095 90 uses a cooler bonded to an upper surface of a chip. The radiator has parts which extend over the chip towards the radiator and which are bonded to the radiator using some form of paste.

The connections between the chip and the receiving surface are maintained at a constant size by using recessed parts in a shock-forming jig.

However, when the connections on a chip are not symmetrical, for example when very few connections are present and these are arranged predominantly on one side of the chip, which is common for chips designed for microwave applications, for example, the placement of the chip with all connections is achieved placed in line not easy. Consequently, the assembly process is difficult and the effort and the resulting electrical connections are often unreliable. It is therefore an object of the invention to provide a mounting arrangement which overcomes the disadvantages of the prior art.

It is a further object of the invention to provide a mounting arrangement which enables semiconductor elements to be mounted in a simple manner using in-chip techniques which are easy to implement and which provide reliable connections even when made automatically.

Summary of the invention According to the invention, a mounting arrangement is proposed in which a semiconductor element having at least one connection ball for directly, electrically connecting the element to a corresponding connection pad on a receiving PCB or similar surface and at least one further raised part with a larger volume than the connection ball, which is adapted to bind with a corresponding cushion having a larger area than the connecting cushion, which is arranged in a predetermined position on the receiving surface.

The raised part is advantageously a solder mass, which when heated will fl surface only to designated positions on the receiving surface. As a result of its larger mass, the raised portion will effectively pull the semiconductor element in proper alignment with the receiving surface regardless of the position of the connecting ball.

The increased volume of the raised portion also alleviates the effects of any mismatches between the coefficients of tennis expansion of the semiconductor element and the receiving surface. Such a mismatch exposes the solder connections to shear forces when the circuit is heated during use. The larger volume of the raised part can absorb most of this force, prevent or significantly alleviate wear and tear of the connection balls over time.

According to a further embodiment of the invention, a package is proposed which comprises a semiconductor element having connection balls on a surface for directly, electrically connecting the element with corresponding connection pads on a receiving surface and a carrier applied to the opposite surface. surface. The carrier is dimensioned to extend beyond the edge of the semiconductor element on at least one side. At least one raised part with a larger volume than said connection balls is connected to this projecting part of the carrier to connect the carrier to corresponding pads with a larger surface area than the connection pads on the receiving surface. Advantageously, the carrier extends beyond the semiconductor element on all sides and a raised portion is located at each corner of the carrier. The carrier suitably serves as a ground plane for the semiconductor element where the raised parts electrically connect the ground plane to the receiving surface.

The carrier can also serve as a cooler. Due to the reinforcing effect of the carrier and the automatic alignment achievable with the larger mass of solder provided by the raised parts, this arrangement allows even thin and brittle semiconductor elements such as GaAs chips to be mounted reliably and easily using fl ip. chip technician.

Brief Description of the Drawings Further objects and advantages of the invention will become apparent from the following description of the preferred embodiments given by way of example with reference to the accompanying drawings, in which: Fig. 1 shows a plan view of the underside of a silicon chip incorporating the mounting arrangement according to a first embodiment of the invention; Fig. 2 is a side view of the silicon chip of Fig. 1 mounted on a receiving surface in accordance with the invention; Fig. 3 shows a side view of a GaAs chip applied to a carrier, both of which are mounted on a receiving surface in accordance with a further embodiment of the invention; 10 15 20 25 30 523 164. G. 4 shows a plan view of the underside of the carrier in fi g. 3; and fi g. 5 shows a GaAs chip with a carrier mounted on a receiving surface in accordance with a further embodiment of the invention.

Detailed description of ritninszama F ig. 1 shows the underside of a semiconductor element or chip 10 carrying some form of integrated circuit (IC). In this embodiment, the IC chip is structurally quite strong; a typical material for the IC chip is silicone. Electrodes are provided on the surface of the chip to connect the integrated circuit with external contacts such as on a printed circuit board. The electrodes consist of metallic, conductive pads exposed or deposited on the surface of the chip and may include, but are not limited to, gold, aluminum, or copper. The techniques for making these conductive pads are well known in the art and will not be described further here. The electrical connection between the electrodes and the PCB is accomplished by soldering using raised solder projections, commonly called balls or balls.

The arrangement of these balls 20, or rather the conductive pads on the surface of the chip, is determined by the configuration of the integrated circuit. Thus, although some effort may be made to distribute the balls 20 asymmetrically across the surface of the chip 10, circuit design reasons may make this difficult if not impossible. The balls 20 are generally first bonded to the IC chip 10 and the chip 10 then applied to the desired receiving surface 40, which may be a PCB or thick film or thin film circuit (see fi g. 2). The receiving surface carries corresponding conductive pads to establish electrical contact with the solder balls.

To simplify the mounting of the IC chip 10 on the receiving surface 40, an arrangement of additional projections 30 on the underside of the IC chip 10 is also provided. To more easily distinguish these projections 30 from the connection balls 20, the term "placement balls" will be used. However, as will become apparent from the following description, the balls 30 have more than just a positioning function. These placement balls 30 are advantageously of a metallic solder material but are of larger volume than the connection balls 20 with the volume ratio of placement balls 30 to connection balls 20 advantageously lying in the range of about 2 to 10. The placement balls 30 are bonded to metallic pads provided on the IC chip. Corresponding well-defined wettable surfaces, also advantageously in the form of metallic pads (not shown), are similarly provided on the receiving surface 40. Due to the larger volume of the placement balls 30, the metallic pads on the receiving surface 40 will be of correspondingly larger area than the conductive pads provided for the connection balls. The surface tension of solder allows it to melt only on metal. Thus, when the IC chip is applied to the receiving surface 40 and the placement balls 30 are heated until the solder fl surfaces again, the balls 30 will automatically search for corresponding metal pads. As a result, the placement balls 30 will actively pull the IC chip in proper alignment with the conductive pads on the receiving surface. Due to the larger size of the metallic cushions that fit with the placement balls, i.e. the larger contact area provided for the placement balls 30 on the receiving surface, the alignment is greatly simplified and is practically automatic. In fact, the relatively large volume of solder produced by the placement balls creates a greater alignment force than would be possible with only connecting balls.

In the execution form shown in fi g. 1 and 2, the connection balls 20 and the placement balls 30 are substantially cylindrical in shape and of equal height when the IC chip is bonded to the receiving surface 40. With the preferred volume ratio mentioned above, therefore, the contact surface of the placement balls 30 will be between about 2 and 10 times the surface of the contact surface of the connecting balls 20. It should be mentioned that the solder balls can initially be applied as spherical balls, which is common in the art. They will then assume a substantially cylindrical shape when the solder fl surfaces again during warming and cause the metallic and conductive pads on the receiving PCB 40 to be of the correct size. It will be appreciated that a true cylindrical shape cannot be achieved in practice. A more probable shape for the placement balls 30 would be thinner.

The choice of solder for the connection and placement balls and the solder paste to be used is determined in part by the material forming the metallic pads on the receiving surface 40 and the underside of the chip 10. Preferred materials include gold, tin and indium and alloys thereof. However, although the preferred material for the placement balls 30 is metallic solder, the invention is not limited thereto. In particular, metals which are not good conductors or a non-metallic conductive material such as a polymer can be used. The placement balls can actually be made of a non-conductive material such as, for example, a resin. In order to maintain the alignment of the placement balls 30, corresponding pads provided on the receiving surface should, of course, be of a material that can be easily wetted by the placement ball material.

The placement balls 30 are positioned on the IC chip in such a way that they provide a stable structure to support the IC chip 10. In particular, the placement balls 30 serve as a platform for holding the IC chip substantially parallel to the receiving surface so that reliable connections can be made between all the connection balls 20 and the corresponding conductive pads provided on the receiving surface 40.

Preferred arrangements include a grid (commonly referred to as a surface group) or zigzag arrangement. In a highly preferred embodiment, placement balls 30 are placed at the edges of the chip 10 and more advantageously at each corner.

This enables excellent stability to be achieved. However, it will be appreciated that in some cases a simple placement ball may be all that is needed to properly align the IC chip on the receiving surface. In particular, this may be the case when the existing connection balls are arranged in an arrangement which provides an almost stable structure for supporting the chip 10. Since the alignment function of the placement balls 30 is caused by the reflux of the placement ball material 30 or solder paste is a important and desirable property of the described arrangement, placement balls 30 can be used in applications where only the additional stability is required. In such a case, again the fate of the solder or solder paste can be dispensed with and the bonding between the placement balls 30 and the receiving surface 40 can be effected using tin compression. Such a technique would also allow metals or other materials to be used for the placement balls 30 which have a very high melting point.

When they are made of an electrically conductive material, the locating balls 30 can suitably also be used as ground connections for the IC chip 10.

The placement balls 30 serve an additional purpose. During use, the chip will heat up. The receiving surface, which normally also includes at least passive circuit components, will also be heated. Both the IC chip and the receiving surface will undergo expansion upon heating with the coefficient of thermal expansion (CTE) depending on the thermal expansion coefficient. on the respective material. Although the materials for both the IC chip and the receiving surface are advantageously chosen to have similar CTEs, a degree of thermal expansion mismatch is inevitable. This will manifest itself as a shear force experienced by the solder connections 20, 30. As a result of their larger volume and mass, the locating balls 30 can absorb a larger portion of this force and in this way substantially reduce the thermal stress to which the connecting balls are subjected. for. The effects of a thermal expansion mismatch will inevitably be greater at the edges of the IC chip 10 than in the center and the greatest effects will be experienced at the corners of the IC chip 10. It is therefore preferred that the placement balls 30 be located substantially at the corner of the IC chip as shown in fi g. 1 and 2. Although the connection and position balls 20, 30 showed in fi g. 1 and 2 are substantially cylindrical when bonded to the receiving surface 10, it will be appreciated that the solder balls 20, 30 may be allowed to regain their spherical shape by utilizing a solder paste which locks at a lower temperature than the reflux temperature of the solder balls themselves. As mentioned above, this can also be accomplished by bonding the placement balls 30 to the receiving surface 40 by thermocompression even if no alignment force is generated using this technique. It is further possible that solder paste may be used with the placement balls 30 to increase their volume for any particular application. When both connection and placement balls 20, 30 are bonded using solder paste, it is preferred that a paste be used for the placement balls, which is less dispensed than that of the connection balls 20 and therefore locks at a slightly higher temperature. This ensures that the placement balls remain higher and maintain the desired distance of the chip 10 away from the receiving surface 40.

A further embodiment of the invention is shown in fi g. 3 and 4. In these figures, the parts already described with references to fi g are denoted. 1 and 2 with the same reference numbers.

F ig. 4 shows a further IC chip 50, which is mounted on a receiving surface 40 and electrically connected to the receiving surface using connection solder balls 30 bonded to the underside of the IC chip 50. A carrier 60 is connected to the top side of the IC chip 50 of a solder compound 51. The carrier 60 serves as a booster for the IC chip 50; this arrangement is thus particularly well adapted for very fragile semiconductor components such as a GaAs chip, which can be particularly thin and sensitive to refraction. The carrier 60 can also serve to distribute heat generated by the chip 50, whereby it should be made of a suitable material. The carrier is slightly larger than the IC chip 50 and extends beyond the chip 50 on at least one side. In the arrangement shown in fi g. 3 and 4, the carrier 60 extends beyond the chip 50 on all sides, but by a smaller margin on a pair of opposite sides than on the other. In this arrangement, the locating balls 30 are not provided on the IC chip but on the carrier 60. In particular, the locating balls 30 are located on the underside of the carrier 60 substantially in the corner surfaces thereof. The underside of the carrier is shown in fi g. 4. This consists essentially of a mask 61 of insulating material through which metallic surfaces are visible. These metallic surfaces comprise a substantially rectangular exposed surface 62 located centrally on the carrier 60 to receive the IC chip 50 and approximately circularly exposed metallic surfaces 63 for bonding to the placement balls 30 either with or without the passage of a suitable solder paste. In many applications, it may be advantageous for the carrier 60 not to carry current. For high speed applications, e.g. radio frequency applications, however, the carrier 60 may incorporate, or serve as, a ground plane for the chip 50. The exposed surfaces 63 for bonding to the location balls 30 will then be electrically connected to the ground plane so that the location balls 30 also serve as ground connections.

In this embodiment, the locating balls 30 are not only larger in diameter than the connecting balls 20 but also higher. The structure of the whole is effectively a chip scale package (CSP) with the IC chip 50 directly, electrically connected to the receiving surface 40 by connection balls 20. Due to the carrier 60, the structure is robust, but the IC chip is not encapsulated, thus allowing assembly using fl ip chip techniques. Accurate and reliable alignment of the chip on the receiving surface 40 is made possible by the placement balls 30 in the same manner as described with reference to the embodiments in fi g. 1 and 2.

When the chip 50 is thin compared to the carrier, the carrier 60 is selected to have substantially the same coefficient of thermal expansion (CTE) as the IC chip 50. If the chip 50 is thicker and thus more robust, the CTE of the carrier 60 and the chip 50 may differ slightly grarm without the chip 50 suffering from harmful effects. It will be appreciated that the effective CTE of the package relative to the receiving surface 40 will depend on the relative thickness of the carrier 60 and the chip 50. Thus, the CTE of the package will be determined by the carrier material if the chip is thin compared to the carrier 60. ; the opposite is true if the carrier is relatively thin. As for the embodiment in fi g. 1 and 2, the placement balls 30 absorb most of the shear forces resulting from a CTE mismatch between the packet and the receiving surface 40. It is also possible in this arrangement that at least one placement ball is provided on the IC chip 50 itself. in an analogous manner to that shown in fi g. 1 and 2. Such a ball would, of course, be dimensioned to hold the chip 50 from the receiving surface 40 with substantially the height of the connection balls 20.

In the arrangement shown in fi g. 3, both the ball balls 20 and the position balls 30 are substantially spherical when they bond to the receiving surface 40. As a result, the ball balls 20 tend to exert an upward force on the IC chip 10.

The position balls counteract this force by pulling the carrier 60 downwards. This compression force is considerably increased by alternating the shape of the bonded placement balls 30. This arrangement is shown in fi g. 5.

I fi g. 5, the placement balls 30 are substantially hourglass-shaped with a diameter in the middle of the balls 30, i.e. midway between the receiving surface 40 and the carrier 60 of about 80% of the diameter of the metallic pads. This shape includes compression forces per se and is designed to counteract the upward forces of the spherical connection balls 20 by pulling the carrier 60 toward the receiving surface 40.

This shape is formed by appropriately dimensioning the metallic pads on receiving surface 40 and carrier bottom 60 and providing the correct volume of solder either as a solder ball or by filling the mass with solder paste.

Since the solder will melt completely on the provided metallized surfaces, the desired shape should be formed naturally. Although the balance of forces described above applies to lower temperatures, the position balls will expand as the temperature rises. This is illustrated in the following example given with reference to the structure shown in fi g. 5.

Example The placement balls 30 are considered to have a height of 247 μm with an expansion value of 27 ppm. 10 ul 20 523 164 12 The expansion of the locating balls must be ironed with that of the connecting ball chain which is composed as follows: Element Height um Material Expansion ppm Soldering compound 51 20 Sn / Au 22 Chip 50 127 GaAs 6 Connecting ball (20) 80 Au 14,5 Solder 20 Indium 20 (boundary layer connection ball / PCB) Total 247 In a worst-case scenario, it is assumed that the device has a cold start at -45 ° C with all elements in balance. The initial voltage effect is eliminated. It is further assumed that the connection balls 20 are around 20 ° C warmer than the placement balls 50.

Thus, a suitable operating temperature would be around 80 ° C for the connection balls 20 and 60 ° C for the placement balls 30.

At these temperatures, the positioning balls will have expanded by 0.70 μm and the connecting ball chain will have expanded by 0.34 μm.

An excess expansion of 0.36 μm must therefore be absorbed by the gold connection ball 20 and its solder. Since these together form a structure of 100 μm, an expansion of 0.36% must be absorbed, which is considered completely probable.

In all the embodiments described above, but especially the one shown in fi g. 5, it is possible that the connection balls 20 are not soldered at all without a reliable dry connection can be sufficiently secure by means of compression forces of the placement balls 30. The feasibility of such an arrangement will of course depend on the application. For example, some high frequency applications may require a wet connection.

Claims (13)

10 15 20 25 I 523 164 14 Patent claims A mounting arrangement for a semiconductor element (10, 50), having on one surface at least one connection ball (20) for electrically connecting the semiconductor element (10, 50) directly to a corresponding connection cushion on a receiving surface (40), characterized in that at least one raised portion (30) having a larger volume than said connecting ball (20) and disposed to bond with corresponding pads having a larger surface area than said connecting pad arranged in predetermined positions on the receiving surface (40) for pulling said semiconductor element (10, 50) in the correct direction to said receiving surface (40). A mounting arrangement according to claim 1, characterized in that upph your raised parts (30) are arranged in an arrangement which forms a stable platform for said semiconductor elements (10, 50). A mounting arrangement according to claim 2, characterized in that said raised parts (30) are disposed in a grid arrangement. A mounting arrangement according to any one of claims 1 to 3, characterized in that said at least one raised part (30) comprises a material intended to float at a predetermined temperature. An arrangement according to any one of claims 1 to 4, characterized in that said at least one raised portion (30) is disposed on the surface of said semiconductor element (10). A mounting arrangement according to claim 5, characterized in that said at least one raised part (30) is disposed at one end of said semiconductor elements (10). 10 15 20 25 30 523 164 15 A mounting arrangement according to claim 5 or 6, characterized in that a raised part (30) is disposed at each corner of said semiconductor element (10). An arrangement according to any one of the preceding claims, characterized in that a flat support (60) of larger area than said semiconductor element (50) is mounted on said semiconductor element (50) on a surface opposite said one surface and at least one raised part (30) is disposed on said carrier (60) for coupling said carrier to said receiving surface (40). An arrangement according to claim 8, characterized in that said carrier (50) serves as a ground plane for said semiconductor element (50), said at least one raised part (30) electrically coupling said ground plate to said receiving surface (40). An arrangement according to claim 8 or 9, characterized in that a raised part (30) is disposed at each corner of said carrier (60). An arrangement according to any one of claims 8 to 10, characterized in that said carrier (60) is a heat distributing plate. An arrangement according to any one of the preceding claims, characterized in that said at least one raised part (30) is dimensioned to form an hourglass shape when it is bonded to said receiving surface (40). A semiconductor package comprising a semiconductor element (50), connection balls (20) disposed on and electrically coupled to a surface of said semiconductor element for establishing electrical contact with corresponding connection pads on a receiving surface, characterized by a carrier (60) disposed on and bonded to an opposite surface of said semiconductor element (50) and at least one raised portion (30) of a larger volume than said connecting balls (20), said raised portion (30) being disposed on said carrier to bond to corresponding pads of larger surface than said connecting pads on said receiving surface to pull the carrier (60) into proper alignment with the receiving surface (40).