Additive manufacturing of standoffs between integrated circuit and printed circuit board

Abstract

A method of attaching a chip and a printed circuit board, comprises additively manufacturing an array of conductive standoffs onto conductive pads on a surface of the chip, and connecting the printed circuit board to the array of conductive standoffs.

Description

ADDITIVE MANUFACTURING OF STANDOFFS BETWEEN INTEGRATED CIRCUIT AND PRINTED CIRCUIT BOARDField of the Invention

[0001] The present Application relates to the field of package manufacturing, and more specifically, but not exclusively, to the use of additively manufactured conductive standoffs between chips and printed circuit boards, which increase longterm reliability of the package.Background of the Invention

[0001] Integrated circuits, also known as chips, may be mechanically and electrically connected to electrical contact pads on a printed circuit board (PCB) by several conventional methods. Such methods include a ball grid array (BGA), a pin grid array (PGA), a land grid array (LGA) that uses sockets with pins, a column grid array (CGA), and a ceramic column grid array (CCGA). The mechanical connection is achieved by applying a solder material or conductive adhesive to the PCB or to the chip, to form a solder joint

[0002] During the connection process, the temperature of the chip and the PCB rises, and correspondingly each of these components expands. Typically, the coefficient of thermal expansion (CTE) of the chip and the PCB are different from each other. For example, the CTE of a PCB is about 15-18 ppm / °C and the CTE of the chip is about 3-5 ppm / °C. As the difference in CTE between the PCB and the electronic component increases, shear stress acting on the joint increases, and cracks tend to occur. During the lifetime of the package, repeated heating andcooling cycles deform the joint, thereby reducing the long-term reliability of the package.

[0003] Various approaches currently exist for minimizing the potential for damage caused by shear stress on the joints. One approach is to apply underfill, corner bonding, or edge bonding (e.g., made of epoxy) between joints, in order to provide mechanical reinforcement to the joints. However, the process of creating underfill or applying corner bonding or edge bonding is complicated, lengthy, and expensive.

[0004] Another approach for minimizing the damage caused by thermal expansion is the use of conductive columns in a column grid array. The column grid array increases the height of the chip relative to the printed circuit board, thereby reducing the shear stress on the joint. Typically, column grid arrays are mounted onto a printed circuit board using a through-hole method or inserted into sockets. These manufacturing processes typically require the use of columns made of a uniform material and having uniform dimensions, including particularly height and thickness.

[0005] Additive manufacturing is a process of using computer-aided-design (CAD) software or 3D object scanners to direct hardware to deposit material, layer upon layer, in precise geometric shapes. Additive manufacturing has been used to manufacture electrically conductive materials made of conductive metals such as copper, silver, and gold. Exemplary techniques for additive manufacturing of conductive metals include material jetting, material extrusion, electrohydrodynamic (EHD) printing, direct ink writing (DIW), laser powdered bed fusion, and ultra-precisedeposition (UPD). The feedstock materials for such additive manufacturing include liquid metals, metal particles, and in situ reactive metal inks.Summary of the Invention

[0006] The uniformity restrictions of column grid arrays render them of relatively limited utility. In particular, due to the manufacturing restrictions, column grid arrays may be installed only on a flat surface. However, often a printed circuit board has ceramic capacitors or other materials attached thereto, around which the chip is attached, and which would prevent a perfectly flat surface for the printed circuit board. In addition, column grid arrays are restricted to one type of material, one size of material, and one manner of attachment of the material to the chip or printed circuit board.

[0007] In addition, column grid arrays are expensive to implement. In order to produce a column grid array, it is necessary to solder or weld each column, individually, to the chip. This process is costly and time consuming. Indeed, for this reason, use of column grid arrays is considered a last resort, when there is no other option that will provide the same functionality.

[0008] The present disclosure introduces the use of additive manufacturing in order to create uniquely shaped and sized standoffs for creating height between a PCB and a chip. The additive manufacturing enables the standoffs to be made of different materials, and of different three-dimensional shapes and sizes. The standoffs thus achieve the goal of reducing the shear stress on the soldering connection, with greater adaptability and utility.

[0009] According to a first aspect, a method of attaching a chip and a printed circuit board is disclosed. The method includes: additively manufacturing an array of conductive standoffs onto conductive pads on a surface of the chip; and soldering the printed circuit board to the array of conductive standoffs.

[0010] In another implementation according to the first aspect, the conductive standoffs are comprised of a plurality of layers of conductive materials. Optionally, the conductive materials comprise one or more of gold, silver, copper, platinum, or conductive polymers.

[0011] In another implementation according to the first aspect, the method further includes forming the conductive standoffs in a substantially cylindrical or spherical form.

[0012] In another implementation according to the first aspect, the method further includes forming the array of conductive standoffs with standoffs having different heights or angles relative to the chip.

[0013] In another implementation according to the first aspect, the method further includes forming the conductive standoffs in a coil-like or spring-like form. Optionally, the step of forming the conductive standoffs comprises forming at least partially overlapping layers of conductive materials, additively manufacturing polymeric spacers in areas between the layers, and removing the polymeric spacers following completion of the conductive standoffs to thereby retain the coil-like or spring-like conductive standoffs.

[0014] In another implementation according to the first aspect, the method further includes, prior to the step of additively manufacturing the array ofconductive standoffs, additively manufacturing the conductive pads onto the surface of the chip.

[0015] According to a second aspect, an apparatus includes a chip having conductive pads thereon; a printed circuit board; and an array of conductive standoffs additively manufactured onto the conductive pads of the chip and electrically connecting between the chip and the printed circuit board.

[0016] In another implementation according to the second aspect, the conductive standoffs are comprised of a plurality of conductive materials. Optionally, the conductive metals comprise one or more of gold, silver, copper, platinum, or conductive polymers.

[0017] In another implementation according to the second aspect, the array of conductive standoffs is formed with standoffs having different heights or angles relative to the chip.

[0018] In another implementation according to the second aspect, the conductive standoffs are formed in a substantially cylindrical or spherical form.

[0019] In another implementation according to the second aspect, the conductive standoffs are formed in a spring-like or coil-like form.Brief Description of the Drawings

[0020] FIGS. 1A-1E illustrate the effect of height of the joint on the reliability of a joint, according to embodiments of the present disclosure;

[0021] FIG. 2 is a schematic depiction of a prior art pin grid array;

[0022] FIG. 3 illustrates a process of additive manufacturing of a standoff, according to embodiments of the present disclosure;

[0023] FIG. 4 depicts a first embodiment of additively manufactured standoffs, with straight or cylindrical standoffs, according to embodiments of the present disclosure;

[0024] FIG. 5 depicts a second embodiment of an additively manufactured standoff with diagonal standoffs, according to embodiments of the present disclosure;

[0025] FIG. 6 depicts a third embodiment of an additively manufactured standoff with pyramidal standoffs, according to embodiments of the present disclosure;

[0026] FIG. 7 depicts a method of manufacture of a fourth embodiment of an additively manufactured standoff with spring-like standoffs, according to embodiments of the present disclosure; and

[0027] FIG. 8 is a schematic depiction of the additively manufactured standoffs of FIG. 7 arranged between a chip and a printed circuit board, according to embodiments of the present disclosure.Detailed Description of the Invention

[0028] The present application relates to the field of package manufacturing, and more specifically, but not exclusively, to the introduction of conductive standoffs that are additively manufactured between chips and printed circuit boards, which increase long-term reliability of the package.

[0029] Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not necessarily limited in its application to the details of construction and the arrangement of the components and / or methods set forth in the following description and / or illustrated in the drawings and / or the Examples. The invention is capable of other embodiments or of being practiced or carried out in various ways.

[0030] FIG. 1A schematically illustrates basic components of a connection between a chip 101 and a printed circuit board (PCB) 102, with solder or conductive adhesive joint 103. In the example of FIG. 1A, joint 103 has a relatively low height H.

[0031] FIG. IB illustrates the effect of heating on the extension of the chip101 and the PCB 102. In the top view of FIG. IB, the package is at room temperature. In the middle view of FIG. IB, the package is increased to 183°C, such as during a reflow or curing process. Both the chip 101 and the PCB 102 expand. Since the coefficient of thermal expansion of the PCB is greater than that of the chip, the PCB expands more than the chip. In the bottom view of FIG. IB, when the temperature of the package goes down again, the chip 101 and PCB 102 retract to their original dimensions.

[0032] Referring to FIG. 1C, the uneven expansion of the chip 101 and PCB102 causes shear stress on the joint 103. Over time, this shear stress may cause a crack 104 to form within the joint 103. The development of shear stress and an eventual crack is illustrated in three views from right to left in FIG. 1C. FIG. ID is a photograph of solder 103 having cracks 104 formed therein. The cracks within the solder reduce the long-term life of the package. Even when the package is stillusable, e.g., because the intact solder still conducts electric current, the presence of the cracks renders the performance of the package less reliable. Typically, during a reflow process, the layer nearest the interface of the copper pads and the solder is a composite of tin and copper. This layer is more brittle than the solder itself, which is a composite of tin and lead, which is why the solder 103 is likely to break at this location.

[0033] FIG. IE illustrates the effect of the introduction of height on the strain exerted on the solder. The stress may be expressed as the following equation:in which osis the stress, E is the elastic modulus of the joint, AT is the change in temperature, ACTE is the difference in the coefficient of thermal expansion of the two materials, L is the length from the center of the assembly to the joint, and H is the height of the joint. Plainly, as the height of the joint increases, the shear stress decreases. In the right view of FIG. IE, a relatively short joint 103a connects between chip 101a and PCB 102a. During heating, the relative expansion of each of the components causes angle di to form between the joint 103a and the PCB 102a. In the left view of FIG. IE, a taller joint column 103b connects between chip 101b and PCB 102b. When the package is heated, an angle ot2forms between solder column 103b and PCB 102b, with a2being less than ai.

[0034] FIG. 2 schematically illustrates a prior art column grid array connecting between a chip 201 and a PCB 202. Each of the columns 210 is of uniformconstruction, including materials, height, and thickness. Each column 210 is individually connected to the chip 201.

[0035] FIG. 3 illustrates a method of manufacture of additively manufactured standoffs, according to embodiments of the present disclosure. The chip 301 is manufactured with pads 311 and, optionally, a solder mask 322. The solder mask 322 serves as a passivation layer. The solder mask has gaps in the region of the pads 311, to permit deposition of conductive material therebetween. The solder mask 322 is useful in order to prevent accidental deposition of conductive material onto locations between the pads 311. When the solder mask is used, it is removed following deposition of the standoffs, using conventional methods. In alternative embodiments, the solder mask is not required, as the additive manufacturing process is sufficiently precise to permit controlled deposition of the solder paste.

[0036] As shown in FIG. 3, an additive manufacturing device 320 deposits droplets 321 of the conductive material onto the pads 311. The conductive pads 311 themselves may be additively manufactured using the same equipment that is used for the manufacture of the standoffs. The droplets are merely illustrated as a demonstrative, and any suitable form of additive manufacturing may be employed. In one embodiment, the additive manufacturing process is aerosol jetting, such as that performed by Optomec®. Another additive manufacturing solution involves Laser Induced Transfer Molding, such as that provided by ioTech. Other possible options for the additive manufacturing include use of a nozzle, screen printing, or dispensing. The conductive material may be, for example, silver, copper, gold, platinum, a conductive polymer, or any combination thereof. Of course, the specificadditive manufacturing technique that is most desirable may differ based on which conductive material is employed.

[0037] Because the standoffs are made in an additive manufacturing process, they may take any shape. Moreover, the standoffs need not be shaped in a uniform manner, and different standoffs in an array may be shaped or oriented differently. For example, the standoffs of FIG. 4 are rectangular or cylindrical, similar to columns of a column grid array. The standoffs 312 of FIG. 5 are angled, with the degree of inclination being higher the further that the standoffs are positioned from the center of the chip. Standoffs 312 are pitched at an angle at room temperature, such that, during thermal expansion of the package, the standoffs straighten out. In the view of FIG. 6, the standoffs 313 take the form of a rhombus. The standoffs may alternatively be spherical, similar to balls of a ball grid array.

[0038] In the embodiment of FIG.7, the standoffs 310 are shaped in the form of springs. The springiness is achieved by forming the standoffs as partially overlapping layers 315 of conductive material. In order to enable the layers 315 to be formed with gaps therebetween, a polymeric spacer 314 is printed, in an additive manufacturing process, in the space between each overlapping layer. This polymeric layer is then removed, using processes known to those of skill in the art (chemical, laser, etc.), leaving the spring-shaped standoffs 315. The springs may be coiled, or may be made of other spring-like shapes.

[0039] Advantageously, the springs provide multidimensional degrees of freedom. The additional degrees of freedom further reduce the shear stress generated on the standoffs. Generally, the strain on an object, when a perpendicularforce is applied, is inversely proportional to the modulus of elasticity of that object. The modulus of elasticity is controlled, in part, by the mechanical properties of the object. Thus, when a standoff that is undergoing strain during heating and cooling of the chip and PCB, the net strain that is experienced by the standoff, and especially at the contact points between the standoff and the chip and PCB, is reduced, due to the flexibility of the spring. In brief, the spring bends but does not break.

[0040] The manufacturing process may proceed for multiple standoffs in parallel. Advantageously, the additive manufacturing process allows for standoffs of different heights and dimensions to be manufactured simultaneously.

[0041] The standoffs may have varied heights and three-dimensional structures, both relative to each other and along the heights of each standoff. For example, some of the standoffs may be cylindrical, others may be spherical, and others may be spring-like.

[0042] Following completion of the array of standoffs, the array is then connected to the PCB using standard methods. The results of such a connection process are illustrated in FIG. 8, using, as an example, the standoffs illustrated in FIG. 7. During this process, soldering or conductive adhesive material 316 may be provided between individual standoffs 310 and the PCB 301. A reflow or curing process may be performed in order to solidify the bond between the PCB, standoffs, and soldering material.

Claims

What is claimed is:

1. A method of attaching a chip and a printed circuit board, comprising: additively manufacturing an array of conductive standoffs onto conductive pads on a surface of the chip; and connecting the printed circuit board to the array of conductive standoffs.

2. The method of claim 1, wherein the conductive standoffs are comprised of a plurality of layers of conductive materials.

3. The method of claim 2, wherein the conductive materials comprise one or more of gold, silver, copper, platinum, or conductive polymers.

4. The method of claim 1, further comprising forming the conductive standoffs in a substantially cylindrical or spherical form.

5. The method of claim 1, further comprising forming the array of conductive standoffs with standoffs having different heights or angles relative to the chip.

6. The method of claim 1, further comprising forming the conductive standoffs in a coil-like or spring-like form.

7. The method of claim 6, wherein the step of forming the conductive standoffs comprises forming at least partially overlapping layers of conductive materials, additively manufacturing polymeric spacers in areas between the layers, and removing the polymeric spacers following completion of the conductive standoffs to thereby retain the coil-like or spring-like conductive standoffs.

8. The method of claim 1, further comprising, prior to the step of additively manufacturing the array of conductive standoffs, additively manufacturing the conductive pads onto the surface of the chip.

9. An apparatus comprising: a chip having conductive pads thereon; a printed circuit board; and an array of conductive standoffs additively manufactured onto the conductive pads of the chip and electrically connecting between the chip and the printed circuit board.

10. The apparatus of claim 9, wherein the conductive standoffs are comprised of a plurality of conductive materials.

11. The apparatus of claim 10, wherein the conductive materials comprise one or more of gold, silver, copper, platinum, or conductive polymers.

12. The apparatus of claim 9, wherein the array of conductive standoffs is formed with standoffs having different heights or angles relative to the chip.

13. The apparatus of claim 9, wherein the conductive standoffs are formed in a substantially cylindrical or spherical form.

14. The apparatus of claim 10, wherein the conductive standoffs are formed in a spring-like or coil-like form.

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