HK1175890A - System and method for securing a semiconductor device to a printed wire board - Google Patents

Description

System and method for securing semiconductor devices to printed wiring boards

Technical Field

The present disclosure relates to semiconductor device packages, and in particular to systems and methods for securing packages to printed wiring boards.

Background

Complex products today often contain multiple devices such as processors and microcontrollers for driving these products. As the number of devices increases and the size of products decreases, printed wiring boards are being used to improve device density. A Printed Wiring Board (PWB), alternatively referred to as a Printed Circuit Board (PCB), houses a number of semiconductor devices connected via circuits or lines in the printed wiring board. These devices are connected to the printed wiring board in a variety of ways. One approach is to solder a Ball Grid Array (BGA) package to conductive pads on the printed wiring board.

However, this current method has its drawbacks. For example, the PWB may not be planar. The BGA is manufactured to tight tolerances to ensure a tight connection with the printed wiring board. Furthermore, for high pin count devices, the pitch or distance between the pins will be in millimeters. Thus, if the PWB is not planar, the BGA attachment may not have a complete connection to the board. Initially, the weak connection may cause intermittent communication failures with other devices on the PWB. Over time, the connection may become completely broken, rendering the device unusable.

Another example is the effect of warping. Most of today's PWBs are constructed as multiple layers comprising multiple materials. These materials have different coefficients of thermal expansion. When exposed to a specified temperature of the system (e.g., -40 degrees celsius to 140 degrees celsius), different layers of the printed wiring board expand and contract at different rates, thus causing warpage of the printed wiring board. Over time, the printed wiring board will become bumpy and the rigid connection of the BGA and solder to the PWB will not accommodate the warpage of the PWB, resulting in a broken connection between the BGA and PWB.

In the above situation, the field engineer may remove the printed wiring board and attempt to replace the disconnected components. There is a risk of damaging the printed wiring board when removing the BGA. There is also a risk of reducing the lifetime of the device or making the device permanently unusable when reattaching the BGA balls and reattaching the BGA to the printed wiring board. Furthermore, replacement of parts is not possible in many applications, thus creating a need for a more secure attachment of the BGA to the printed wiring board.

Disclosure of Invention

According to a specific embodiment of the present invention, a system and method for securing a semiconductor device to a printed wiring board by a compliant spring is provided. In a particular embodiment, the semiconductor device package includes a Ball Grid Array (BGA). The BGA is coupled to a plurality of flexible springs that include one or more turns and are configured to be coupled to the BGA. A spacer configured to align and separate the springs is attached to the BGA and the spring assembly. A soldering aid configured to align with solder on a Printed Wiring Board (PWB) is attached to the PWB. The BGA, spring and spacer assemblies are coupled to conductive pads on a Printed Wiring Board (PWB).

Technical advantages of one or more embodiments of the present invention may include improved flexibility in the x, y, and z axes of the connection between the BGA and the PWB. The improved flexibility in all directions provides several advantages. First, the flexible connection provided by the spring can minimize the disconnection and shorting of the BGA to the PWB's multiple connections. Second, if the PWB becomes uneven due to warping or shrinkage and expansion caused by extreme temperatures, the springs can be adjusted without compromising the connection between the BGA, springs and PWB.

These advantages may reduce disconnection and shorting of the connection between the package and the printed wiring board, thus reducing the need for field service. Further, the method and system may improve product reliability for reduced maintenance needs. This is an important advantage in applications such as military and aerospace where maintenance possibilities are minimal.

Other technical advantages will be readily apparent to one skilled in the art from the following figures, descriptions, and claims. Moreover, while specific advantages have been enumerated above, various embodiments may include all, some, or none of the enumerated advantages.

Drawings

For a more complete understanding of the present invention, and the features and advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1A is a diagram illustrating an example of a disconnection between a Ball Grid Array (BGA) directly attached to a Printed Wiring Board (PWB) due to movement in the x or y axis;

FIG. 1B is a diagram illustrating an example of a disconnection between a BGA directly attached to a PWB due to warpage in the z-axis;

FIG. 2A is a diagram illustrating an assembly process for attaching a spring to a BGA, in accordance with one embodiment of the present invention;

FIG. 2B is a diagram illustrating an assembly process for attaching the BGA and spring assembly of FIG. 2A to a PWB according to one embodiment of the present invention;

FIG. 3 is a diagram illustrating a complete attachment of a BGA and a PWB using a spring in accordance with one embodiment of the present invention; and

FIG. 4 is a flow diagram illustrating an assembly process according to one embodiment of the invention.

Detailed Description

Fig. 1A is a diagram illustrating an example of a Ball Grid Array (BGA) directly attached to conductive pads on a board, such as a Printed Wiring Board (PWB). BGA100 includes semiconductor devices that communicate with other semiconductor devices on PWB 120. The BGA communicates with other devices via balls 130 attached to conductive pads 140 on the PWB 120. If there is movement in the x or y direction, there may be a disconnection 110. If the BGA100 is not securely attached to the conductive pads on the printed wiring board, the operation of the device may be intermittent or completely interrupted. These disconnections 110 cause product failures in the field. In some applications, such as military or aerospace, repair or rework of printed wiring boards is not possible. Where repair is possible, there is a risk of damaging the conductive pads 140 on the Printed Wiring Board (PWB)120 during device removal. There is also a risk of damage or reduced device life during removal from the printed wiring board, reattachment of BGA balls 130, and reattachment to PWB 120.

Fig. 1B is a diagram illustrating an example of a Ball Grid Array (BGA)100 directly attached to conductive pads 140 on a printed wiring board 120. A disconnection 110 may be present if the movement in the z-direction is caused by device warpage or shrinkage and expansion due to extreme temperatures. If the BGA100 becomes disengaged from the conductive pads 140 on the printed wiring board, the operation of the device may be completely interrupted. These disconnections 110 cause product failures in the field. In some applications, such as military or aerospace, repair or rework of printed wiring boards is not possible. Where repair is possible, there is a risk of damaging the conductive pads 140 on the Printed Wiring Board (PWB)120 during device removal. There is also a risk of damage or reduced device life during removal from the printed wiring board, reattachment of BGA balls 130, and reattachment to PWB 120.

FIG. 2A is a diagram illustrating an assembly process for attaching a spring to a BGA, in accordance with one embodiment of the present invention. The BGA 200 may comprise ceramic, plastic, or any other material to provide protection to the circuitry inside the BGA or to withstand the temperatures required for assembly or the final product. The BGA 200 includes a number of balls 226 for electrically coupling the BGA 200 to a circuit board, such as a Printed Wiring Board (PWB). The balls of BGA 200 may include a metal alloy such as tin and lead that is adapted to melt and bond to conductive pads of a PWB at a particular temperature. To begin the assembly process, the BGA 200 is rotated upside down and then the first spacer 208 is aligned with the balls 226 on the BGA 200. The first spacer 208 comprises any material suitable to withstand the temperatures required during the reflow or assembly process, such as Durastone or polyamide. The first spacer 208 includes a hole 220, the hole 220 having a diameter sufficient to allow the set of springs 216 to slide through and make contact with the balls on the BGA 200. The spring 216 comprises a flexible wire such as phosphor bronze and is plated with a suitable material such as silver that facilitates attachment via soldering. In some embodiments, silver plated phosphor bronze springs may be used. The material can be used to solve embrittlement problems. For example, the use of gold plated springs may compromise the quality of the connection due to embrittlement. Gold can become dust in the solder and create voids in the connection. Silver plated phosphor bronze springs do not have these problems. This material may be used in particular embodiments to meet standards such as the joint industry standard IPC J-STD-001D, which lists specific requirements for avoiding embrittlement problems. These standards are often required for certain applications, such as military or aerospace. Alignment pins 212 may be used that fit inside the holes 202 of the first diaphragm 208. Alignment pin 212 comprises any material suitable to withstand the temperature requirements of an assembly or reflow process, such as aluminum or steel. With the spring 216 and pin 212 in place, a second spacer 228, similar to the first spacer 208, is positioned on the first spacer 208 such that the spring 216 is positioned through the apertures 220 of the spacers 208 and 228. In some embodiments, the combined height of the two baffles 208 and 228 is less than the height of the spring 216. Aluminum plate 224 is placed over spring 216 to ensure flatness of the final assembly. Additional details of the assembly process will be described in fig. 4.

Fig. 2B is a diagram illustrating an assembly process for attaching the BGA and spring assembly of fig. 2A to a Printed Wiring Board (PWB), according to one embodiment of the present invention. The PWB230 has conductive pads 234 configured to align with the springs of the component 242, the component 242 including the BGA 200, balls 226 and springs 216 as assembled in fig. 2A. PWB230 is a multilayer board that includes circuitry for connecting a plurality of devices attached to the PWB via soldering techniques. The pads 234 comprise a material, such as solder, that will adhere to the springs when the printed wiring board is placed by an assembly or reflow process. The resulting assembly 242 in the example of fig. 2A will remove the spacers 208 and 228 during the cleaning step of the assembly process. This will be further described in fig. 4. The spring 216 of the assembly 242 is then positioned through the aperture 220 of the cleaning partition 238. The springs 216 of the component 242 are seated on the conductive pads 234 of the PWB 230. Soldering aids 236 may be used to ensure alignment between the PWB230 and the BGA spring assembly 242. A thermal protector 246 is then placed over the component 242 to protect the component 242 from the heat of reflow soldering required during attachment of the spring to the pad 234. As described in more detail in fig. 4, aluminum plate 250 provides weight to ensure a flat connection during reflow soldering.

Fig. 3 is a diagram illustrating a complete attachment of a BGA and a PWB using a spring according to one embodiment of the present invention. The BGA 200 is attached to a spring 216. The spring 216 is attached to a conductive pad 234 on the PWB 230. Like the spacer 238, the soldering assistant 236 is held on the printed wiring board. These springs 216 promote flexibility in the z, y, and z directions. This flexibility may minimize reliability issues that necessitate repair and rework. Furthermore, this may reduce reliability issues causing product failure in high strain applications such as military or aerospace.

Fig. 4 is a flow diagram illustrating an assembly process for attaching a BGA to a PWB as shown in fig. 2A, 2B, and 3 according to one embodiment of the present invention. In step 400, the BGA is placed bottom up so that the balls are exposed. In step 402, a first (or bottom) spacer is placed on the ball so that the spacer is flush with the package. In step 404, flux is applied to the spacer. In step 406, flux is scraped so that all of the holes in the septum are filled with flux. In step 408, alignment pins are placed in the outer holes of the spacer plate. In step 410, a second (or top) spacer is placed on the alignment pins. The second (top) baffle is then pushed to be flush with the bottom baffle in step 412. In step 414, a spring is positioned in each spacer aperture. The flux applied in step 404 will hold the spring in place against the ball of the BGA. The height of the spring may be greater than the height of two partitions that are flush together. In step 416, an aluminum plate is placed on the spring. The aluminum plate ensures good contact between the spring and the BGA ball. Which will further ensure that the contact area between the spring and the conductive pad on the PWB is flat. In step 418, the assembly is reflowed. During reflow, the assembled parts may be placed in an infrared convection or "pizza oven". As an example, the assembled part may be slowly advanced through several thermal zones that allow the solder to slowly heat up and slowly cool down. By way of example only, the industry standard goal is 4 degrees celsius per second to avoid thermal shock, although this may be different in other embodiments. During the reflow process, the BGA solder balls melt onto the springs with the aid of the flux. The solder will only partially travel onto the spring such that one or more turns of the spring are uncovered to maintain spring flexibility. In some embodiments, two turns are left uncovered, however, more or fewer turns of the spring may be left uncovered by solder from the BGA balls. In step 420, the aluminum plate, first (top) spacer and second (bottom) spacer are removed to allow for cleaning and removal of flux and other possible byproducts of the assembly process. In step 422, a BGA soldering aid may be placed on the pad area of the PWB. For example, a Kapton BGA soldering aid may be used that is laser cut into a pattern that matches the pad pattern on the PWB. The cutout exposes the pad for bonding. The thickness of the Kapton aid can vary, but it is typically thick enough to create a cavity on the exposed pads of the PWB. In step 424, flux is applied to the BGA soldering aid. In step 426, the flux is smoothed so that all cavities are filled. In step 428, a cleaning diaphragm is placed on the spring. The spacer is not attached to the PWB or BGA. The purpose of this is to ensure that the springs do not short together during extreme vibrations such as satellite transmissions. The combined spacer and BGA spring assembly is placed onto a soldering aid in step 430. In step 434, a strip of any material suitable to withstand the heat of the reflow or assembly process is placed over the BGA assembly to secure it in place on the PWB. A thermal protector is placed on the package in step 436 to ensure that heat from the next reflow process does not affect the connected springs and BGAs. In step 438, the aluminum plate used in step 416 is placed on the thermal protection from step 436. An aluminum plate is placed over the assembled parts as in step 416, and the parts are placed again by reflow (step 440). The reflow in step 440 is similar to the reflow step 418, however, in this case, the springs are attached to pads on the PWB. After reflow, the connection between the spring and the PWB is complete (step 442) and the aluminum plate and thermal protector may be removed from the assembly.

Although fig. 4 discloses a specific number of steps employed for the method for securing the semiconductor device to the printed wiring board, these steps may be performed by more or fewer steps than those shown in fig. 2 to 4. Further, although fig. 4 discloses a particular order of steps employed for the method of securing a semiconductor device to a printed wiring board, the steps comprising the method of securing a semiconductor device to a printed wiring board may be accomplished in any suitable order.

Although the present invention has been described in preferred embodiments, various changes and modifications may be suggested to one skilled in the art. By way of example only, embodiments may have a different number of pins, the configuration of the arrays may be different, the pitch of the arrays may be different, the size of the balls may be different, the size of the springs may be different and the materials may be different. It is intended that the present invention cover such changes and modifications as fall within the scope of the appended claims.

Claims (16)

1. A system for securing a ball grid array, BGA, to a printed wiring board, PWB, said system comprising:

a ball grid array comprising one or more balls;

a plurality of springs, each spring comprising one or more turns and configured to attach to a ball in the ball grid array;

a spacer comprising a plurality of holes configured to align and separate the springs; and

a printed wiring board including a conductive pad;

wherein said springs are soldered to balls of said ball grid array through said conductive pads of said printed wiring board PWB.

2. The system of claim 1, wherein each spring comprises a flexible wire.

3. The system of claim 2, wherein the spring comprises silver plated flexible phosphor bronze wire.

4. The system of claim 1, further comprising a soldering aid configured to align solder on the printed wiring board.

5. The system of claim 4, wherein the welding aid comprises Kapton or polyamide tape.

6. The system of claim 4, wherein the soldering aid comprises one or more holes configured to align with pads on the printed wiring board; and

the aperture is configured to provide a cavity for containing a liquid.

7. The system of claim 1, wherein the septum comprises polyamide or Durastone.

8. The system of claim 1, wherein the bulkhead includes alignment pin holes and the system further comprises:

a second spacer comprising a plurality of holes and alignment pin holes configured to align and separate the springs; and

disposed between the bulkheads in the alignment pin holes of the bulkheads.

9. The system of claim 1, wherein the printed wiring board comprises one or more layers; and

the top layer includes conductive pads configured to be electrically connected.

10. A method for securing a ball grid array to a printed wiring board, the method comprising:

aligning a plurality of springs with a plurality of balls on a ball grid array;

separating the spring by a partition;

soldering each ball on the ball grid array to one of the springs;

aligning the springs with pads on a printed wiring board; and

attaching the spring to a pad on the printed wiring board.

11. The method of claim 10, wherein aligning the plurality of springs with a plurality of balls on a ball grid array comprises:

aligning one or more spacers flush with the ball grid array BGA package; and

positioning an alignment pin configured to maintain a position of the one or more baffles.

12. The method of claim 10, wherein attaching a plurality of balls on a ball grid array to a spring comprises:

applying flux to the one or more baffles configured to maintain the position of the one or more springs; and

reflow is performed to fuse the balls of the ball grid array BGA to the springs.

13. The method of claim 12, wherein the reflow soldering is performed in a convection oven comprising a plurality of heating zones.

14. The method of claim 10, wherein aligning the plurality of springs with pads on a printed wiring board with a soldering aid comprises:

applying a soldering aid to the printed wiring board;

applying flux to the welding aid; and

the ball grid array is seated with springs and spacers attached to the welding aid.

15. The method of claim 10, wherein attaching one or more springs to pads on a printed wiring board comprises:

disposing a thermal protector on the ball grid array after attaching the springs to balls to protect the ball grid array and springs through a reflow process;

placing an aluminum plate on the thermal protector to ensure a planar connection; and

reflow is performed to attach the spring to the printed wiring board.

16. The method of claim 15, wherein reflowing includes using a convection oven configured to include a plurality of thermal zones.