Space Transformer Performance Study (Final report)

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Gwen Joleen Jefferson
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1 Space Transformer Performance Study (Final report) Introduction As the pitch of devices becomes smaller, the complexity and cost of the test board becomes significantly higher. In some cases they can no longer be constructed using standard manufacturing processes. This cost increase is due to the reduction in yield that is seen with very high aspect hole drilling and plating. Paricon Technologies has developed a means to solve this technology bottleneck 1 by segmenting the interconnection structure into two components; a standard test board with design rules that allow cost effective manufacture and a small pitch transforming board that translates the pitch from the test board to the pitch of the device under test (DUT). These 2 boards are interconnected by a layer of PariPoser elastomeric conductive material which allows electrical and mechanical performance that can closely approximate that of a single board. Figure 1 presents a schematic view of the space transformer (ST) structure. Figure 1 In addition to reducing the cost and extending the capability of test boards, the use of the PariPoser contact system has also been shown to greatly reduce the rate of test board contact wear due to the well known grinding phenomena caused by conventional contacts. This can greatly extend the life of the DUT board providing further cost reduction. Several companies have incorporated this technology into their fine pitch device testing strategy. Paricon was commissioned to undertake a study to both optimize the physical structure as applied to their system and demonstrate the cycle life capability to either end of life or 1 million cycles. The statement of work is provided as Attachment 1. The final report on this project is presented. Conclusions The data shows that the space transformer structure, with the PariPoser contact interface, is capable of being used for more than 2 million test cycles with little increase in contact resistance. Specifically, the average contact resistance started at ~13 milliohm, dropped to a low of under 8 milliohm and climbed to 9.1 milliohm after 2.1 million cycles but never returned to the original average resistance. The maximum contact initially measure was 24 milliohms. At the end of the study the maximum contact was 28 milliohms. They were not the same contact. At 1 million cycles, the maximum contact resistance was Covered by Patents 7,077,659 and 7,249,954 among others

2 milliohm. This level of stability over 2 million cycles seems to be better than most test contacts available. Apparatus The data acquisition system, load cells, other commercially acquired equipment utilized in this study are listed in Attachment 2. A custom DUT board was designed that matched the 32x34 footprint of the 0.8 mm contact pads used in the actual test boards. Pad height and plating also met the design requirements. Figure 2 presents a photograph of the DUT board. Two hundred fifty eight contacts in the 1088 contact array are utilized in pairs to measure the ST contact resistance using the Kelvin Method. The 4 traces associated with each pair are routed to 4 contacts in the 50 pin connectors which are connected to the data acquisition system. Figure 3 presents a picture of the distribution of the connections utilized on the 32x34 array. This distribution was selected to provide a quality mapping of the contact resistance across the interface with key focus at the corners and middle. Note that each square in the figure represents a pad location in the array. Figure 2a

3 Figure 2b Figure 3 The ST board is a double sided board which incorporates the same pad structure at the PariPoser interface as used in the ST board. The groove in the board and the PariPoser frame are identical to that provided to the customer. Figure 4 is a photograph of the bottom side and top side of the space transformer. As seen in the picture of the top side, adjacent pads are connected with short leads to complete the 2 contact daisy chains.

4 Figure 4a Figure 4b Figure 5 provides a schematic view to indicate the daisy chain interconnection. This shows the relationship between the bottom side contacts and the leads on the top of the ST board. It should be noted that each measured chain contains 2 contacts, 4 plated thru holes and the connecting trace. Figure 5 The stack up included the test socket provided by the customer. The boards and socket were assembled with guide pins and corner mounting as done with the actual board. The static corner load used by the customer was defined as being 24 pounds per corner. The ability to do this with total accuracy using the Belleville washer structure provided was a concern. As a result we chose to use springs that were calibrated by load cells to assure we operated at the required load. It should be noted that based on past experience, we do not think that the accuracy of this load is overly critical. Figure 6 presents a photo of the assembly. Not shown is a thich Mylar sheet placed between the socket and the top of the ST. This was done to prevent any potential shorting issues between the socket contacts and the ST traces. The DUT board /ST / connector system was mounted to an aluminum plate which housed the heaters for heating the system to 110 C. Care was taken to not overly constrain the mounting structures due to a concern over thermal mismatch between the board and aluminum plate. Although the data looks good, the means to mount the board to the backing plate in a manner that deals with potential thermal mismatch is an area for future evaluation.

5 Load Cells Tekscan sensor Figure 6 The aluminum plate is integrated into a frame that houses an air cylinder for applying the dynamic cycling load. Figure 7a shows the final setup and Figure 7b shows the system fully insulated. Shown in the figure is a plastic block mounted in the socket to simulate a device. A short movie showing the cycling action is available and can be provide. Figure 7a

6 Figure 7b Overview The study was divided into two components. The first was to develop an understanding of the static load and dynamic load across the 0.8 mm pitch contact face. This was done using a combination of load cells and an array load sensor. These components are listed in Attachment 2 and the setup shown in Figure 6. Figure 9 shows the load distribution developed across the surface with the 24 pound load established at each corner. Figure 10 shows the same setup with the addition of the 24 grams per contact dynamic load applied by the cylinder. A comparison of the two figures shows the increase in load at the center. Note that the local variability is most likely caused by the sensor pixels (sencells) not being on the same pitch as the board contacts. Also the dark areas on the topo plots are due to the necessity of punching holes through the sensor interconnect so that it could fit on the hole pattern of the ST. The data shows that with the presence of a solid backing plate, the load distribution at each contact meets the design requirements for quality interconnection.

Figure 9 Figure 10 After completing the load analysis, the load sensors were removed and the system reassembled using the

The 110 C temperature cycling was initiated using a load that was ~2x the minimum load.

This was done at 1, 10, 100, 1000, 10,000 etc cycles to allow the generation of a logarithmic plot.

7 Figure 9 Figure 10 After completing the load analysis, the load sensors were removed and the system reassembled using the known, required spring length to re-establish the static load. The 110 C temperature cycling was initiated using a load that was ~2x the minimum load. This was done in recognition that the 32x34 array should have 24 grams per contact load for this study not the connector which had ~1/2 the number of contacts. Periodically the cycling load was held at the applied dynamic load and the resistance of the contact pairs measured. This was done at 1, 10, 100, 1000, 10,000 etc cycles to allow the generation of a logarithmic plot. Figure 11 shows the raw data for the 258 contacts measured in pairs. As shown in Figure 5, each data point consists of 2 contacts in series along with 4 plated through holes and a strap interconnecting the pair. The plot shows the data to 2,161,000 cycles. The average resistance remains very stable over the life of the study.

8 Two Contact Chain Resistance vs Cycle Count (Raw Data Including 2 PariPoser contacts, 4 PTH and Trace) Average Minimum Maximum StDev Resistance (ohms) ,001 1,000,001 1,500,001 2,000,001 Cycle Count E Figure 11 The distribution of the contact pairs plus PTH resistance of the 129 pairs at 110 C after 100,000 cycles is shown in Figure 12a. Here the data is provided in Milliohms. The resistance data after 2.1 million cycles is shown in figure 12b. The topo plot shows how little the resistance has changed over the 2 million cycles. The data of Figure 11 was modified to remove the impact of the 4 PTH and strap by removing a collective resistance of 7 milliohms for the combination. This is less that the smallest direct measurement of the stack without the PariPoser contact. The resultant was divided by 2 and the standard deviation adjusted to reflect the data being taken in pairs. This is show in Figure 13a. The corrected data shows that the average individual contact resistance has dropped from 13 to 7 milliohms over the 1st million cycles and increased to ~9 milliohm after 2 million cycles. This is consistent with previous studies done by Paricon and others. The same data is presented in a topo format in Figure 13b.

9 Figure 12a Figure 12b

10 0.05 Space Transformer Resistance vs. Cycle Life Single Contact Data Average 0.04 Minimum Maximum StDev Resistance (ohms) ,000 1,000,000 1,500,000 2,000,000 2,500,000 Cycle Count E Figure 13a Figure 13b Further analysis of the decrease in resistance over time was done with the raw data. Here, the initial resistance reading for each pair was subtracted from each subsequent reading to provide a better focus on the detailed change in resistance. The results of this are shown in Figures 14a and 14b. These plots shows that there have been 8 contacts that had an increase in resistance in excess of 2 milliohm over the life of the study little

11 change was seen until after the 1 million cycle mark. The worst of these is the lower left corner where the resistance of a contact pair rose by 11 milliohms from a low resistance of ~4.5 milliohms per contact. All of the individual pairs that rose more than 2 milliohm over the study are marked in brown. Al but one of these is in the central area of the ST and they were also in the same bank of connectors. This may be just a coincidence but will be checked to be sure that the contact on the cables are not an issue. Change in Contact Resistance vs. Cycle Count Change in Resistance (ohms) ,000 1,000,000 1,500,000 2,000, Cycle Count Figure 14 Figure 14b

12 Post Experiment Analysis After Completing the 2 million cycle study, the space transformer was disassembled and the contact pads were inspected and photographed. Samples of the best and worst performing contact pairs are shown in Attachment 2. The photographs have been grouped in a manner that allows them to be viewed in their functional pairs. For some pairs pictures were taken through the PariPoser sheet after removing the ST board but prior to removing the PariPoser contact from the DUT board. The following observations were made. All proposed conclusions as should be viewed as being preliminary. None of the contact pads show any wear after the 2 million test cycles. The silver touch marks on the pads are indications of the location of the BallWire contacts touching the pad. This is a transfer of silver to the gold pad which has no impact on the pad performance. The size and number of silver touch marks is correlated to the resistance of the interconnection. More touch marks indicates more BallWires participating in the interconnection. Heavier silver transfer seems to indicate more intimate contact. Contact pair 22 started very low at 18 mω and rose to 29 mω over the 2 million cycles. This includes 4 PTH and strap (7mΩ). Although this started as one of the lowest (~5 mω per contact) it also had close to the greatest increase. The reason for this is not obvious but this pair is at a corner location. Contact pair 20 remained low and stable changing by only 2 mω over the life of the study. This pair has very well formed touch marks on each contact pad. It is also located next to pair 22. Contact pair number 98 started at 49 mω and rose to 63 mω over the study. This pair is representative of the highest resistance seen with the average corrected resistance going from 21 to 28 mω. Note that the BallWire touch marks are much lighter on this contact pair than on the lower resistance pairs. Again, the reason for this is not fully understood. Contact pair 9 showed a high reduction of resistance over the life of the study dropping from 38 to 18 mω. These contacts also have good touch marks but perhaps not as good as 20 and 22. One of these pads has a particle imbedded in it (see arrow). There were only ~8 observations of particles being torn from the PariPoser contact as it was being removed. This may actually be an indicator of the care used to remove the PariPoser sheet since it was well stuck to the board after the 2 million cycles. If less care is used, more particles would be torn from the matrix. Removing the PariPoser contact from the assembly probably is the major cause of contact damage. The pictures of the PariPoser material seem to show a correlation with the size of the touch marks. After 2 million cycles, there is clear indication of permanent deformation which is freezing the shape of the structure prior to disassembly. The circular marks on the PariPoser sheet indicate that the upper and lower pads were misaligned by a few mills. The BallWire columns are also distorted (perhaps) more for the lower resistance contacts indicating that there was more concentration of force at these sites. This distortion is not in a smooth bow format as indicated in our literature but more of a random squeezing outward of the particles from the column center line. This should have been expected. Conclusions The study clearly demonstrates the ability of the PariPoser contact incorporated into the ST geometry to provide low contact resistance over a long cycle life. The definition of end of life needs to be understood to fully determine the useful life of this system. Pad wear is virtually eliminated using the PariPoser contact allowing the life of the test board

13 to be greatly increased. The added cost benefit of having test boards that do not wear out opens the door to a new approach to test board design. Specifically, it should be possible to modularize the test board to have standard, base test boards that can be shared with multiple device designs. Customization can be done using one or more stacked ST boards which become low cost wear members.

14 Attachment 1 Statement of Work Paricon will develop a test capability that emulates the mechanical behavior of the test process being done by the customer. It will consist of a DUT board, PariPoser contact, adaptor board and pseudo device structure. The system will be configured to monitor the resistance of ~120 contacts in pairs using the Kelvin measurement system. These pairs will be distributed at the 4 corners of the array and the middle so as to provide an accurate test sample. Several additional contacts will be configured in an extended daisy chain to provide further information. The plating system on the contacts will be the same as used on the customer s test boards.. The structure will be housed in a structure that allows the continuous cycling of the pseudo part in the system at an applied load that approximates the test load used in production.. A load measurement system that allows the load at each contact pad will be employed to measure the actual applied loads. A load cell will also be used to measure the total load. Note that it will not be possible to measure the individual contact pad load and resistance at the same time. The ability to heat the ST / DUT board interface will be employed. A data acquisition system with meters capable of resistance measurements to under 3 mω will be used to collect the interface resistance data. Appropriate acquisition and data analysis software will be employed to provide a quality understanding of the data. Assembly of the system will incorporate a continuous load between the adaptor board and the DUT board through the PariPoser contact. The first study will be to look at the relationship between this DC load and the resistance to determine the minimum load for quality resistance contact. The second portion of the test will be to cycle the pseudo device to apply a cyclic load to the system that emulates the performance seen in production. This will be done at a temperature of 110 C. The resistance of the contacts will be monitored as a function of cycle life. The goal will be to conduct 1 million cycles as measured by a cycle counter on the system. This will be executed on 2 samples using 2 DUT, 2 adaptor boards and 2 PariPoser contactors from 2 separate manufacturing lots. The wear on the DUT board and Adaptor board pads will be evaluated by microscopic photography as a function of cycle life. A report on the life performance and pad wear will be the deliverable for this project. Paricon will develop the board designs, acquire the boards and construct the test capability. Paricon will also use its data acquisition and measurement capability. It will prepare the software needed to acquire data conduct the experimental data.

15 Attachment 2 Equipment List

16 Attachment 3 Contact Analysis BJ33 ST BJ34 ST BJ33 PP/Board BJ34 PP/Board BJ33 Board BJ34 Board Contact Pair to 29 mω

17 BG33 ST BG34 ST BG33 Board Contact Pair to 18 mω BG34 Board

18 AQ10 ST AQ11 ST AQ10 PP/Board AQ11 PP/ST AQ10 Board AQ11 Board Contact Pair to 63 mω

19 AD33 ST AD334 ST AD33 PP/Board AD34 PP/Board Contact Pair 9 38 to 18 mω

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