By Rudy Sedlak, RD Chemical Co., Mountain View, Calif.
The problem: Test/burn-in boards are expensive, and as they are used, the sockets develop increasing resistivity to such an extent that after about 5000 hours of use, they must be discarded. This results in a very high cost per hour of use. The answer to this problem is now available, and it is relatively easy and inexpensive. Specifically, when test/burn-in boards are new, they have a resistance of less than 10W, and usually less than 5W. Over time, as a result of picking up oxides from the leads of the devices tested, this resistance can increase to more than 100W, at which point the contact becomes unusable. When enough contacts fail, the whole board is discarded. Early Results Were Promising This issue was initially addressed, and although early results were promising, the conductivity increased rapidly when the boards were put back into service, or other problems were created, and it appeared that cleaning up the test sockets was not worthwhile. On further discussion, we determined that the actual problem was oxides, not base metals, and the chemistry of the approach was refined to remove only oxides, without touching any actual metals. This proved to be the answer, but there was a twist to proving it. Test boards loaded with contact sockets were cleaned up and given to an independent party for test, and nothing was heard for months. Then a request came in for more loaded boards to be cleaned. This was done, and the boards were submitted. Again, months went by. Finally, the independent lab opened up, and told us what was causing the delays. The independent lab did not believe the results of its own testing, and so they retested and retested and retested. Cleaned Boards Performed Better The reason they did not believe the results of their own testing was that the cleaned test boards performed better, with lower resistivity, than brand new boards. Specifically, cleaned sockets demonstrated a consistent 2W resistance. Further-more, the boards were then put into use, and lasted at least as long as new boards before they needed to be cleaned again. The resistivity/conductivity life limitation of the test boards/sockets had been extended indefinitely; they will fail from mechanical issues before they become unusable. This was achieved through the use of an aqueous chemical wash, six minutes at 120°F, followed by a DI water rinse and dry. What are the limitations/downsides? Not all soldermasks can tolerate this cleaner, and the cleaner cannot touch aluminum metal, so if stiffeners are used on the boards, the stiffener and rivets must be stainless steel. This new approach will help a little in lowering costs, and will enable the industry to continue to deliver more technology for less money.
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