METHOD FOR IDENTIFYING AND MIGRATING TIN WHISKER RISKS (17-Nov-2009)

METHOD FOR IDENTIFYING AND MIGRATING TIN WHISKER RISKS

FIELD OF INVENTION

[0001]               The invention relates to method of identifying tin whisker growth risk on a board and mitigating this risk using a structured approach.

BACK GROUND OF THE INVENTION

[0002]               ROHS compliance of electronic components calls for moving from lead based solders, or component lead finishes to lead-free based finishes. One of the alternative to lead is usage of tin based alloys. Tin(Sn) is known to exhibit a phenomenon called whiskers. These whiskers are hair like structures that grow on the surfaces. The whiskers can be of any shape, length and can grow irrespective of the external environment conditions; tin whiskers grow even in vaccum. Tin whiskers can grow from few micrometers to upto 10mm. Typcially they are known to grow to less than 1mm.

[0003]               One of the major problems that arise out of the whiskers growth in electronic components is the possibility of these whisker growing on adjacent IC pins and shorting these pins. If these pins are at different potentials e.g. power and ground, it can create a permanent or temporary short between power and ground pins that can lead to the product failures in the field.

[0004]               Another case may be where the state of a signal is permanently altered to a logic level '1' or '0'.Considering the unpredictable nature of the tin whiskers, it is very important that these components be tested for the tin whisker growth. Tin whisker testing typically runs for a period of 5000 hours ( as per iNEMI guidelines). This test is only a testing technique and a 'Pass' result on this test does not guarantee that tin whiskers will not form over the product life. This, however, is an acceptable testing mechanism for component level qualification. The problem arises in a typical board where different type of active and inactive components (with different component finishes) are used. In this case, not all the components may have tin whisker tests results to support 'no whisker growth' on these components. In this case, carrying out a tin whisker test at board level does not address this problem, as there are multiple equivalents available for these components from different manufacturers and with different finishes.

[0005]               Some of the existing solutions include: carrying out tin whisker testing as per iNEMI guidelines. If the test passes, the component is marked as 'No tin whisker risk'. Alternatively, components are used with a component pitch more than a specified distance as per iNEMI guidelines. e.g. for a IC component with Matte Sn finish, if the component lead spacing is greater than 4mm, it does not present a tin whisker growth risk and can be used without any tin whisker testing. However this is practically not possible as IC component packages like TQFP (Thin Quad Flat Pack) with a lead spacing of ~0.23mm use...