Premature failures in solder joints between I/O pads and flex-circuit board
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Integrity testing of Sn-Pb solder attachments to Cu/Ni/Au substrates shows that after several thermal cycles the joint fails with very low peel force. The three different thermal excursions experienced by the solder joints during manufacturing and testing cause significant changes in the gold distribution. Analysis reveals that Au plating at the solder area is dissolved into solder and forms Au-Sn intermetallic needles. The amount of Au dissolved into the solder increases with each thermal excursion, because Au available outside the solder area diffuses into the solder bumps. This translates into an increase in the number and length of Au-Sn intermetallic needles, while the composition of the needles stays close to AuSn4. A thin layer of Au-Ni-Sn intermetallic compound starts to form after the second thermal excursion and grows into a thick layer after the third thermal cycle. The thick Au-Ni-Sn intermetallic compound contains approximately 21 wt.% Au, and the high gold intermetallic embrittles the joint. Separation at the interface of this intermetallic layer and solder bump occurs during the peel testing, and the force required for this separation decreases as the gold content increases. The thick intermetallic compound of Au-Ni-Sn is the cause for premature failures. To prevent these failures, it is recommended that the Au plating at the solder area be thinned, and that a larger solder bump be used to reduce the Au concentration in the solder to less than 0.3 wt.%.
Keywordssolder joints thermal excursions intermetallic needles AuSn4 Au-Ni-Sn
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