Solder Attachment of Leaded Components to Thick Film Hybrids

The mechanical reliabilty of solder connections between leaded appliqued components surface mounted to thick films is discussed. The test vehicle was a three-leaded device, known as soldered-on-transistor (SOT) which was soldered using 60Sn-40Pb to standard PtPdAg thick films. The amount of solder per connection was varied between 0.14 and 3.1 mg while the solder pad area on the metallization was held constant at 35 x 35 mil. The cross-sectional area of the SOT leads was 4 x 16 mil. SOT´s were electrically and mechanically tested initially and after accelerated stress testing. It is concluded that about 1.1 mg of solder per connection is optimum although solder amounts between 0.6 and 2.0 mg were shown to be acceptable. Solder amounts as little as 0.14 mg per connection were shown to be clearly inferior. Acceptable long-term reliability is predicted for the optimum solder mass connections based on accelerated constant temperature aging and accelerated temperature cycling.