Enhancement of TSOP solder joint reliability using encapsulation

The Thin Small Outline Package (TSOP) is a relatively new type of surface mount component which has a low profile. The unique dimensions of the package allow for it to be used in applications where there are both space and weight constraints. The reduced component profile dictates a shortened lead length, which translates to a less compliant lead when soldered to the card to form the overall coefficient thermal expansion (CTE) of the card. In the case of a TSOP attached to an FR4 card, the low compliance in the leads result in high strains in the solder which contribute to the reduced reliability of the TSOP joints. Encapsulation of the TSOP solder interconnections has been shown to greatly improve the joint reliability. The type of encapsulation material is a commercially available glass filled epoxy resin. IBM has implemented the TSOP encapsulation process in order to achieve the reliability objectives for several products that incorporate TSOP components. Experimental work has been done on both standard 0.062" thick FR4 cards and also thin cards having a nominal thickness of 0.015", The improvement in the encapsulated TSOP solder joint is attributed to a dramatic change in the failure mechanism as compared with an unencapsulated package. In the case of bare leads attached to the card, the solder joint fails as a result of high localized fatigue strains at the heel of the TSOP which is where the crack initiates. Once initiated, the crack propagates in a relatively short period towards the toe of the solder joint, which results in an electrical failure. However, when the solder joints are encapsulated, there is no longer any type of localized fatigue strains on the solder joints. The encapsulant firmly holds the entire package to the card, and uniformly disperses the strains over the encapsulated area between the component and the card. In this paper experimental data will be presented on both standard and thin card cross sections which show the dramatic improvements in TSOP reliability when the solder joints are encapsulated