Alternative Test Solution for Low Power, Small Footprint Leadless Package

Test socket design is a critical element in the semiconductor product development and test development especially for QFN leadless type packages with pre-plated lead surface finish. The pre-plated leads tend to be recessed into the package as the leadframes are pre-taped prior to mold and during molding, mold transfer pressure compresses the tape and causing the molded package to "bulge" and protrude a little higher than the lead. This is also evident if mold bleed is found on the exposed lead surface. However, this phenomenon occurs more frequently on silicon-based tape. Thermoset type tape tends to gasket around the leads when laminated and hence, causing the leads to "protrude" after molding. Other causes can be due to assembly misprocessing which caused the lead to twist or tilt which result in lead been non-planar particularly at wire bonded area. Recessed leads make test socket pin contact with the leads a major problem as pins may contact the "higher" package surface instead of the leads. Variation in design geometry, DUT (Device under test) lead finish, and contamination of the leads further aggravates the problem and makes the socket design more critical and more sophisticated especially when the footprint of the package are getting smaller and smaller and is leadless. Different design of sockets provides specialized electrical and mechanical parameters needed for specific steps in the product testing process. Frequent cleaning of the contaminated socket, criticality in test set-up alignment and retests due to poor contacts are a major contributor to test output losses and a remedy is greatly desired. The search for a solution led to conductive elastomer where high-density fine wires are molded into the elastomer material at fixed pitch. This conductive elastomer is used to improve the signal integrity of testing. The embedded wires are extremely small hence is especially suited for packages where the leads are small. The elasticity of the elastomer is also well suited for recessed leads or lead burrs as it easily absorbs the differences in height. This will eventually reduce the over rejection of open or short failure at test. As the wires are evenly pitched, set-up and alignment is a breeze as all that is required is to overlay it on the test contacts and held in place with a socket doing away with extensive alignment time. It is also easy to clean as the contaminants can be easily removed from the wire surface with sticky tape.