The present invention relates to the packaging of integrated circuits (ICs) and more particularly to a method of assembling a Quad Flat Package (QFP) type device.
A QFP is a surface mounted IC package with leads that extend from its four sides. The leads often are shaped or bent into a “gull wing” form. QFP is a mature technology, having been in use since the 1970's, and therefore has the advantages of mature infrastructure of manufacturing tools and materials, as well as lower cost, compared to BGA packages, in the lead count ranging from 48 to 208 IOs. QFPs are formed using a lead frame, and it is expected that the same lead frame can be used to package chips with different die sizes. However, different die sizes means different loop lengths for the bond wires that electrically connect the die or chip to the leads of the lead frame. For example, when the die size is small and the wire pitch is fine, the wire loop length must be extended, which can cause wire sweep during encapsulation, where the longer wire loop cannot withstand the transfer pressure of the liquid mold compound.
One solution to address the longer wire length issue is to increase the inner lead length. However, for cost savings, lead frames have been becoming thinner, such as from 8 mils (that is to say, 200 microns) to 5 mils (that is to say, 125 microns), so longer leads made using thinner metal are prone to bounce during wire bonding, which can lead to poor joint strength between the bond wire and the lead, and bring residual stress to the heel of the second bond, which risks heel break during field application.
It would be desirable to have a reliable method of assembling QFP type devices with a thin lead frame that can accommodate various size dies.