Structure deformation of leadframe in plastic encapsulation

Compact commercial products make our life better but bring more challenges to IC assembly engineering. To satisfy the requirements, package body becomes thinner and pitch of I/O becomes finer. Although leadframe assemblies have been well developed and plastic encapsulation is quite popular for semiconductor packaging, new problems emerge to the packages due to the compact requirement. During molding process, the viscous epoxy molding compound (EMC) flows over wires and drags them for a distance, then induces the well-known problem called wire sweep. The same phenomenon also happens to the paddle and leads of leadframe and makes them shift, which was rarely mentioned or discussed in the past. Excessive deflection for one lead could be sufficiently severe to cause lead contact and damage the device. Beside, large paddle deformation could cause wire or paddle exposure and decrease the yield. These issues become more and more important when the package becomes thinner and has finer pitch. Therefore, a numerical simulation method was developed to predict the lead deflection of leadframe assemblies, encapsulated with EMC, after molding process. Furthermore, the displacement, stress and strain distribution of the structure were calculated through some simulation technique. X-ray detection and optical microscope inspections, after decapsulated the package, were also introduced to investigate the lead shifting phenomenon. The experimental results showed satisfactory consistency with the simulation. Via this method, engineers could get a lot useful information for the leadframe design, particularly to the thin package during molding process.