Lead-free 0201 assembly and reliability

Abstract

The many challenges with 0201 assembly can be attributed to the solder paste volume, pad design, aperture design, board finish, type of solder paste, pick and place and reflow profile. A design of experiment study was carried out to investigate the effects of these parameters on assembly defects and reliability. The test vehicles for the study consist of pad layouts for 2000-0201 components. Five different test vehicles were used, with the same pad layout and non-solder mask defined pads, with HASL, Ni/Au, pure tin (Sn), immersion silver (ImAg) and OSP finish. Four different pad shapes are designed on each of the test vehicles (rectangular, oval, modified home plate and double trapezoid). The pad areas for all four shapes are maintained the same. The pads are oriented both in the horizontal and vertical directions. Electroformed 3 mil and 4.65 mil thick stencils were used for printing the solder paste. The stencil was designed to obtain two distinct aperture-pad combinations (matched and unmatched). Three solder paste types (lead-based, lead-free and anti-tomstoning) were used in the study. Two test vehicles assembled for each experimental run, one with resistors and the other with capacitors, provided an understanding of the difference in the process for these two common passive devices. This paper discusses in detail the influence of a few key parameters and defects associated with them using both leaded and lead free solder alloys. The most recent data on shear test results after isothermal aging at 150/spl deg/C are presented.