Lead Free 0201 Assembly and Thermal Cycle/Aging Reliability

摘要

The many challenges with 0201 passive component 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 (DOE) study was carried out to investigate the effects of these parameters on assembly defects and reliability.rnThe test vehicles 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, ENIG, Pure Tin, Immersion Silver 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. Pads were 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 (tin-lead and anti-tombstoning and lead free) were used in this investigation.rnTwo 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 the 0201 component using both leaded and lead-free solder alloys. A large number of these assemblies were subjected to isothermal aging at 150℃ and thermal cycling in the range of -55 to 100℃ to establish their reliability. Shear tests were carried out at various aging intervals up to 500 hours to determine the effects of aging damage on strength relative to virgin assemblies. Similarly, shear test data generated before and after 1500 cycles and data for ENIG and ImAg are compared. Weibull plots are given for reliability to establish solder joint fatigue behavior for the lead free assemblies compared to lead-based solder as well as data correlation for various sets of data. In addition, photomicrographs taken using an optical microscope at intervals during thermal cycling to establish damage progress are given. Scanning electron microscopy (SEM) analysis before and after cross sectioning are also performed to reveal microstructural changes and intermetallic formation at 1500 thermal cycles are also included.