THE INFLUENCE OF PRINTED CIRCUIT BOARD THICKNESS ON THE THERMAL FATIGUE RELIABILITY OF QUAD FLAT NO-LEAD PACKAGES

摘要

The Pb-free solder joint attachment reliability of two Quad Flat No-Lead (QFN) packages with different body sizes and die to package ratios (DPR) was measured for three different printed circuit board (PCB) thicknesses, 1.2 mm, 1.6 mm, and 2.4 mm. The reliability testing was performed using accelerated thermal cycling (ATC) profiles of -40/125 °C and 0/100 °C in single zone chambers. Testing was performed in accordance with IPC-9701 guidelines using daisy chained components and test boards to allow in situ resistance monitoring of the solder joints during testing. The initial surface mount solder joint assembly quality was documented with x-ray inspection and metallographic cross-sectional analysis using optical and scanning electron microscopy to characterize attachment quality and microstructure. The same metallographic techniques were used for failure mode analysis and for characterizing microstructural evolution during cycling. The failure data are reported as characteristic lifetime (the number of cycles to achieve 63.2% failure) and slope from a two-parameter Weibull analysis. One of the QFN components showed the expected decrease in reliability with increased board thickness, while the second QFN component had less of a reliability dependence on PCB thickness. The QFN component coefficients of thermal expansion (CTE) were measured using Digital Image Correlation (DIC), and the CTE of each PCB was measured using a thermomechanical analyzer (TMA). Component and PCB warpage were measured using the shadow Moire warpage measurement technique. The ATC results are discussed in terms of the QFN attributes and the effect of PCB thickness on reliability. Additional QFN components currently are under test and those results will be reported at a future date. Finite element analysis (FEA) has been incorporated into the program to aid in correlating the relationship between the package attributes, PCB properties and thickness, and cycles to failure in each of the test profiles.