High drop performance interconnection: Polymer cored solder ball

The reliability life of solder joints is a well-known key topic for the electronics industry. Board level temperature cycling test is most common test method and provide an approximate means of relating the results from these performance tests to the reliability solder attachments for the use environments and conditions of electronic assemblies. Recently, due to environmental debates, SnPb solder has replaced by lead free materials. The two most common compositions are Sn3.0Ag0.5Cu and Sn4.0Ag0.5Cu, respectively. Since small and thin package is the future tendency, the structure of shorter interconnection of chip scale package (CSP) with ball grid array (BGA) is customarily occupied for handheld products. In virtue of recognizing package quality for actual status, board level drop impact is a great concern to simulate mishandling during usage. As well known, the common Sn3 ~ 4Ag0.5Cu of lead free solder is poor drop performance due to higher Young's modulus by high volume of Ag3Sn. Lower silver content solder was introduced to replace Sn3 ~ 4Ag0.5Cu solder to satisfy trend requirement. But, the drop performance is not still enough to satisfy the end user demand. In this study, two kinds of the solder ball were investigated, as the pure solder ball - SnlAg0.5Cu and polymer cored solder ball. There are three major layers for polymer cored solder ball. The cored material is polymer. Then, 1 um thickness of Cu was coated on the surface of polymer cored. The outer layer is 10 um thickness of plating SnlAg0.5Cu. A TFBGA 9times8 with 300 mm of ball diameter was used a test vehicle. The surface finish of substrate is Ni/Au. The same lead free flux (water soluble) and ball attach reflow was used for two type solder ball. The peak temperature of reflow profile was 240degC with 50 seconds time above 217degC. Ball shear and cold ball pull test were used to measure the solder joint performance after ball attached. OM and SEM was done the appearance inspection. The non-clean type of Sn3Ag0.5Cu solder paste was used to mount package on the PCB. The reflow profile parameter was the same as the ball attached reflow. After SMT procedure, use X-ray to check the solder joint quality. Subsequently, board level drop and temperature cycle test were performed as per JESD22-B111 test method and IPC-9701 standard, respectively. The PCB is 1/6/1 stack-up with OSP (organic solderability preservative) surface finish. For board level drop test, there must be 15 daisy chained integrated circuits spaced equally on PCB. The PCB must experience a peak acceleration of 1500 G's on each drop for duration of 0.5 milliseconds. For board level temperature cycle test, the test condition is TC 3 (- 40~125degC) with 33 pes daisy chained packages. Weibull distribution analysis was used to plot the curve between of cycle and failure or drop time and failure. Some documents show board Level TCT belong lower strain test, compared with drop test. The characteristic life depends on the standoff height, solder ball composition, package size, I/O number and surface finish. According to the cross-sectional SEM images, the polymer cored solder ball has higher standoff height than SnlAg0.5Cu. The standoff height of polymer-cored solder is 250um. But, normal solder ball is only 180um. That's why polymer cored solder ball has better characteristic life (44802cycles) than SnlAg0.5Cu (3036 cycles). The drop test results showed polymer cored solder ball also has better performance than SnlAg0.5Cu. The characteristic life of SnlAg0.5Cu and polymer cored solder ball are 362 and 467 times, respectively. It indicated that soft polymer cored could reduce the impact during drop test. The failure mode of polymer-cored solder ball was different from SnlAg0.5Cu solder ball. The major crack of polymer-cored solder went into the bulk solder initially and along the copper layer.