SMT007 Magazine

SMT-July2017

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96 SMT Magazine • July 2017 joint microstructure that has poor solder joint integrity in many product use environments. Three industry solutions have emerged as ac- ceptable methodologies to address the potential solder joint integrity issue of a mixed metallur- gy (i.e., lead-free parts used on a tin/lead assem- bly, condition). The first solution is to send the lead-free BGA component to an external ser- vice provide to be "reballed" (i.e., the lead-free solderballs are removed and replaced with tin/ lead alloy solderballs). The reballing process has been shown to be reliable, provided that strictly controlled process procedures are followed [2, 3] . The advantage of a reballed BGA component is that it is transparent to a tin/lead soldering pro- cess; the disadvantages are the cost and time re- quired to reball the BGA component. The reballing of a BGA component requires the control of several key process parameters: moisture sensitivity level, solderball removal/ attachment temperature/time and cleanliness of the reballed BGA component. Functional component testing is necessary to ensure that no process or component defects result from the reballing process. The following example il- lustrates how conducting functional assessment due diligence prevents the introduction of de- fective reballed BGAs into products. A BGA component was found to have func- tional errors during engineering prototype test- ing. The BGA component in question had been procured from the component supplier as a lead-free BGA and was subsequently reballed us- ing a eutectic tin/lead solder alloy. An X-ray of the suspect BGA revealed excessive solder joint voiding. Industry investigations [4] have shown that BGA voiding, in general, is not a solder joint integrity issue but is a clear indicator of a pad design or soldering process problem. The BGA pads did not contain any microvia tech- nology so it was initially believed that an issue relating to a solder paste deposit or reflow pro- file was the voiding root cause. Figure 2 shows the excessive voiding observed in the BGA sol- der joints during X-ray assessment. Metallographic cross-sectional analysis was conducted to verify that the observed solder joint voiding was a solder process issue. The cross-sectional analysis revealed component copper pad thicknesses so thin that, during the assembly reflow process, the molten solderball made contact with the BGA laminate material, USING LEAD-FREE BGAs IN A TIN/LEAD SOLDERING PROCESS Figure 1: Global solder usage trends.

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