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Written by Ursula Marquez de Tino   
Thursday, 31 July 2008 19:00

Why lead and Pb-free are often an unhappy marriage.

Soldering Components with Pb-based alloy finishes, put into a Pb-free process, don’t always go willingly. Problems can result, as in the instance recorded here.

It seemed innocent enough: Double-sided boards, including surface mount and through-hole components, were assembled using reflow and wave soldering processes. For both processes, SAC alloys were used and the PCB surface finish was copper OSP. The majority of components used, with the exception of a QFP, had Pb-free finishes. Because of this, the QFP joints were carefully inspected.

However, following assembly, defects were observed in the QFPs. Poor solder joint strength was observed when subjected to pull testing. A 50% decrease in pull strength was observed: The pull force after reflow was recorded to be 12 N for each individual lead of the QFP versus 6 N after the wave process. What went wrong?

Boards were sent to the Unovis Lab for failure analysis. Some cross-sections of the QFP leads showed a gap or separation between the solder joint and lead (Figure 1). This gap lowered the force required to break the joint during pull testing, seriously compromising joint strength.

Image

Other cross-sections of the leads did not show gaps, but instead a lack of intermetallic formation between the lead and solder joint. This lack of intermetallic translates into poor solder joint formation and a weak connection.

An elemental mapping analysis was done on the joints using the SEM/EDX (Figure 2). The analysis showed lead component in the QFP finish had diffused into the Pb-free solder joint, creating a new alloy (SnPb), with a melting point of 183°C. Also, intermetallic formation was observed only between the board and solder joint.

Image

When the boards were subjected to the wave, a maximum topside temperature of 200°C was measured when the boards were over the wave. This high temperature recorded near the QFP was owed to the presence of thermal through-holes. At this temperature, the SnPb on the solder joints becomes molten and flows away from the lead, leaving behind a gap or lack of intermetallic.

Poor pull strength is the result of lead contamination and its diffusion into the solder joint, leaving an exposed lead material or lack of intermetallic formation. Other possible causes for the presence of gaps are board or component warpage. To avoid the unintended double reflow of the QFPs, the customer fabricated a specific heatsink. In general, it is important to pay attention when mixing SnPb into Pb-free applications. Secondary reflow can affect solder joint integrity and assemblies will not comply with RoHS requirements.

Ursula Marquez de Tino is a process and research engineer at Vitronics Soltec, based in the Unovis SMT Lab (vitronics-soltec.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

 

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