Secondary Reflow of Lead-Free Joints
01 July 2007
Control of board temperature is very important during wave soldering to avoid secondary reflow on the topside of the board. This can occur when a board passes over the solder wave and the heat is transferred through the board by conduction. Correct topside profiling can eliminate this issue by use of a process control tool to monitor changes in contact time and temperature.
Secondary reflow occurs when the temperature of the reflowed alloy is reached or exceeded during wave soldering. Secondary reflow of tin/lead joints and joint separation can be exaggerated by:
High solder wave temperatures
Long wave contact times
Higher pre-heat temperatures
Variation of board thickness or density
Sag and flex of the printed board during processing
Sealed nitrogen chambers
It is understood that some companies have experienced problems over the last couple of years with secondary reflow of lead-free joints during wave soldering. This has led to separation of the termination from the bulk of the joint. In the past the same phenomenon has occurred with traditional tin/lead alloys when the top surface of the board reached or exceeded 180oC.
Lead separation has been seen in the past on large components where the board and the component body can flex against each other during wave contact effectively pushing corner joints apart and leading to an intermittent joint. The lead can be left sitting on the surface of the joint like a ball in a cup. On some occasions with tin/lead joints the lead and the bulk of the joint has been seen to separate from the pad surface. The first conclusion is this is a solderability issue with the PCB plating but it may not be.
In the case of a lead-free process it is assumed that if tin/lead is used on the PCB or on the components and it does not fully dissolve into the bulk of the solder, the layer at the interface could weaken or reflow at a lower temperature than the paste alloy. The same problem has been highlighted by researchers and early users of lead-free with Bismuth alloys, who have experienced lead contamination issues as well.
Practical testing by the author has shown this to occur on tin/lead components soldered with SAC paste and then reflowed again with a peak temperature of 175-180oC. Simulations conducted by NPL and measurements taken on peel strength at elevated temperatures clearly shows a reduction in strength of the joints. SEM examination of the joint interfaces show the difference in joint separation with a hot tear on the joint surface as opposed to a brittle fracture that occur when mechanically separated.
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