Issue link: https://iconnect007.uberflip.com/i/1546933
78 SMT007 MAGAZINE I OCTOBER 2026 • Precisely align and attach the BGA to the interposer • Align the completed BGA/interposer stack to the PCB • Reflow using a developed thermal profile Inspect the completed assembly using X-ray, including oblique views when necessary, along with visual and electrical or functional testing as appli- cable. Because the interposer adds another substrate and solder interface, thermal profiling becomes particularly important. The additional material affects heat transfer, temperature uniformity, and the time required for each solder interface to reach the desired temperature. Using Mixed Alloys to Control Reflow One useful strategy is to use different solder alloys at the two interconnect levels. This staged approach is often referred to as step soldering because the assembly sequence takes advantage of alloys with different melting temperatures. For example, the BGA can first be attached to the interposer using a lead-free alloy such as SAC305, which has a liquidus temperature of approximately 217°C. The completed BGA/interposer assembly can then be attached to the PCB using eutectic tin- lead solder, which melts at 183°C. The second reflow profile can therefore be devel- oped to fully reflow the lower-temperature tin-lead interface while keeping the upper lead-free BGA- to-interposer joints below their liquidus tempera- ture. This reduces the likelihood of remelting the first solder interface during final attachment and helps preserve component alignment and solder- joint geometry. This is particularly useful after the BGA and inter- poser have already been aligned and inspected as a subassembly. Instead of subjecting both interfaces to another complete high-temperature reflow cycle, the second process can be focused on forming the K N O C K I N G D OW N T H E B O N E P I L E Figure 3: Example of a completed BGA-interposer-PCB stack.