Head in Pillow effect BGA

We had a crisis with one BGA causing huge yield losses due to this defect. Nothing we could do with the profile got rid of it. Changing paste didn't get rid of it. It was the BGA warping in reflow, caused the corners to lift up when the solder was molten then the part flattened out as the solder cooled, causing the head in pillow. Thermoire measurements simulating our profile showed what was happening. We had to work with the BGA supplier to resolve it at their end. BGAs naturally want to warp because of the CTE differences of the various materials in the construction, but there are things BGA suppliers can do in their process to reduce it (mold material, mold preheat, clamp pressure, cure time, interposer copper design, etc...).

We've also had instances where it was our fault. Either the stencil apertures were too small or we'd some how let print get out of control and there was insufficient solder to help make up the gap due to warpage. In those cases we either purchased new stencils with proper apertures to assure enough paste or figured out what our manufacturing site had let get out of hand in the print process.

I've also wondered if a paste that exhibited superior hot slump performance would help this problem, but I haven't pursued it yet. Good luck.

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Tell us about: * Temperatures and times of the thermal profile * Where the thermocouples were mounted * Materials involved * Where in the process the 672 BGA component is soldered * Processes that follow that soldering

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