Finding of my problem posted couple of days ago: There is some problem with the solder paste. It can be overcome by: 1. Changing it to another paste date code 2. Changing the reflow profile to be more "triangular" by lowering down first few preheat zone. The paste supplier tested a good lot using J-Std (I told them to dump the first test mentod which is IPC). Both passed spec but some "hair/tailing" was observed for the problematic paste.
Thanks to Dave F & Charley Qin for their reply. ________________________________________
Has anyone come across PLCC-44 & above misalignment after reflow. I tried to solve the problem by eliminating factors one by one until at one stage, I open up the oven when the board is half way through the oven (in preheat zone)and found the PLCC already misaligned. I saw slumping of the solder paste and decided to lower the first 4 preheat zone temp by 10 to 50 degree C. This eliminated the problem. As this product had been running for quite some time w/o such problem, I suspect the root cause is not the preheat profile but the solder paste involved. I sent the solder paste back to the supplier for slump test but the result is negative (within their spec of > 10% slump). I am finding out their test std used but I think those lab test is different from real reflow as the heat transfer rate of reflow oven is much higher.