Stuffing boards with pulled QFP chips

On 04/01/2017 11:27 PM, Chuck Guzis via cctalk wrote: OK, this is going to be long. I do small-scale production of mostly motion control boards, but also some nuclear instrumentation. Mostly 0603 to 0805 passives and SOIC up to 0.5mm pitch QFP chips. I use a lot of FPGAs in 144 pin packages. I have a Philips CSM84 pick and place machine. This is an old-style machine, with mechanical alignment jaws for centering and rotation, no vision, except it has a "one pixel camera" called a beam sensor to pick up the fiducials on the PCB. It steers the XY gantry around to map out the fiducial spots. I make my own solder stencils using my existing PCB fab gear (which I almost never use for making PCBs anymore, just too much mess.) But, a solder stencil is basically a PC board without the glass epoxy substrate. I use .003" brass shim stock, laminate dry film photoresist, expose, etch in ferric chloride and, voila, a stencil. A BIG part of the trick is to know how to reduce aperture size for specific chips. As the lead pitch gets finer, the apertures have to get smaller, or you end up with solder bridges. So, after applying the solder paste with the stencil, and then running through the pick & place machine, I put it in the GE toaster oven. I bought the biggest toaster oven they had at WalMart ten years ago. I got a thermocouple ramp and soak controller and a roll of thermocouple wire on eBay. The ramp and soak controller can follow a profile of the sort: Start at room temp ramp to 180 C at 30 C / minute hold at 180 C for 1 minute ramp to 245 C at 30 C / minute hold at 245 C for 1 minute ramp to original temp at 50 C / minute One of the tricks I found out very fast was the thermocouple doesn't absorb IR the way a board does, the first board I did came out warped like a potato chip and nearly black. It occurred the me to poke the thermocouple junction into a through-hole in the board, and then it all worked. I still have some slight problems with some areas of the oven running a bit cooler or hotter, so I have to tweak the peak temp setting sometimes to get all the boards soldered. I was lucky to find really FINE thermocouple wire on eBay, so that the junctions are small enough to fit even fairly small through holes. I've done well over 1000 boards with this system. Oh, and most of it is RoHS, too. Jon