Stuffing boards with pulled QFP chips

[Jon Elson]

On 04/01/2017 11:27 PM, Chuck Guzis via cctalk wrote:
> What do you do about boards with SMT components on *both* 
> sides? I can't see how it would work with a toaster oven.
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