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# estimate solder thickness

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estimate solder thickness | 7 June, 2011

I am building a rugged digital video camera product that essentially is just a sandwitch of 3 parts:

#1: front piece is machined aluminum with C-mount thread #2: middle piece is 1/16" printed circuit board #3: back piece is machined aluminum

These 3 pieces are bolted together with M3 bolts at the four corners (with clearance holes in #1 and #2 and threaded holes in #3).

On the middle of #2 (the PCB) is an image sensor in an iLCC package. Essentially the iLCC package is just like a QFN except package with a quartz window over the image sensor chip inside.

I need the distance between the front surface of #1 to be exactly the correct distance to the image sensor. The specifications for the position of the image sensor surface in the package is given in the image sensor specification, so I can calculate the appropriate thickness of the front of the case #1 to the PCB #2.

However, the melted solder under the iLCC contacts will raise the image sensor up slightly. Assuming I have a 0.005" thick stencil (which I do), how thick is the bottom of the chip likely to be once the solderpaste is melted and hardens in the reflow oven?

Will it be 0.005" or something less or more? The openings in the soldermask are just a hair larger than the openings in the solderpaste stencil (about 0.001" ~ 0.002" smaller).

I'm guessing 0.004". But that's a guess.

Does anyone know?

Thanks

estimate solder thickness | 7 June, 2011

Boots: Here's several methods for guessing at the amount of solder on your connection: * First, 'Down & Dirty' ** Assume that the solder is flat, which is reasonable since it's 'flattened' by the component package ** Assume 100% paste transfer from the stencil to the board ** You know the stencil aperture volume ** You know the metal content of the paste ** Assume 100% termination coverage ** ... and you can calculate solder thickness

* Second, improve 'Down & Dirty' by refining your assumption of 100% paste transfer. In this, you'd make paste volume measurements, do statistics, and develop a more likely estimate of amount of solder paste on the pads.

* Third, bag 'Down & Dirty' and its variants. ** Make a board with 100 sets of iLCC termination outlines, just like those for your component. ** Print solder paste on the board ** Place dummy components just like the one you'll be using ** Reflow the board ** Section each component pad ** Measure the solder thickness, do statistics, and develop a most likely solder thickness estimate.