estimate solder thickness under SMT pads

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bootstrap
 
Posts: 73
Joined: Wed Jun 02, 2010 8:47 pm

estimate solder thickness under SMT pads

Post by bootstrap »

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

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alphatronique
 
Posts: 231
Joined: Fri Jun 25, 2010 8:30 am

Re: estimate solder thickness under SMT pads

Post by alphatronique »

HI

i do something similar in past and not have care of the solder thinness (camera based 2d/3d bard code reader)
since lens mount have focus adjustment and most of time mechanical variation will be bigger that solder induced error

i check back my datasheet and the sensor i have used MT9M111 have height 0f 1.17 +/- 0.1mm tolerance
so solder past was really not a issue

what did your particular CCD chip tell about this ? i assume it quite similar

Best regard

mikeselectricstuff
 
Posts: 164
Joined: Fri Jun 11, 2010 9:21 pm

Re: estimate solder thickness under SMT pads

Post by mikeselectricstuff »

I'd think you'd want a method of manually adjusting the sensor to lens distance by a lot more than the solder thickness to deal with lens tolerances etc.

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