Mounting heights

For space contracts we have to mount chip capacitors/ resistors etc 0.1 to 0.4 mm from the pad height.

This is to compenstae for the CTE differential in the substrate/component and also to ensure cleanliness.

Does anyone have any new ideas on how this could be acheived. I have performed trials using cured Amicon blob tops under the components and this appears to work but I get inconsistent heights.

reply »

| For space contracts we have to mount chip capacitors/ resistors etc 0.1 to 0.4 mm from the pad height. | | This is to compenstae for the CTE differential in the substrate/component and also to ensure cleanliness. | | Does anyone have any new ideas on how this could be acheived. I have performed trials using cured Amicon blob tops under the components and this appears to work but I get inconsistent heights. | If there�s a great number of parts to handle maybe applying dots under the components before assembly might work but maybe the machine for this isn�t invented yet and the $$ I can�t figure out. It just shot through my mind the moment I read this mail

Good luck

Wolfgang

reply »

| For space contracts we have to mount chip capacitors/ resistors etc 0.1 to 0.4 mm from the pad height. | | This is to compenstae for the CTE differential in the substrate/component and also to ensure cleanliness. | | Does anyone have any new ideas on how this could be acheived. I have performed trials using cured Amicon blob tops under the components and this appears to work but I get inconsistent heights. | Bob: Sorry, no new ideas, just old ones.

Consider discussing this with your contract administrator to determine the specific requirement and the degree it is enforced.

Actually, it's pretty hard to solder chip components flush to the board. They all float somewhat. Just don't bury the suckers in the paste and push on.

My2�

Dave F

reply »

| For space contracts we have to mount chip capacitors/ resistors etc 0.1 to 0.4 mm from the pad height. | | This is to compenstae for the CTE differential in the substrate/component and also to ensure cleanliness. | | Does anyone have any new ideas on how this could be acheived. I have performed trials using cured Amicon blob tops under the components and this appears to work but I get inconsistent heights. | Let's see, that's 0.004" to 0.016". Dave's right, they don't sit flush to the board. Typically, the ends are larger in size than the body and the solder will do some pushing up. Go look at what you're doing now, measure from the center of the component to the board - I believe that you'll find you're at least .003" if not more in height. Measure component size by itself in the center, then height from board to top of component center installed.

I would also go look at that spec again in regards to the height from the pad or if it's the height from the board, totally different. You may find that it's the height from the board - cleanliness was the clue. You REALLY NEED to READ these specs, there's usually some qualifiers that'll help you.

If you're only 0.003" (I doubt that you're less), then ask for a variance, or more money. They'll grant a variance before they'll give you money. Or ask for 2 oz. copper on the PCB, still more money.

Once you've sorted that out, you may find you're already compliant without all that work you've done.

Hope this helps, Scott

reply »