Force to Compress Adhesive Bond

Your title is misleading.

Do you want to compress the mask, the squeegee or the part after placement.

This is done every day in high volume production. The mask is just large enough to clear the pads. So hole location and tolerance is important. Consistent pressure is required. No bowing allowed.

First, the parts do not need any compression during bonding.

The metal mask is applied to the surface of the board and the squeegee pressure and speed is the key.

Depending on the quantity of boards to process hand squeegeeing may be the way to go. A light touch and plenty of adhesive ahead of the squeege is the best approach.

Trial and error is the best guide.

The process can be automated, if the quantity allows.

Here's a couple of sites.

Remember, there's always enough time to remove the adhesive and do it over if you get too much paste on the pads. It won't take much.

Solder Paste Stenciling - SparkFun Electronics

PCB Solder Stencils - Sunstone Circuits

We want to compress the PCB on to the anode after applying the epoxy. The intention is to ensure that we get a good connection but excess pressure might cause the epoxy to spread too far & short to the adjacent pad.

We had in mind a stencil similar to those used for PCBs & hand squeegeeing the epoxy.

These will be very much 1-offs until we prove the whole device & thereafter, a few hundred devices.

Since you are in uncharted territory, consider this. Include glass or metal microspheres/beads in the solder paste. This may require special processing to mix in the beads, but will limit the amount of squeeze-out of the paste by controlling the amount of compression of the joint.

While I have used this method with adhesives bonding, I have never done it with solder paste.

That's a good thought, I've seen that in other adhesives to control the bond thickness. There might be some concern about the beads reducing the contact area of the pads as they are so small but I guess the beads could be quite widely dispersed. What we are considering is not a solder paste, it is a conductive adhesive.

Sorry, conductive adhesive. I knew that. This might still work, since conductivity does not rely on continual pressure, once the resin has cured.

Just a thought.

That's right, no pressure required once it has cured but the electrical resistance of the joint will depend on how many conductive particles contact the 2 surfaces. Introducing spacer beads might compromise this. At the moment I'm not sure what the optimum joint thickness would be.

Highly Conductive Silver-Coated Precision Microspheres ...

There are options.

Conductive Nickel Metal Coated Hollow Glass Microspheres 0.69g/cc - "Conductospheres™"

That's interesting, having a predetermined joint thickness would allow us to predict the spread of the epoxy without detracting from the conductivity.

Yes the question is how many ?

http://www.google.com.ar/patents/US5410806

Sorry, the worst possible example to give is a patent.

Patents are written to be unusable by those not "well schooled in the art."

OP would be better served by visiting a silk screening shop nearby and seeing how T shirts are decorated, or signs are printed.

Seemed clear enough to me....

Trial and error method is the best. You could estimate however the volume of adhesive (LxW of panel)xH, (you could minimize this) of adhesive. Pressure does not guarantee that epoxy will be distributed well on the surface. There should be a means of applying the epoxy evenly on the pad. Pressure would be a little help for the pressing the pads even on the whole area.

We can estimate the volume of epoxy & try to control this by way of the hole size and thickness of the stencil used. I agree that pressure will not necessarily be even but we still feel that pressure will be required to get a good connection.

Have you considered a Z axis connector? You often find them in calculators to connect the display to the PCB.

https://www.google.co.uk/search?q=Z+axis+connector&ie=utf-8&oe=utf-8&gws_rd=cr&ei=OzvbVKmmH-a07ga8jIHYBA

This would work if we had a single row of pads to connect but not with a square array of pads.

We are also looking at using ACF sheets which contain conductive particles & work in a similar way to the Z connectors. I'm currently building a jig to test the application of the ACF.

You could go here:

https://event.webcasts.com/starthere.jsp?ei=1051680

We've taken advice from various manufacturers about suitable adhesives but, as usual, our situation is one that they have not encountered before.

We will be trialing some conductive epoxy with the conductive microspheres & I will report back on how this works. We will also be trying the ACF film to see which is best.