Fine pitch paste release problems.

Are you using "on-contact" printing? (is the stencil touching the PCB during squeegee travel)

reply »

What paste are you using?

reply »

square apertures provide better paste release than round in most applications. See IPC 7525 Stencil Design Guidelines

reply »

Personaly I would try a different solder paste. Give Indium a call and get a sample of their SMQ92J No clean. You will be impressed. Alpha and Kester also have good products but we have a long history with this paste and we have printed millions of boards with .016" pitch or micro BGA's parts with no problems.

This message was posted the Electronics Forum @

reply »

Are your stencils electropolished? I still haven't used an electroformed stencil, but I understand they can be worth the extra money, because of the better release characteristics.

reply »

Check :

1) design aspect ratio / are ratio required to fabricate the stencil you r using.

2) check if the stencil maker is using your checked aspect ratio / area ratio? if not the same, what are they using? any QA report confirming actual dimensions matches their supplied theorectical calculations shown to you?

3) check if your storage conditions and paste control is proper and correct? ours is 4-7degC for our paste.

4) check if your humidity and temperature control for the printer machine environment is somewhat 22-26degC / 40-60RH%?

5) check if your printer setup coplanarity is even and squeegee contact to stencil level? try running a dial gauge indicator to check if there are "humps" along the surface area the squeegee is travelling during printing? also check for PCB levelness (warp/bow/twist?).

6) follow the advise to get square stencil aperture openings.

7) see if electrofoam stencils are a viable option for u? such stencils do help.

note: many a time, lousy design can still be overcome with some creative process engineering techniques. Yeh, i know, its a bandaid.

Good luck!

reply »

One thing that will also cause this is very slight stencil mis-alignment. Sometimes the paste will have a tendency to hang up in the apertures if you are over hanging just a little on the solder mask. Some solder mask are very slick and will not pull the paste out of the apertures like the component pad will.

reply »

I have had a similar problem in the past, since I have never worked with a semiautomatic printer there could be a few things I recommend that will not work for you. 1. Make absolutly sure that you have proper support under all of the BGA's. 2. Print at a very low speed but do not go below 12mm/sec. 3. Use the slowest separation speed possible. 4. Spend the money for an electoformed stencil, I have conducted tests that proves they are worth the money and last 6-8 times longer than stainless. 5. Use stainless steel squeegees if you are not using a step stencil. 6. Make absolutely sure the board is properly fixtured and cannot move during the print cycle. 7. Increasing your print pressure could also help you gasket to the pad better and force the paste down to the pad so you can get a good release. 8. Verify that you are within the rated temp/humidity levels for the paste you are using.

reply »

Your area ratio for this aperture is fine (.78) usually any value greater than .66 will provide a reliable paste release. I would make sure however that the stencil is polished. I agree with other posts regarding your setup and/or paste type is probably the culprit.

Russ

reply »

Hi Grant Petty,

I read about your problem, You might have problem with the design. First on the pad. What is the diameter of the BGA pad in your PCB. We have the same size of PCB and we use 0.584mm Pad dia. and a 10% reduction on our aperture. We use a semi-auto printer as well but has no problem with regards paste release on any solder paste brand.

reply »

Check your stencil apertures making sure they are trapazoidal and smooth. I have hadjobs where the laser stencil was improperly cut (poor laser settings leaving a rough hole even after electropolish. Be sure to specify a .0005 trapazoid in your stencil Docs. I was using squares for everything but stopped specifying them when I did not see a big difference. I would still use square apertures on applications where higher paste volumes are desireable such as cerammic BGAs

reply »

Hi: Your stencil specs are good; Did you check the stencil thickness? It would be worth a ck since there may be an error from the stencil mfr

If the paste is not releasing with good stencil specs, these are possible causes:

1 The paste isn't touching the pads (never gets there) - try flooding hard (like ten deg angle w squeegee, fast and hard). Clear with a firm 90 deg stroke (straight up w the sq)

2 If everything is very clean; sometimes the stencil will gasket against the pc soldermask, and air will trap in the aperature preventing the paste from touching the pc lands. To check this, skew stencil off alignment a little and try printing.

3 Microscopic dust on the pcb can cause similar prob. Be sure boards are clean.

Transition Automation Inc.

reply »

I am breaking a cardinal rule with this reply, but here goes. I believe almost every one is missing this one. Roger has hit the nail on the head. The gentleman is useing a clam shell printer and when the stencil lifts (too fast) and at the angle of separation--we have solder left in the aperatures. Another engineer who is trying to use a printer that was not designed or built for bga's. Yes I know some people can use them and I know most don't work correctly. Too many of these print and process problems return to the same problem that we see all year--wrong printer for the end results. Some may agree and some may not, anyway good luck.

reply »