Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Sudden paste release problem

Grant

#32546

Sudden paste release problem | 9 February, 2005

Hi,

We have been running the same stencils for about a year, 5 thou thick, 1-mm pitch BGA's and every-things been ok, but then suddenly over the last week, 2 different products have had paste release issues with the BGA.

It's odd because nothings changed. We have various opinions on what's happening, and I thought it would be good to get some ideas from the forum as well.

Could it be dried paste in the holes. We clean with stencil wipes, and the stencils look clean, but we don't ultrasonically clean, and never have. So we could have dried paste in the holes, even though they look clean when looking by eye.

Could it be a change in the solder paste formulation. We have not changed, and we use Koki, which has been excellent in the past.

One of the guys had an idea that as the stencil's have worn, they might have burred over the holes a little, and that's grabbing the paste. The stencils are a bit worn.

What is the symptoms of a worn stencil?

Anyway, I hope someone has some ideas!

Regards,

Grant

reply »

Mark M.

#32547

Sudden paste release problem | 10 February, 2005

Check your humidity levels there as well. If the airs to dry it would dry out your paste as well. We had to go out and buy some floor humidifiers for our place.

reply »

#32554

Sudden paste release problem | 10 February, 2005

Don't kid yourself, something has changed. * Worn stencils should produce better release and greater paste deposits than less worn stencils. * Consider inspecting the effectiveness of your cleaning process with a X10 magnifier. * Temperature in your screen printer needs to stable. Paste suppliers say that 1*C change can affect paste viscosity by about 10%. * Paste needs to be worked * Paste needs to be at room temperature * Paste may have changed -- Solder paste viscosity is too high -- Solder paste is too tacky -- Solder paste particle size is too large -- Some flux formulations are very sensitive to humidity

reply »

#32556

Sudden paste release problem | 10 February, 2005

I am curious as to why you are using Koki ? Why are you not using a main stream paste that is proven in the industry? Indium,Kester,Multicore,AIM....

It has been my experience that when you start having paste-releasing problems out of the blue, it is usually a chemistry problem with the paste. IE-inconsistent chemistries batch to batch. I would get on the phone immediately with your paste manufacturer.

I have never in all my years seen a stainless stencil that was worn out. Maybe damaged, but not worn out.

Another thing: I am still amazed to hear that people "wipe" their stencils off. You need to invest in some type of a washer that uses pressurized fluid to blow through the apertures. It is IMPOSSIBLE to get a stencil totally clean by wiping. It is very possible that your release problem is due to residue on the aperture walls. (If you insist on the wipe method, get yourself an eye loupe or a microscope)

reply »

Grant

#32574

Sudden paste release problem | 11 February, 2005

Hi,

That's what I am thinking, and we might have a dirty stencils and cannot see the problem. We are going to get a magnifier, and check it out. I did not think the stencil could bur, but I didn't want to shoot the guys down when they had an idea on what it might be, so I thought I would ask.

I am going to check the cleaning on the stencil, and we might check humidity, because it's Australia, so the air is dry, and this happened within a few days on two different products.

We use Koki because have found it to be extremely good. Almost no residue, solders really well. It's always worked well for us in the past. Thanks for the feedback.

Regards,

Grant

reply »

Reflow Ovens thermal process improvement