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

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Charles Stringer

#13227

Peelable solder stop | 23 December, 1998

Can anyone recommend a solder stop that can be applied by our PCB supplier. Our current material form Tamura Kaken is causing problems.

The material should be capable of withstanding 1 Surface mount IR reflow cycle and 1 wave solder cycle. We need the material to be able to prevent the soldering of 3mm diameter PTH.

The problems we see are with run though of the material when trying to screen over the large holes. This leaves pips of stop on the top side of the board that have to be cut off prior to screen printing. Another problem is that the run through can get left on the printer jig and ends up on the surface mount pads of the next board to be printed with stop,

We also use a white peelable latex that is applied after the surface mount part of the build. This involves a lot of manual labour and the possibility for misapplication is higher. In addition there is a 2 hour cure time which increases the process time for PCAs and gives us the problem of storing 'wet' boards.

We have looked at a peelable from Peters but this had odour problems after wave soldering that we were unable to overcome satisfactorily. We also looked at a UV curable that was excelent but expensive (the green bit we put it on was cheaper)

Greetings for the festive season and thanks in advance for any suggestions.

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Dave F

#13228

Re: Peelable solder stop | 29 December, 1998

| Can anyone recommend a solder stop that can be applied by our PCB supplier. Our current material form Tamura Kaken is causing problems. | | The material should be capable of withstanding 1 Surface mount IR reflow cycle and 1 wave solder cycle. | We need the material to be able to prevent the soldering of 3mm diameter PTH. | | The problems we see are with run though of the material when trying to screen over the large holes. This leaves pips of stop on the top side of the board that have to be cut off prior to screen printing. | Another problem is that the run through can get left on the printer jig and ends up on the surface mount pads of the next board to be printed with stop, | | We also use a white peelable latex that is applied after the surface mount part of the build. This involves a lot of manual labour and the possibility for misapplication is higher. In addition there is a 2 hour cure time which increases the process time for PCAs and gives us the problem of storing 'wet' boards. | | We have looked at a peelable from Peters but this had odour problems after wave soldering that we were unable to overcome satisfactorily. We also looked at a UV curable that was excelent but expensive (the green bit we put it on was cheaper) | | Greetings for the festive season and thanks in advance for any suggestions. | | | Charles: Thanks for your warm greeting.

The best solution to your peelable mask problems is to redesign your boards to eliminate post-process components and materials.

First, a couple of questions:

1 I�m uncertain why you apply peelable mask both before SMT processing and after SMT processing. Is "before SMT processing" for Product A?? And "after SMT processing" for Product B?? 2 Ya know, 3mm (0.12 inch) is a rather large hole to expect a sloppy goop, like peelable mask to fill with no run-through.. Do you wonder (1) if you�re ask too much of peelable mask to plug this hole? (2) the reason that you�re plugging this sized hole in the first place?

I assume you are using vinyl/latex peelable solder mask because either you�re:

1 Struck with the cost saving of using no-clean � OR 2 Saving money by cleaning with a closed loop wash system with either a RO filter or a carbon bed � OR 3 Limited by some wacky environmental restriction.

� because otherwise you�d be using a water soluble temporary mask.

PRINTED PEELABLE TEMPORARY MASK: Yup, printed peelable temporary mask is a pain to remove, because it�s so thin that it breaks during removal and lodges in through holes during printing. The lodged material often breaks, leaving plastic in the through hole, which solders poorly. I figure either there�s a little extra solder at the edge of the through hole that necks-down the opening and prevents the mask from pulling free or the adhering force of the mask in-side the through hole is greater than tensile of the mask. What ever �

DISPENSED PEELABLE TEMPORARY MASK: Similarly, dispensed peelable temporary mask is a pain to remove and it takes longer to cure, because it ends-up being thicker than printed mask. So to speed-up curing, you end-up baking the boards in a hot box and potentially goofing-up the solderability of your boards. What�s nice about dispensed mask is, you can leave "tails" of material that make it easier to get a handle (purchase) on the mask to peel it from the board.

The key to effectively printing and dispensing peelable masks is controlling the mask cure mechanism. A little bit like your very nice UV curable, some peelable masks are heat cure, some are moisture cure.

SELECTIVE SOLDERING: Have you considered selective soldering? The two approaches are:

1 Solder fountain. 2 Specialized, peculiar wave solder pallets.

I�m not keen on solder fountains, because of cost of machines and chimneys, limited use, manual handling requirements, and tough process control requirements. But then again, I haven�t truly given them a lot of thought either. In many applications, a well designed selective wave solder pallet is a good middle ground between re-designing the board and total labor cost of peelable masks.

THERMAL TAPE: So, what�s your experience with tape? Yes, I know that�s hand work. So is routing peelable mask from through holes and scraping mask from stencils and the top-side of boards.

RUBBER PLUGS: So, what�s your experience with re-useable hard rubber plugs? See proceeding paragraph �

Ya know, you reduce a cost here and you increase a cost there. And it just becomes a juggling act, ever seeking balance, yet always keeping the juggled objects off the floor. So, there maybe some combination of approaches that meet your requirements.

TTYL

Dave F

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