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Release difficult from thick stencil

Wister

#2681

Release difficult from thick stencil | 24 October, 2000

Our important customer demanded us to use 12mil thick stencil,it is too difficult to release solder paste from stencil.it is not helpful even I ask stencil supplier polished the openings.The flux content is 10%. I want to ask if the polyurethane squeegee blade or Pro-flow head is useful to release easily? Rgs, Wister

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#2682

Re: Release difficult from thick stencil | 24 October, 2000

Maybe someone has a good explanation for this, but I'm wondering why your customer would dictate the equipment you use in your process. I thought typically contracts define the quality standards and you decide how to get there....show's you what I know about contract manufacturing.

Anyway, if you're trying to print on lands any smaller than 18 mils with that kind of sheet metal you're at risk of paste release problems. The rule of thumb I use (and seems to be the norm, based on the scads of papers and posts on this topic) is a 1.5/1 aspect ratio between land width and aperture width.

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#2683

Correction of my previous post..... | 24 October, 2000

I meant aperture width and stencil thickness, not land width and aperture width. Sheesh.

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#2684

Re: Release difficult from thick stencil | 24 October, 2000

Sorry,I can't help you with your problem, but would it be possible for your customer to take a little time to explain the reasoning behind their demands? I am sure we would all like to know. What are you putting on these massive deposits? Trends in both PTH & SMD are leading to less & less solder, not more. Good luck with difficult customer.

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#2685

Re: Release difficult from thick stencil | 24 October, 2000

In response to your questions: * Will a polyurethane squeegee blade help? NO * Will a Pro-Flow head help? NO, but it might be a cool excuse to buy one. Unfortunately, you will look like a jerk, if you talk your boss into buying one and then he /she finds out that it doesn�t work.

Wister there�s too much surface area on the walls of your apertures and that prevents proper release of the solder paste. The approach you take to squeegee work will do nothing to affect that problem.

The problem you have is that your stencil is twice as thick as you normally would make it. So in order to deposit the same amount of paste as you would with a thinner stencil, you need to pinch your apertures to maintain the same volume relationship on the inside of your aperture. When you do that, you ruin the aspect and area ratios [check the SMTnet Archives] necessary for good paste release. As you have noticed that these numbers are so blanked-up by using a thick stencil that polishing doesn�t do blank.

Here�s some things to try: 1 Try a Type 5 paste. This is a dumb idea, but it just may allow you to avoid doing what you should do, which is #3 below. 2 Increase your printer separation speed. This is a dumb idea, but it just may allow you to avoid doing what you should do, which is #3 below. 3 Sit down with your customer, explain the problem, and request that they either * Pay for the additional cost of using the thick stencil ... OR * Get the blank otta the way and let you do yer blankin� job!!!

Your important customer is a jerk!!! [And unfortunately you have to deal with it] Your customer should describe to you the quality and performance of the product that they expect your to deliver and not interfere with your ability to deliver product. It took many years for the US military to realize that by prescribing both process and quality requirements, they were creating an untenable situation for their suppliers [but then again that appears to be the purpose of rule making bureaucracies]

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Steve A

#2686

Re: Release difficult from thick stencil | 27 October, 2000

Wister,

I agree with the other comments as far as challenging the customer, and minding proper height/width or better yet area aspect ratios. Let's assume you need to go with a .012" stencil for arguments sake. Talk to your stencil supplier about how aggressive they can be with the stencil wall angle. While there is the natural trapezoidal angle from laser cutting that is slight, there are also processes to exaggerate that angle and thus reduce your effective paste contact with stencil walls. For kicks and giggles you could also nickel plate the stencil for improved paste release characteristics. Good luck.

Steve A

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Fraser

#2687

Re: Release difficult from thick stencil | 1 November, 2000

Is it possible you could simply over print to achieve the same paste volume? Supply me the numbers and I will do the calculation for you.

Cheers

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Fraser

#2688

Re: Release difficult from thick stencil | 1 November, 2000

Steve, someone somewhere is feeding you a line!! if you increase the draft angle from || to / \ on your laser cut stencil you increase the surface area of the side wall and hence increase the likelyhood of the paste sticking in the aperture. Whats more you increase the horizontal proportion component of the side wall = z*tan(x) where z is the foil thickness and x is the draft angle, you also decrease the paste release. Think about it - if you have some solder paste sandwiched between 2 parallel plates (=) the paste is as likely to stick to one as the other - as the angle of one of the plates tends towards 90 degrees wrt the other (_|) and is released at 90 degrees the paste will obviously tend to stay on the one perpendicular to gravity. Thus the perfect stencil will have side walls at right angles to the surface plane.

Cheers

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