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MLF / QFN Lead Free Shorting

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#48916

MLF / QFN Lead Free Shorting | 6 April, 2007

Hi,

Anyone seeing difficulty is soldering MLFs or QFN with lead free solder? I have an MLF with 3 rows of pads on each of the 4 sides of the part and then the large center ground plane in the middle. The part is 20 mil pitch. Had good luck with lead solder using 0.010" X 0.020" with a 0.004" stencil. Built the product for 2 years with little trouble. We are now using a 0.010" X 0.015" with a 0.004" stencil with lead free solder and we have 20% failure rate. We are finding a few short at X ray with the lead free. Guess I am going to try 0.010" X 0.010" with a 0.004" thick stencil next.

I grow tired of hearing these so called "Experts" state that you can do finer pitch soldering with lead free since lead free wets less than lead solder. This is not what we are seeing with MLFs. By the way, my printing is right on so this is not a print problem.

Chris

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Haris

#48925

MLF / QFN Lead Free Shorting | 7 April, 2007

Hi I dont think there is a stencil problem or a printing one but i think you should check you paste specification regarding 20 mil pitch i.e. its ball size in microns so that you know Is that paste can be use for this type of components or not. Secondly check you solder paste profile which should match with you board profile.And also check your Activation zone temperature as you have mentioned that there are few short circuits. bye Muhammad Haris

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Haris

#48929

MLF / QFN Lead Free Shorting | 9 April, 2007

And check also your component company,sometimes component quality varies with the change of the company.

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#48930

MLF / QFN Lead Free Shorting | 9 April, 2007

Hi Chris,

So you run eror free for 2 years, then change stencil aps and get problems. Why would you go smaller? Try using your old stencil with your no-lead.

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#48934

MLF / QFN Lead Free Shorting | 9 April, 2007

Hi,

We were running with few defects with lead solder paste. The lead solder paste with stencil 0.010" X 0.020" and 0.004" thick stencil produced near defect free PCB assys.

0.010" X 0.020" (0.004" thick) lead = defect free 0.010" X 0.020" (0.005" thick) lead free = bridges 0.010" X 0.020" (0.004" thick) lead free = bridges 0.010" X 0.015" (0.005" thick) lead free = bridges 0.010" X 0.015" (0.004" thick) lead free = bridges

Placement is perfect. Printing is perfect.

This part is not fun. Like I said, 3 rows of 0.020" pitch on each of the 4 sides of the MLF and then the center ground pad.

We will try 0.008" X .012" (0.004" thick)ElectroFormed stencil next. Maybe have to go to type 4 (-400 +500 mesh) paste.

Maybe we will switch back to lead solder.

Chris

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#48937

MLF / QFN Lead Free Shorting | 9 April, 2007

How wide are your pads? What paste are you using? What is your board finish? Could you please post a link to the datasheet for the component?

We place a lot of MLFs, both in a Pb and Pb-free process. Our standard stencil design for MLF components of that pitch is "No reduction in the length of the pad, 1 mil per side reduction in the width of the pad, 5 mil stainless electropolished foil" So in your case, if the pad is .010" x .020" then you'd want your apertures to be .008" x .0020".

What do you do for reduction on the center ground pad? We do .020" rounds in a checkerboard pattern with ~.040" column spacing and ~.025" row spacing.

We don't have bridging issues.

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bart.lozie@page.be

#49018

MLF / QFN Lead Free Shorting | 13 April, 2007

Hello,

why focuss only on the printer and the paste, maybe the problem isn't there, maybe the problem is in the oven, is the component a havy one? because leadfree solder can't lift as mutch as a leadcontaining solder.

regards Bart

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