PCB Stencils, thickness and solder paste

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djoshi
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Posted by djoshi: Fri. Jul 17, 2020 - 12:09 PM
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Hi All

 

I am awaiting for the delivery of my solder stencil printer, therefore am i preparing a stencil order for some pcbs.

 

I usually order PCBS from a company called PCBWAY, they also offer affordable framed stencils.  Has any one used there stencil service?

 

They offer various thickness.

 

Does the thickness of the stencil reflect the type of paste to use?

 

My PCB software, outputs the top solder paste layer in the Gerber file.

 

Is it a good idea to scale each pad by 10-15%?  Or should the solder paste be on entire pad?

 

Thanks

 

DJ

Thanks

Regards

DJ

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Learning and Information, General Electronics
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Moe123
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Posted by Moe123: Fri. Jul 17, 2020 - 12:38 PM
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IIRC, the thickness for the stencils varies but I've been using sometime 100um and also 130um, both are fine if the footprints you are using are one of these common one, eg. SOIC, SOT..etc. However, If the pads are quite small, like the new nordic nRF modules then its better you go with the 100um.

 

another point is the solder paste, there are several types depending on the temperature for the reflow, I tend to use low temperature solder paste for my prototypes.

 

Good luck!

 

Regards,

Moe

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avrcandies
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Posted by avrcandies: Fri. Jul 17, 2020 - 07:45 PM
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A common mistake is to cause too much paste to be applied.  A little goes a long way, in a properly controlled process.  Sometimes people try to make up for bad process control by jacking up the amount of paste, gaining only sad results with high variance.

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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djoshi
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Posted by djoshi: Fri. Jul 17, 2020 - 08:23 PM (Reply to #3)
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So i guess the stencil limit the amount of paste that is applied?

 

And if you have variation of components small and large, then how would you select the correct thickness?

Thanks

Regards

DJ

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avrcandies
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Posted by avrcandies: Fri. Jul 17, 2020 - 08:48 PM
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And if you have variation of components small and large, then how would you select the correct thickness?

You can make some tweaks to the hole size 

 

http://www.surfacemountprocess.com/solder-paste-printing-process.html

 

http://www.circuitinsight.com/pdf/printing_solder_paste_quality_assurance_methodology_ipc.pdf

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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ki0bk
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Posted by ki0bk: Fri. Jul 17, 2020 - 08:48 PM
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djoshi wrote:
And if you have variation of components small and large, then how would you select the correct thickness?

 

Experience!    Sort of a catch22 issue here, but it depends on your processes, pnp, reflow etc.

 

How many units will you build?  10, 100, (my guess somewhere between these two numbers, you will know) 100000? 

Jim

 

 

(Possum Lodge oath) Quando omni flunkus, moritati.

"I thought growing old would take longer"

 

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DocJC
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Posted by DocJC: Sat. Jul 18, 2020 - 01:10 AM
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When you get the printer please post a picture of it and some of the stencils you print!

JC

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valusoft
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Posted by valusoft: Sat. Jul 18, 2020 - 04:12 AM
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I have used PCBWay to produce pcbs and matching stainless steel stencils. My Eagle software has the option of producing gerbers for the stencil which are automagically adjusted in  size to suit the pad sizes. I did not have a solder paste printer. I used the "credit card screeding" method for applying the paste to the pcb. Worked well for me.

Ross McKenzie ValuSoft Melbourne Australia

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djoshi
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Posted by djoshi: Mon. Jul 20, 2020 - 09:12 PM
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Hi

 

This is the stencil we are going to have

https://neodenusa.com/fp2636-framed-stencils

 

And our stencil will the following:

 

 

 

Attachment(s): 

Image icon stencil.jpg

Thanks

Regards

DJ

Last Edited: Mon. Jul 20, 2020 - 09:11 PM
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Kartman
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Posted by Kartman: Mon. Jul 20, 2020 - 10:02 PM
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From a distance the layout around the cellular module doesn’t look too good - unless you have components on the bottom of the board.

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meslomp
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Posted by meslomp: Tue. Jul 21, 2020 - 05:45 AM
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we have learned that if you do automated placement you could do with a 100um stencil, but when we hand pick and place a board then we use 130um for some reason then the components have less unconnected or short pins.

 

Normally solder paste should be over the entire pad. exceptions are for instance the pads underneath QFN packages or BGA, but they most of the times have a recommended footprint and paste opening description.

 

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djoshi
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Posted by djoshi: Tue. Jul 21, 2020 - 10:21 AM (Reply to #10)
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Hi Kartman, all power related caps are close to the module, the other component are the transistor circuits for the uarts , GPIO etc. 

Thanks

Regards

DJ

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djoshi
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Posted by djoshi: Tue. Jul 21, 2020 - 10:24 AM (Reply to #11)
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Currently our stencil is 0.12mm , which is 120um. 

 

When you refer to underneath QFN, i presume you mean the GND pad. There we usual just make a few small squares instead of placing the paste entirely.

 

Is it advisable to reduce the aperture about 10%.

Thanks

Regards

DJ

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meslomp
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Posted by meslomp: Tue. Jul 21, 2020 - 11:17 AM
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djoshi wrote:

Currently our stencil is 0.12mm , which is 120um. 

 

When you refer to underneath QFN, i presume you mean the GND pad. There we usual just make a few small squares instead of placing the paste entirely.

 

Is it advisable to reduce the aperture about 10%.

120um will be OK.

Yes I meant the GND pad underneath. we also just place a couple of squares as the manufacturers specify them.

We had a datasheet a couple of years ago were they had put up a special design with circles and squares. Almost like a crop circle.

Unfortunately I cannot find it back, needles to say we did a rough guess as to how much paste was actually put on and just put in a couple of squares.

Not much later a revision update of that specification had also just a couple of squares, so I guess more people had second thoughts about the design.

 

We do not use aperture reduction. Not sure what the factories do as they design their own stencil and take our drawing as a base for them to work from.

 

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djoshi
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Posted by djoshi: Tue. Jul 21, 2020 - 01:03 PM
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Yes, i normally place few small squares as well.

 

 

I will be ordering some PCB soon, so i am thinking i could order a 100um version as well and will keep the aperture the same size as the pads.

 

 

Thanks

Regards

DJ

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samay
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Posted by samay: Tue. Jul 21, 2020 - 02:48 PM (Reply to #15)
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The stencil manufacturer we use reduces the apertures very slightly, though I'm not sure by what percentage. Probably 5-10%. Typically our components are 0805 or bigger (TSSOP ICs, no QFNs), and stencil thickness is 0.2mm, which has worked well for us.

-Sam

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