Author Topic: Solder paste coverage for large pads?  (Read 6934 times)

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Offline knotlogicTopic starter

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Solder paste coverage for large pads?
« on: July 13, 2017, 02:22:06 am »
Is there any rule of thumb when deciding the size of the solder paste aperture for large pads, especially like with exposed pad QFN packages?  I've seen recommendations to break up larger areas into a number of smaller ones, but nothing that talks about how large/small to make the smaller areas, or what percentage coverage to aim for vs the original.
 

Offline mrpackethead

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Re: Solder paste coverage for large pads?
« Reply #1 on: July 13, 2017, 02:35:25 am »
Almost certainly ( at least for any decent manufacturer ) there will be recommended solder paste patterns for a package in datasheets etc.  The rule of thumb would be to use those.
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Offline knotlogicTopic starter

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Re: Solder paste coverage for large pads?
« Reply #2 on: July 13, 2017, 05:26:19 am »
Almost certainly ( at least for any decent manufacturer ) there will be recommended solder paste patterns for a package in datasheets etc.  The rule of thumb would be to use those.

Looks like Atmel might not be one of those decent manufacturers...  ;)  Or I've been doing a terrible job of searching for them.  But I've also been looking at various power LED packages from smaller manufacturers, and.... it's messy.  There's a 2835 package that appears to be an industry standard, but there seem to be rather different PCB footprints between manufacturers!

evb149, thanks for the list!  That's going to take me some time to get through though.  70% is rather small though, not to mention 50%!  I was guessing 80% but now that seems way too much.

I've been using EAGLE thus far, and IIRC the paste layer is automatically shrunk a certain amount relative to the copper layer, although it's possible to delete it and insert one's own during footprint creation.  I need to check that.  (I also need to start using CircuitStudio, but that's a different matter entirely...)

I'm not doing any commercial manufacturing with this, and am probably going to go with OSH Stencils for my first time round.
 

Offline jmelson

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Re: Solder paste coverage for large pads?
« Reply #3 on: July 14, 2017, 07:54:42 pm »
For QFNs with exposed pad under the chip, the amount of solder paste must be reduced a LOT, or the chip will pull down and squeeze the solder out to the rows of pads, shorting everything out.  Many people make "window frames" by making a number of squares with clear stripes between them.  This arranges for the solder to be spread across the pad but still reduces the total solder volume to prevent the shorts.

Larger parts can use 100% aperture to pad size, as the lead pitch is reduced, then you have to make the apertures smaller to prevent bridging.  I sometimes have to go down to about 50% area for 0.5 and 0.4mm lead pitch.

Jon
 

Offline ion

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Re: Solder paste coverage for large pads?
« Reply #4 on: July 14, 2017, 09:26:04 pm »
If you're looking at Atmel, they do have this Application Note:

http://www.atmel.com/Images/Atmel-8826-SEEPROM-PCB-Mounting-Guidelines-Surface-Mount-Packages-ApplicationNote.pdf

Section 2.3 has some information on paste stencils.
 

Offline mrpackethead

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Re: Solder paste coverage for large pads?
« Reply #5 on: July 15, 2017, 12:13:05 am »
If you're looking at Atmel, they do have this Application Note:

http://www.atmel.com/Images/Atmel-8826-SEEPROM-PCB-Mounting-Guidelines-Surface-Mount-Packages-ApplicationNote.pdf

Section 2.3 has some information on paste stencils.

this is probably the best note i've seen on it.. nice post.
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Offline d1wang

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Re: Solder paste coverage for large pads?
« Reply #6 on: January 16, 2019, 02:30:33 am »
I was wondering the same thing and found this thread.

While many manufacturers have recommendation on the stencil design, all involve rectangular stencil apertures. So far I haven't found any actual drawing on a complex stencil (like this) from any semiconductor manufacturer:


I am in the process of creating a Web tool for calculating the paste masks and the total paste area. Let me know if you find any mistake or have any suggestion.
 

Offline IconicPCB

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Re: Solder paste coverage for large pads?
« Reply #7 on: January 16, 2019, 05:13:16 am »
It is counter productive to talk about aperture area when in fact solder volume is the issue to hand.
 
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Offline mrpackethead

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Re: Solder paste coverage for large pads?
« Reply #8 on: January 16, 2019, 06:43:35 am »
It is counter productive to talk about aperture area when in fact solder volume is the issue to hand.

I would be significnatly concerned about creating voids with the circular patterns in that stencil.    The variation in height because of the use of solder mask is also an issue.
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Offline d1wang

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Re: Solder paste coverage for large pads?
« Reply #9 on: January 16, 2019, 07:18:10 am »
I have searched the Web for information on this. I haven't found any formula or a ballpark target of the solder paste volume. I would appreciate it if someone can provide a formula/number that people can use as starting point.

The idea here is to have some acceptable result (without the added cost of via plugging). The following comes from TI's application note:

Quote
Another solder-mask tenting technique is to tent from the top of the board. The via solder-mask diameter must be 0,1 mm larger than the via hole diameter when tenting. Another variation is to create a crosshatch pattern of solder mask to create a predictable area of coverage. Trials have shown that via tenting from the top is less likely to produce random voids between the exposed pad and the PCB pad.

Does anyone have experience with via-tenting on exposed pad?

I haven't read anything that mentioned height variation. How big of a problem is the height variation?
 

Offline ar__systems

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Re: Solder paste coverage for large pads?
« Reply #10 on: January 17, 2019, 03:15:19 pm »
I did vias on the exposed pad and masked via on the bottom side. Works fine. I notice that at least some of lead free pastes don't really flow that much (or at all, really). In particular GC10 practically does not flow. So sucking solder into via might be much less of an issue than you think.
 

Offline d1wang

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Re: Solder paste coverage for large pads?
« Reply #11 on: January 18, 2019, 12:25:32 am »
Good to know. What's the drill size of your via hole? Do you know if the stencil apertures avoided covering paste on the via holes?
 

Offline mrpackethead

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Re: Solder paste coverage for large pads?
« Reply #12 on: January 18, 2019, 12:37:32 am »
tenting the bottom side is not a good idea. It can result in outgassing.. which can do all sorts of nasty things when you reflow.. the gas thats trapped in the via will expand when heated.

I have good results with not tenting either side,  I put my solder paste in squares to avoid the holes.. and with GC10 it really doe'stn seem to move that much, and i don't get wicking.




« Last Edit: January 18, 2019, 01:31:11 am by mrpackethead »
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