Low temp solder paste... Good, Bad, Complicated?

[Bill Gray]

What are people's experience with Low Temp Solder Paste?

I'm doing a single sided board, but it seems like the lower processing temperatures will allow me to reflow faster, reduce the risk of over cooking things, and will likely reduce the reflow fumes a bit?

Still, it seems that Low Temp is not the norm.  I've read a bit about brittleness of high bismuth alloys, but I haven't been able to find my discussion online about *actual problems*

Is there a reason not to default to Low Temp given the choice?

Thank you,

Bill

[Randy Cox]

I’ve gone through a bunch of different pastes including from cheap stuff out of China off of eBay.  My go-to paste is Loctite GC 10 (2023628)  2023628 Loctite | Mouser  .  The stuff is awesome.  Long shelf life.  Great consistency.  Heck, I’ve had paste on my spreader for several days and used it before it ever dried out.  I don’t make a habit of that, but the days are gone about worrying if your paste dried up between stencil, placement, and re-flow.  Don’t think I’ll ever change now.

 

Randy Cox

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[aleksandar berta]

I use on a few boards Interflux LMPA Q6 paste and observe two important things; first , use only lead free coating on boards. Lead with bismuth (from paste) make alloy which melt on very low temperature (think under 100C). And second, generally  solder joints melt on lower temperature so don't use it for designs in which components warm up to higher temperatures. I haven't other issues. I successfully use GC10 also, based on recommendations from forum. Personally don't use other paste, so can't compare with them.

And you have wright, soldering process are faster with LMPA, and paste have much lower price.

regards

Aleksandar

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[Jarosław Karwik]

Well, you need to have precise oven regardless of paste melting point - although bismuth  with ~120 may put less stress on components.

But all should take lead-free 230 deg as this is 99.9% of cases. If you are worried about it buy vapor reflow oven which uses Galden ( so you get 230 deg by design)

See https://www.imdes.de/produkt-kategorie/kondensations-reflow-loetanlagen-en/?lang=en  - this are small volume cheap version of professional ovens using such technology.

I have tried bismuth for dual side boards, but with careful design ( only small passive on the other side) you make second reflow with already soldered components - they will be kept in place because of surface tension.

This works for most  R/C components without any glue and it is used by professional assembly shops.

[3d.m...@gmail.com]

You can actually get away with fairly large parts without glue too.  I've been able to do large SO6 optocouplers, small bridge rectifiers, SO8, pushbutton switches, and SMB diodes, without losing any so far.  It seems to depend on component mass, and the lead contact area, but I haven't worked out an actual formula yet, though I suspect one is already available.

[fred.d...@gmail.com]

I ordered some of the Loctite GC 10 you mentioned and I just want to thank you for that.  It's the best paste I've ever tried.  It has a completely different feel than the FCT NL932 I've been using.  Much more viscous, yet stencils really well and never sticks to the stencil when it is lifted.  Precise and consistent paste on the pads every time.    It was like it was some advanced non-newtonian substance discovered at Area 51.    I had a fraction of the rework and touchup that I normally have.   Thank you again.

[Peter Chaisty]

Hi

Did you use T3 or T4 grade ?

I am interested in trying this out too.

Peter

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[Dave Park]

As long as the low temp paste isn't bismuth bearing, or you're SURE your entire process is free of any lead contamination, you're fine with almost anything.

I have found from bitter experience that when they say "even trace amounts" they mean at the forensic level. 

Dave

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[Fred Davison]

I actually bought the T5.  SAC305T5.   Here's the link:  https://www.mouser.com/ProductDetail/Loctite/2446444/?qs=TiOZkKH1s2TRGB2eeCh2QA%3D%3D

I have a QFP-144 part that has always given me trouble and this solder performed better than the others I have tried.

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[Jim Pruitt]

The GC10 is lead free.  Is there a pasted that is leaded (not lead free) that has a longer shelf life?

Thank you.

Jim Pruitt

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[3d.m...@gmail.com]

I haven't found a leaded paste that has a specified longer shelf life, but I've had good luck with Kester EP256 paste.  It usually lasts me at least a year as long as it is kept refrigerated.  After that, it seems to leave more solder balls floating around.  It is certainly less expensive than GC10, but because it required refrigerated shipping, this actually makes it more expensive for me to get.  Depending on where you are located, this may not be the case though.

My go to paste that I'm switching over to from having run leaded for decades is GC10 T4.

[Bill Judge]

Any idea how reliable GC 10 is for long life applications and/or high humidity applications?

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[Peter Chaisty]

Thanks Fred

I'm going to give that a try, the fact it has such a long shelf life is great too.

Peter

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[3d.m...@gmail.com]

No idea, but probably similar to other SAC305 pastes.  I'd recommend cleaning off the flux residues for high humidity applications, but that would probably be necessary for most pastes.  Leaving flux on the PCB is never a good idea if the humidity is high.  Conformal coating the board might be beneficial as well.

[ma...@makr.zone]

I'm just a hobbyist: what is "Conformal coating"?

_Mark

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[3d.m...@gmail.com]

Conformal coating can be a thick or thin insulating coating that is applied to circuit boards by brushing, dipping, or spraying, to protect them from the environment or handling.  There are a number of different versions, but often they are silicone, acrylic, or urethane based, which leaves a somewhat soft coating that can be removed if necessary (though some coating are not that flexible).  They are a great solution to provide extra protection in high humidity situations, where the humidity could react with ionic contamination on the board surface to create current leakage paths that are not desirable.  In particular, this can be a problem in circuitry that has high impedances, where any additional leakage current might have a large effect in disturbing signals within the circuit.  MG Chemicals makes a number of different varieties of these products, and I've used a number of them over the years: https://www.mgchemicals.com/category/conformal-coating/.

[Peter Chaisty]

I just tried it on some boards.

It's amazing ,, solders beautifully, no solder balls left over.

It's very thick / viscous but goes a long way.

That's all I am ever going to use in future.

Thanks for the tip Fred

Peter

 

On Sun, Feb 21, 2021 at 8:16 AM fred.d...@gmail.com <fred.d...@gmail.com> wrote:

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