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Solder Paste?

Started by JohnBurton, November 15, 2016, 04:15:41 am

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JohnBurton

Hello,
I'm interested in what solder paste you use for your reflow oven or any observations on it.

I built a reflow oven project inspired by your projects a while ago. Mine is quite a lot more primitive and only exists as a collection of bits plugged into a plugboard thing... but it seems to work well enough. I can put ordinary solder on the PCB with an iron and put a component on top and it reflows well.... but if I have to manually put solder on every pad with an iron I might as well just hand solder!

So I have major problems with solder paste.
I bought a tube of paste, but it was so hard that I could hardly squeeze any out of the tube. Following advice I heated it a little which helped a bit but even then it was so stiff I struggled to get any out and certainly couldn't spread it.
I bought a second tube, this was thinner but seemed to have somewhat separated into a fluid and hard pieces that wouldn't stick to anything.

Both were tubes I bought from ebay so I think it likely I bought bad solder paste :) Live and learn...

I'm just wondering what experiences other people had though, and what they bought and where from?

Andy Brown

November 15, 2016, 02:20:37 pm #1 Last Edit: November 15, 2016, 02:22:16 pm by Andy Brown
I don't use solder paste. I tried some of that 'mechanic' stuff you get on ebay without the benefit of a stencil and it sort of worked but was really messy. Without a stencil you will get it on the soldermask where it will form little annoying solder balls when reflowed that all have to be cleared off. Some years later I decided to try again with some quality (£20 a syringe) stuff from Farnell and even got a laser cut stencil. It was still messy and with all the faffing around with alignment it would have been just as quick and much less messy to just tin the pads with an iron, which it is what I've always done and continue to do today.

Then there's the expense. Good paste is orders of magnitude more expensive than solder wire, has a limited shelf life and needs refrigerating which I cannot do in a household fridge used for food! It might just be me, after all it's because of the posts and how-to guides on the internet that I tried (twice) at all but I'm leaving paste to the pick-and-place assembly line and sticking to my ancient soldering iron!

PS. You can add liquid flux to solidified paste to bring it back to life, though maybe not with the same properties as it had when it was new.
It's worse than that, it's physics Jim!

JohnBurton

ah i see. i just assumed you used paste.
i shall try what you do some more. it took a long time but i suppose practice helps.

JohnBurton

Just wanted to say thanks!

I tried just putting some ordinary solder on the pads with a soldering iron and then reflowing (after putting lots of liquid flux on the board) and it worked pretty much perfectly. My test board just had 8 0805 LEDS and 8 0805 resistors but I figure if they all solder perfectly then everything else is likely to.

Now to use it to build my "real" reflow controller instead of it being hacked together with various things plugged together :)

Andy Brown

It is quite a reliable technique. I find that I can go around a board with the iron in my right hand and the solder wire in my left and quickly dab over all pads down to 0603 size.

The TSOP, QFP and QFN ICs I do by loading up the iron with solder then dragging the molten blob across a row of pads leaving behind little bumps of solder on each. If I've got my flux right then this normally works fine. Sometimes though I have to go back and either undo a bridge or add more solder to a pad (usually the first one in a row). You don't need braid to undo a bridge. Normally if you smother the bridge in flux, clean your iron then just touch the bridge the solder will shrink back to the pads. Usually.
It's worse than that, it's physics Jim!