SMT with solder paste

[DigiTan]

A while ago I read from Seattle Robotic's Global Encoder Newsletter that describes how to perform professional-quality SMT work using solder paste and a conventional oven. I'm no career skeptic, but the going rate for this water-soluable solder paste is about 42 U.S.D., so I want to be absolutely sure this works before I go running for my checkbook. Basically, I have 2 questions about this SMT technique.

(1) First, I'm just curious to know if anyone here has ever reproduced the quality in the article. If so, did it take any practice? (2) Second, I wanted to know if there are any cheaper alternatives to water-soluable pastes. If not, is it possible to buy solder paste "over-the-counter" within the U.S. (hardware stores, etc.)?

 

[bmcculla]

You can do professional looking work with just a fine tipped soldering iron, some flux and a roll of solder wick. It looks like the method discribed in the article still needs cleanup with solder wick.

**broken link removed** This is the method I use for mounting fine pitch components - the results are quite professional looking especialy if you take the time to clean off the flux.

 

[Oznog]

There are far better ways. Zephyrtronics makes a very good paste at a bargain price ($10/tube). You need needle tips and a plunger too.
http://www.zeph.com/chem_direct.htm

You can talk to their engineers about how best to pull it off at home. You must first get the board to 300 deg for a minute to cause the flux to separate from the paste. http://www.zeph.com/pap1.html

Now the soldering iron does work, but a component may get pushed around by the iron so maintaining alignment requires technique. It's also a bit prone to bridging and may need cleanup with solder wick or a desoldering vac tool.

I did a bit of work with the iron, and felt quite "empowered" by being able to do SMD work. But I've since found something that works MUCH better:
Weller's Portasol kit has a catalytic hot air blower (not the blowtorch! the hot air blower!) with very small diameter airstream, which basically does exactly the same thing as Zephyrtronics' $1000 SMD hot air blower, but for around $25! Pick up one off of eBay. It works like a charm. You don't need to touch the component, so as long as you align the pins correctly in the first place, you won't push it around. The solder naturally balls up and you won't bridge pins unless you go overboard on the paste. Just zap the paste and it works every time!

**broken link removed**

 

[Optikon]

You can do professional looking work with just a fine tipped soldering iron, some flux and a roll of solder wick. It looks like the method discribed in the article still needs cleanup with solder wick.

**broken link removed** This is the method I use for mounting fine pitch components - the results are quite professional looking especialy if you take the time to clean off the flux.

Of course, if you have to do 500 components, a paste and bake method starts looking attractive..

 

[DigiTan]

Well it looks like I just saved about $42. Now I just have one last question about the bake method. Does it also work well for hidden-lead packages like LCCs? And how successful is it?

 

[Oznog]

Well it looks like I just saved about $42. Now I just have one last question about the bake method. Does it also work well for hidden-lead packages like LCCs? And how successful is it?

Baking only separates and flows the flux. That stage will not flow the solder for LCC any more than it will for another pkg.

You cannot crank up a regular oven high enough to flow the solder, even if you could, it will take too long and the heat will soak deep into and damage the board and/or ICs. It should be exposed to these temps as briefly as possible, hot air stream melts the solder and is gone long before the silicon die inside rises a lot.

I did goof the alignment on my last tiny MSOP-10. I noticed that the Portasol was more than capable of melting the solder on both sides at once (and keep it on for a moment longer and it will blister the board). So a small pkg, sure it could reach hidden leads, but it looks like the heat's going to have to go through the component or the board if the lead's not at the perimeter. I'd avoid using pkgs like BGA, I've never seen any component I'd even want to use only available in such a pkg anyways.

I assume from your previous posts that the LCC you want is the ADXL202. I have done that one by hand with a soldering iron with no problem at all. The pins aren't close together which is the main reason an SMD pkg needs the hot air. Just get the alignment right with the first pin and you're good to go.

 

[bmcculla]

Circuit Cellar ran an article a month or two ago about modifying a toaster oven to do reflow soldering. The toaster they used had quartz heating elements to do the reflow soldering. They had it temperature profile and everything.

I've tried a cheap toaster oven and if you crank the temp all the way up it will reflow the solder in under 5 minutes. I used the recommended solder paste from PCBExpress. They also sell laser cut solder stencils if you are going to do a bunch of boards.