Brads Electronic Projects Forum

Since I just wrote this up elsewhere, I figured it'd be good to recycle while it's on my mind.

A prominent member on that forum, "BigDumbDinosaur," has poor eyesight and asked me to solder the SMT parts to his hobby computer board. Here are the SRAMs, in SOJ-36 with .050" lead spacing:



I did them with my 30W iron with a 1/8" chisel tip that covers three pins at once. I didn't use any extra flux, or a magnifier, or any solderwick, or anything special. I don't do one pin at a time either. Here's the method:

• Tack two opposite corners just enough to hold the IC in place. It doesn't have to look nice yet, but you do need to get the IC straight and centered before continuing.
• Put some solder on a third corner, at the opposite end of one of the first ones you did. This will hold the IC in place in the next step.
• Now go down the other side, starting with the corner which so far got no solder. Flood that side, making one big bridge all the way down, moving the soldering iron back and forth, making sure all pads get wetted too.
• Do the same thing to the remaining side which up to now only had the corners soldered. Now you'll have both sides all bridged, with lots of extra solder.
• Go back to the first side you flooded. Hold the board vertically, with that row of pins vertical, and start with the soldering iron at the top of the row, and re-melt and move it down slowly. The extra solder will come off on the iron. You might have to shake some off before you get to the bottom of the row, if there's a threat that it will drip onto something it shouldn't. Continue down the row. Each pin should be left with the right amount of solder, just perfect, with no bridging, in spite of the large size of the tip. If I have trouble with an area, I'll put more solder on it again and repeat.
• Do the other side the same way.

After it's cooled, I chip off most of the rosin with any small tool, preferably one that's not too sharp so it won't scratch the board, then finish with a Q-tip dampened with acetone to get the slight remainder of rosin off. That way I don't need a vapor degreaser, or to dunk the whole board in solvent, etc..

It may seem very counterintuitive, but the results in the picture speak for themselves. I don't think I've done finer pitches than .050" this way. They might require something a little different.

Someone might be afraid that the heat would damage the IC. Not so. A couple of jobs ago, in the mid-1980's, I saw transistors actually operating at over 350°C. They wouldn't last long at that temperature, but it did not instantly destroy them. The soldering method I'm describing above won't get the die (ie, the actual silicon chip inside the IC) anywhere near that hot, since at a maximum, only one row out of two will have molten solder all over it while the other row is relatively cool, and the die is not right at the pins anyway, but farther in. 60/40 tin/lead solder melts at 183°C and is completely liquid by 190°C, and again, the die won't get that hot.

They are some clean solder joints Garth

What solvent / cleaner are you using after soldering Garth?

brad wrote:What solvent / cleaner are you using after soldering Garth?

After it's cooled to room temperature, I chip off most of the rosin with any small tool, preferably one that's not too sharp so it won't scratch the board, then finish with a Q-tip dampened with acetone to get the slight remainder of rosin off. That way I don't need a vapor degreaser, or to dunk the whole board in solvent, etc.. The only flux I used was what's in the solder.

acetone and q-tips are fantastic for bringing them up all nice and shiny!

Here's another one, an SOIC. These were taken with a very cheap camera.

First I tack one corner, and get the IC really straight, then tack the opposite corner, and again make sure I haven't rotated it out of being straight. Then I do the two remaining corners. Then I flood a whole side with solder, then the other: Then I hold the board vertical and slowly go from the top of a row to the bottom with the big soldering-iron tip as described above. The excess solder comes off on the tip, leaving this: Again, my apologies for the quality of the super-cheap camera. And I don't have enough hands to photograph the process while I'm doing it. Maybe I should try to coordinate the next time with someone who can help.

Sorry for a late reply here..

The picture quality is good, and the soldering joints have turned out perfectly, looks very clean

I also use Acetone to finish off my boards!..

I have been soldering some fine pitch 0.5 MCU's and 0603 components recently, they have been fun used a lot of desoldering wire on these MCU's...