Andrew L Weekes
Reverse Engineer
Here's some advice for soldering SMD components, especially multi-legged IC's...
*Updated 10/3/19 with suggested flux pen*
On an SOIC package the bevelled edge does show pin1 - with the device the correct way up and the bevel to the left hand side, pin 1 is top left - this is important, any dots you see near a pin are just consequential. for other packages some are marked - always read the data sheet to be sure.
Now for some trade secrets...
Soldering SMD is a bit like making love to a beautiful woman (sorry, a bit of a tangent there), try again....
Soldering SMD is a bit like designing audio - what one thinks will work and has been told through years of education and experience is often wrong, so here's Andy's top tips for S&MD success: -
1. Buy a flux rework wand
These are a large felt tip pen, filled with rework flux - get a no-clean one for less mess (e.g. https://uk.rs-online.com/web/p/solder-fluxes/2513637/)
2. Don't use tiny soldering iron tips
Trying to solder each pin of an SOIC individually is a waste of time, and very fiddly, the technique to use means use of a surprisingly large soldering iron bit, preferably a 'horses hoof' shaped one of the sort you'd use for ordinary precision leaded soldering. Something about 3-4mm wide is perfect.
3. Use the 'drag soldering' technique
No, not dressing up in your wife's clothes (although if you feel comfortable, then who am I to argue...) but a technique where we throw out the first golden rule of soldering, i.e. that of applying solder to the joint, not the soldering iron.
Well, with SMD that's not the way to do it, so here's a better way, that sounds like a recipe for disaster, but once mastered will have you fitting SOIC's quicker than the time it takes to find your cutters to clip the legs on a standard DIP package
The reason for golden rule no.1 is that the flux evaporates / burns off once the solder is applied to the iron. Being the intelligent and thoughtful bunch that all Pink-fishers are, you will have heeded no.1 above and bought a flux wand.
Here's the secret, please listen carefully, I will repeat this only once
1. Apply flux to SOIC land pattern of PCB.
2. Place device accurately on pads, and check the orientation
3. Check the device orientation again
4. Hold the device on the pcb using a suitable tool, by applying pressure from the top (tweezers, screwdriver, all work well).
5. Dab some more flux on the legs of one side of the device, make sure you don't move it - if you do, realign it.
6. Take your iron in your free hand, apply a nice dollop of solder to the tip.
7. Apply the iron to the first leg you've applied flux to and drag the tip along one side, spending about 0.5s per leg, until you reach the end. Don't worry about shorts, poor joints or anything at this phase
8. Repeat 5, 6 and 7 on the other side of the device.
9. Have a good look, you will have either: -
a. Perfectly soldered joints down both sides
b. Shorts between some legs
c. Unsoldered legs
d. A mixture of b and c
10. If you have b / c, then simply apply more flux to the offending side of the IC, and drag a clean iron tip along the entire side as before - the iron will now pick up excess solder and you should get perfectly soldered joints, indistinguishable from those done by a production reflow method.
If any joints need a bit more solder, repeat 10, but add a small dab of solder to the iron first.
It sounds scary and takes a few tries to get right, but works really well, with a bit of practice you'll get the hang of it.
Andy.
*Updated 10/3/19 with suggested flux pen*
On an SOIC package the bevelled edge does show pin1 - with the device the correct way up and the bevel to the left hand side, pin 1 is top left - this is important, any dots you see near a pin are just consequential. for other packages some are marked - always read the data sheet to be sure.
Now for some trade secrets...
Soldering SMD is a bit like making love to a beautiful woman (sorry, a bit of a tangent there), try again....
Soldering SMD is a bit like designing audio - what one thinks will work and has been told through years of education and experience is often wrong, so here's Andy's top tips for S&MD success: -
1. Buy a flux rework wand
These are a large felt tip pen, filled with rework flux - get a no-clean one for less mess (e.g. https://uk.rs-online.com/web/p/solder-fluxes/2513637/)
2. Don't use tiny soldering iron tips
Trying to solder each pin of an SOIC individually is a waste of time, and very fiddly, the technique to use means use of a surprisingly large soldering iron bit, preferably a 'horses hoof' shaped one of the sort you'd use for ordinary precision leaded soldering. Something about 3-4mm wide is perfect.
3. Use the 'drag soldering' technique
No, not dressing up in your wife's clothes (although if you feel comfortable, then who am I to argue...) but a technique where we throw out the first golden rule of soldering, i.e. that of applying solder to the joint, not the soldering iron.
Well, with SMD that's not the way to do it, so here's a better way, that sounds like a recipe for disaster, but once mastered will have you fitting SOIC's quicker than the time it takes to find your cutters to clip the legs on a standard DIP package
The reason for golden rule no.1 is that the flux evaporates / burns off once the solder is applied to the iron. Being the intelligent and thoughtful bunch that all Pink-fishers are, you will have heeded no.1 above and bought a flux wand.
Here's the secret, please listen carefully, I will repeat this only once
1. Apply flux to SOIC land pattern of PCB.
2. Place device accurately on pads, and check the orientation
3. Check the device orientation again
4. Hold the device on the pcb using a suitable tool, by applying pressure from the top (tweezers, screwdriver, all work well).
5. Dab some more flux on the legs of one side of the device, make sure you don't move it - if you do, realign it.
6. Take your iron in your free hand, apply a nice dollop of solder to the tip.
7. Apply the iron to the first leg you've applied flux to and drag the tip along one side, spending about 0.5s per leg, until you reach the end. Don't worry about shorts, poor joints or anything at this phase
8. Repeat 5, 6 and 7 on the other side of the device.
9. Have a good look, you will have either: -
a. Perfectly soldered joints down both sides
b. Shorts between some legs
c. Unsoldered legs
d. A mixture of b and c
10. If you have b / c, then simply apply more flux to the offending side of the IC, and drag a clean iron tip along the entire side as before - the iron will now pick up excess solder and you should get perfectly soldered joints, indistinguishable from those done by a production reflow method.
If any joints need a bit more solder, repeat 10, but add a small dab of solder to the iron first.
It sounds scary and takes a few tries to get right, but works really well, with a bit of practice you'll get the hang of it.
Andy.
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