SMD soldering - any tips for surface mount?

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Patrick
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SMD soldering - any tips for surface mount?

Hey everybody:

I'm re-working some circuit boards, and need to replace the xtal/oscillator driving a PIC. I'm fairly good with soldering, but I've never done SMD. These are six pad devices, they connect to Vss, CLKIN and CLKOUT. I suppose it's possible to cut the traces, and feed CLKIN from an externally mounted device, but there are some reasons I'd rather not do that.

What advice can you give me for the removal/replacement of these guys? Special tools? Techniques for getting them off the board? I have one or two boards I could sacrifice for destructive testing, but would like to avoid that if I could. To complicate matters, one of these boards is dipped in plastic (for weather proofing, I suppose) - it's a real bitch to work on.

Any suggestions or help would be greatly appreciated.

Thanks,

Patrick

happyhobit
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Four techniques

Hi Patrick,

Four techniques come to mind.

Compressed air, heat the pads and blow the solder off. (Solder-Sucker?)

Dental pick, heat and pry, sequentially, carefully.

Hot air gun and wire brush, good for stripping board, probably not what you want.

Crush and pick.

For the one with the coating, (Conformal?) you can try a strong solvent, lacquer thinner or acetone, a wire brush and just clean the area you’re working on. Stinky, don’t do inside or near flame.

Jay

Fechter
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Tiny little suckers

The conformal coating can usually be scraped off with a small straight blade screwdriver. Solvent might work, but that would be messy. Depending on the type, rubbing alcohol might make it easier to remove without melting it.

If you can heat all the leads at the same time, then you can free the component. This is tough if the leads are on both sides. I've had reasonably good luck using solder-wick. You have to jam it tightly into the fillet at the tip of each lead and heat. After wicking out the solder, then try to pry the leads sideways to break them loose. Avoid pulling straight up, since that tends to lift the foil

The dental pick or really tiny screwdriver might work good if you can heat all the legs on one side and try to lift one side at a time. Make sure all the leads are heated before applying upward force. Don't pull too hard to avoid lifting traces.

The components are often sort of glued onto the board. This is what holds them in place during soldering. They usually come off pretty easy when they're hot.

If a trace lifts, use an x-acto blade to trim off the loose part. Then use a single strand from some stranded wire to make a jumper to the intact part of the trace. If you need to solder where there's soldermask (that green stuff), use a small blade to scrape it off first. Tinning helps.

Once you're ready to put the new part in, you need to hold it down while soldering. I use a toothpick or my small screwdriver. My smallest soldering iron at home is way too big, so I took a couple inches of solid copper house wire (14ga.) and wrapped it around the tip of my iron and brought out a "stinger" about a 1/2". The "stinger" gets onto the tiny pads without bridging to the next pad. If you bridge by accident, you can try to "pull" the bridge away from the part with the iron. Solder wick can help there too.

I put a tiny dab of paste flux on the leads, then make a tiny ball of solder on the tip of the stinger. Just touch it to the joint for a coulple of seconds. It helps if you can see good. I use magnifier goggles when they're really small.

Patrick
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Conformal coatings

Hey guys,

Thanks for the tips. Decided to sacrifice one conformal board - just used a dremel and ground off the existing part. That's when I started to look into the coatings. There are four or five main types; this particular board uses a silicone based product. Looking around, I found some products that are specifically made for working this type of coating. An example:

http://www.microcare.com

The product is Veri-Clean. There are some others with similar names.

So now I'm going to try these solvents to remove the coating from the leads, etc. On the board itself, it can be fairly easily scraped off with a fingernail. But on those teeny SMD leads - ecch! Using a $x lens, I see a bunch of little globs of the stuff sticking to the leads.

Any experience with this type of coating, or solvents therefor?

Thanks again.

Patrick

Fechter
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Solvent

A solvent made for that particular formulation would be ideal. Just from experience, gasoline is about the only thing I've noticed that instantly destroys silicone. Cheap and easy to get too. Not recommended for indoor use. Tiny bits of residue might burn off when you go to resolder the part.

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