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Old 16th October 2012, 09:06 PM   #1
CECCO is offline CECCO  Italy
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Default SMD soldering.

Hi.

I need to start solder small smd parts like TQFP and so on with iron or hot air gun.
I'm gonna place an order at DK for the needed parts like flux and solder.
The solder paste they have is Kester and Chipquik, which is the best?
And also for the flux, Kester or Chipquik?
About the solder removal braid, ther's a thousand of them. What type do I need for small smd parts?
Eventually the price won't be a problem, the quality is mandatory.

Thanks.

Last edited by CECCO; 16th October 2012 at 09:22 PM.
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Old 17th October 2012, 05:53 AM   #2
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Unless you are going to be using an oven or a hot air gun almost exclusively, you don't need solder paste. Just buy the very small solder (0.016"??). It about 31AWG size.
Either of those flux brands should be good. More important is whether you prefer water-soluble, no-clean, highly reactive, etc.
Same with braid. Any name brand ~3mm stuff should work fine.
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Old 17th October 2012, 07:23 AM   #3
Mooly is offline Mooly  United Kingdom
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Have a read at this,
Working with SMD. How to do it without specialised tools.
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Old 18th October 2012, 05:34 AM   #4
seanvn is offline seanvn  Viet Nam
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Let me tell you to use lead free solder paste. I would regard fine powered lead as an extreme hazard. There are some very nice lead free low melting point solder pastes out there. For example mp. 130C. However pastes don't keep well. I would second the idea of using fine solder wire.
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Old 18th October 2012, 11:02 PM   #5
CECCO is offline CECCO  Italy
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Thank you guys.

At the end, I ordered different types of staff just to see the difference between them.
Seanvn, I wouldn't care much about the lead of the paste since I use a fume extractor. Further, I heard that the lead-free paste/solder joint is not so reliable.
Anyway I'll post images about the outcome of my work (hopefully in a couple of months).
Wish me good luck!
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Old 19th October 2012, 07:35 AM   #6
marce is online now marce  United Kingdom
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solder paste does not contain powdered lead, it contains balls (spheres) of tin/lead solder. Having worked in this industry and been heavily involved in PCB design fabrictaion and assembly, i have never heard of the lead in tin lead solder being a hazard, and as for the solder paste, even if you snorted it I doubt much would happen, it is combined with tin!!
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Old 18th February 2013, 09:35 PM   #7
SyBorg is offline SyBorg  United States
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I have done a lot of work with SMD parts over the years. All of the boards my company produces are SMD, and when I do new designs I make the first few prototypes by hand. The finest pitch parts I have worked with are 025 pitch, which may be a larger pitch than what you are working with, but the principle still holds.

The trick is to use a cheap toaster oven. Don't use the good ones, they heat up too quickly. And, don't use your wife's toaster oven. Find a cheap one at a garage sale or thrift store.

Apply solder paste to the SMD pads using a toothpick or small wire. It takes very little paste to make a good connection. If you apply too much, the solder will just ball up and with fine pitch parts, it will bridge to the adjacent pad.

Place the parts on the board using tweezers doing your best to line things up straight. It is not that critical for most parts, but more so for the fine pitch ones. When the board is heated, the surface tension of the wet solder will help to self-align the parts on the pads. If the fine pitch parts are not aligned well to start, they may self-align one pad off.

Place the board in the center of the oven and turn it on to "Bake" at 200 deg F for 4 minutes. This will bring all the parts up to a low working temperature. If you err on the time, err on a longer soak time at 200 deg.

Raise the temperature setting to 325 deg F for 2 minutes. This will allow the internal temperature to begin to rise without becoming too hot.

Finally raise the temperature setting to 450 deg F. Watch the board carefully. In my oven the solder will start to melt after 45 sec to 1 minute depending on the lead size of the part. Heavy parts with large leads will take longer as their thermal mass in higher. Watch the time closely. From the time the solder melts, leave the oven at 450 for only 30 seconds, then turn off the oven and open the door. This will allow sufficient time for the solder to wet the joint without overheating the part's die.

The boards will be hot, and the solder joints will need time to cool. Let the board sit for 5 to 10 minutes until it is cool enough to handle.

Inspect the board closely using a lighted magnifier if you have one. If there are solder bolls, just pick them off using tweezers or small-nose pliers. They should pop right off. Sometimes an SMD part like a small resistor may ride up on top of the solder, never making a joint. Touch this up with a fine tipped solder iron. On fine pitch parts if tere was too much paste, you may find a solder bridge across tow leads. Use some fine solder wick and a fine-tip iron and the bridge should come off easily.

Good luck!

Last edited by SyBorg; 18th February 2013 at 09:37 PM.
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Old 18th February 2013, 09:50 PM   #8
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All good suggestions, and I'll second the toaster oven idea. I double checked the thermal curves for reflow, and I was surprised at the good fit to be honest. I've used a toaster oven, and flux pen on a BGA217 before, successfully.

Also good to use is a paint stripper. Seriously. Get some tinfoil, mould it to shape maybe around a large fitting, so most of the air is vented away from the work and use an AA battery / pencil / pen diameter sized object (10 - 15mm) to pruduce a duct for hot air so you can rework smaller parts.

SyBorg's post is good information, personally, I use a solder paste syringe with a fine nozzle on. Keep the nozzle blocked between jobs with some enameled copper wire of the right size so it doesn't dry out. Lay a fine bead around the chip, and touch up the pins afterwards with a fine iron as SyBorg says. I've used this for 0.4mm pitch TQFP100s before (PIC32MX) as well as DFN / QFN and LGA packages. LGA I'll make a bigger footprint for, so I can touch the iron to it and make sure I can manually reflow after it's been hot aired / ovened on...

And I would recommend lead based paste over lead free - the Xbox red ring of death was apparently due to lead free paste and brittle / poor solder joints.

Just as a final tip, when you've got a board in the oven, make sure you place a blob of paste in all four corners so you can see the temperature has reached melting point all over. Things like SMD inductors tend to take a little longer to reach temperature also, due to the higher thermal mass.

Hope that helps too!
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Old 18th February 2013, 09:57 PM   #9
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Nice! Think Ill find an oven!
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Old 24th February 2013, 08:03 PM   #10
tvrgeek is offline tvrgeek  United States
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I used a needle point tip in my Weller station and it was OK. Salvation Army never seems to have a cheap toaster oven. Besides, solder paste is really expensive. I have one of those Radio Shack de-solder bulb jobs, but have not found a free aquarium pump for it yet. I also need a used dissecting microscope as I am blind as a bat. I just can't see the board. I keep wondering if those USB micro-cameras will work.
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