solder and flux

Matthew

Hello. I'm familar with using the solder which contains a core of flux. Because I'm planning to solder small components, I'm going to have to go with pure solder without a core of flux.

My question is how do I go along adding the flux when I'm soldering. What does flux do? What form does it come in? When do I apply it when soldering?

Thanks,
Matthew

KenM

Flux can be obtained in a liquid form, put it on the pads to be soldered. Messy, very effective.

Flux also comes in a paste form.

Flux helps to remove impurities from the surface of the pads/leads when soldering.

I have used rosin core solder when soldering the smallest of surface mount components. Also adding liquid flux to the pads before soldering helps a great deal.

Are you looking for fluxless solder only because of the size of the solder?

Matthew

Yea, I've only seen medium sized diameter rosin core solder, and I'm afraid it'll be too large to solder a uSOP/uMAX package. What do you think?

Forrest

Solder is applied to the soldering iron tip, not to the component directly. See you're previous message for more info http://forums.parallax.com/showthread.php?p=528618

kb2hap

how thin of solder are you looking for I use a very thin rosin core solder I got from jameco
the size is .020"

http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&storeId=10001&catalogId=10001&productId=120243

not sure if you looking for thinner but that works for me

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DTQ

Paul Baker

Forrest hit the point, in traditional soldering, the solder is applied to the heated join, thats because the thermal capacity of the larger pins and bonding surfaces require it. But in surface mount you apply the solder to the iron and with flux already applied to the join, it heats quickly and the solder on the tip flows, so solder diameter isn't much of an issue.

Forrest

Assuming you're just learning to soldering SMT chips (SOIC, SSOP, TQFP, etc), here's some gerneral guidelines:
1. Pick a soldering iron tip that's approximately the same width as the leads that you're trying to solder. The mini-wave tip works best, but if that's not available choose an oval shaped tip that's cut at a 45 degree angle. A 15 watt to 30 watt iron works well. Bigger is NOT better.
2. Use water soluble flux - the gel type works the best. Low viscosity flux contains a lot of alcohol which will burn off quickly - which is NOT desirable when you're trying to learn to solder. As a begginner you should use a lot of flux. When you get more experienced - you'll need less.
3. Clean your tip on a moistened sponge often and when ready to solder the chip - load up the tip with as much solder as it will hold without dripping.
4. Solder the whole side (entire row of leads) of the chip at once using one smooth motion.
5. Use a solder bulb or soldering braid to clear shorts. You may need to apply a little more flux to the joint to clear shorts.
6. When you're happing with the results, clean the board/chip in warm water and rinse thoroughly. The best way to do this is hold the board under a gentle stream of water and use a toothbrush. Dry with air (canned air works well) or lint-free wipes. You can use cotton swabs but they'll leave little strands everywhere - which is not a functional problem.

Paul Baker

Forrest, do you use deionized water to clean the flux off your board afterwards or distilled or tap? I presently am using no-clean, but it still leaves a mess which I find myself having to use alcohol and cloth to clean it. Ive though about switching to water soluble but deionized water is not the easiest to come by.

Also do you work with moisture sensitive components, if so how do you clean it? My present board has 5 components that came packaged in silica gel, but the directions accompanying them stated if the part was to be used in reflow, to pre bake it to reduce the moisture content. I don't know if the moisture sensitivity was with respect to reflow or a more general issue.

Beau Schwabe

To amend to item number 4.... I usually 'tack' down opposite corners of the chip first with a small bit of solder. Then before soldering
the remainder of the pins, I check to make sure I have proper alignment.

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Beau Schwabe - Mask Designer III

National Semiconductor Corporation
(Communication Interface Division)
500 Pinnacle Court, Suite 525
Mail Stop GA1
Norcross,GA 30071

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Forrest

Tap water is fine for most 'commercial' applications. If you're selling electronics to the governement - you'll need to to use deionized water as a final rinse. No clean flux isn't water soluable, and if you reheat no-clean flux a few times you'll eventually end up with charcoal (which is NOT solderable). Water soluble is a lot more forgiving and comes off with water.

One item that's important to soldering is plenty of light and magnification. I do all my soldering under a tabletop magnifier (3X)/lamp - which you can find at office supply stores for around $25.

In my experience in a production manufacturing environment moisture sensitivity and related problems are rare. Keep in ming virtually all SMT parts are packaged in air-tight reels, trays, etc and they're not exposed to the air until seconds/minutes before they're soldered. The majority of these can be cleaned with water after soldering. There are a few exceptions - some prototype chips are NOT hermetically sealed and shouldn't get wet. There are also some RF capacitors and filters that shouldn't get wet because it can slightly alter the RF characteristics of the circuit.

As an electronics hobbyist - I'd just keep all components (especially chips) sealed in their ESD envelopes until I was ready to solder them.

Post Edited (Forrest) : 3/10/2005 11:40:43 PM GMT