# Soldering small pins on IC to a board

## Main Question or Discussion Point

I need some guidance on how to solder those tiny IC pins to their electronics board. I have seen some videos on YouTube where they basically slide the iron across the pins and they all solder perfectly without any bridging between them. How is this possible? Is there a capillary force involved? One would think sliding an iron across all the pins would connect them all in a circuit with the solder but it doesn't. Either way I'm working with really small pins here I believe about 0.3mm thick. Any help is appreciated.

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Tom.G
Agreed, not easy. My limited experience in doing it is most successful as follows:

1) Practice on some junk parts
2) Use a temperature-controlled soldering iron
3) Make sure both the leads and the pads are clean and not oxidized
4) Apply liquid flux to the parts
6) Use small gauge solder, 0.029 Inch/0.7mm or smaller

Several years ago I knew a fellow in the computer repair business that regularly used that approach. He made it look easy!

Yeah they make it look so easy. I'm going to definitely practice on some junk pieces. What kind of flux should I get? Is it easy to find it in stores or do I need to go online?

Tom.G
Something that states it is for Electronic usage. Electronics parts stores are rare these days. If you can find any near you, call and ask them if they carry liquid soldering flux. One online source is:

http://electronics.mcmelectronics.com/search?cataf=&view=list&w=flux&x=16&y=12

A two ounce jar is around $10. The ones with a brush in the cap are convenient. You might also want to order some "Acid Brushes" for cleaning the board after soldering. I get several and cut the bristles on one of them to half length for scrubbing the tough spots. An old toothbrush works too. Rubbing Alcohol from the drug store is a reasonable cleaning solvent and it's usually cheaper than anything else. And yes, you do have to clean afterwards. The flux residue absorbs moisture from the air and is partially conductive. As an example, one time I worked on a computer motherboard and failed to clean the flux off. The area was around the CMOS backup battery, a coin cell. They typically last a few to several years. Not this one, it lasted three weeks. So did its replacement. Had to remove the motherboard and wash off the flux! The fluxes labeled "No Clean" are less active than the rest of the Electronic fluxes; personally, for surface mounted parts I don't quite trust the No Clean claim. The next strength up are those labeled Rosin, they are often, but not always, adequate; it depends on how clean the surfaces are. The ones labeled RMA (Rosin Moderately Activated) will handle most things in electronics. There are also Water Soluble fluxes that don't need a solvent to clean off. The few times I've tried them I have not been impressed. Avoid the Acid fluxes and those for use in plumbing, they attack and dissolve board traces and component leads. Thanks for the information. I'm really glad you told me about cleaning the flux off because I would have probably left it there, so that's definetly something I will keep in mind. How about the contacts and oxidation? The parts are all new, but in case of oxidation would I just scrape the layer off? Also for cleaning the flux, is a solvent like rubbing alcohol necessary or can I just use a brush? Tom.G Science Advisor You're welcome. Scraping the oxidation off is OK but it's hard to get it all without damaging tiny parts. I generally use a stainless steel brush that is on one end of a Soldering Aid. It's easier and more gentle to tiny surface mount pins and circuit boards. Soldering Aids are usually double ended and come with various combinations of point, reamer, brush, fork, scraper, hook and other shapes. My personal preference is one with reamer & fork and one with reamer & brush. The ones with a wood handle seem to be generally better overall quality the the plastic handle versions, but they are harder to find. A solvent and brush are needed. You won't get flux off with just a brush without removing some board material and/or components. It dries out and hardens from the soldering heat. That would be like trying to wash off your dry, muddy hands with a butter knife instead of soap and water. Ok great makes sense. I still don't have an adjustable soldering iron. I'd like to avoid the bad quality ones, how much would a decent one cost? I've seen some for around$20 but I'm not sure.

I would suggest that even more important than getting a station with temperature control is to get a proper chisel tip. Conical tips can work, and have their place in fiddly situations, but heating a flat surface is best achieved with a hot flat surface.

The drag technique you have apparently seen used with surface mount ICs is not difficult at all if the board is properly prepared with solder mask, which counteracts bridging. Try it out with larger packages like SOIC first.

In the video they did use a flat tip so I'm guessing I should stay away from conical tips in that case, I suppose that's more for individual pin soldering. Is solder mask just flux or is it something else?

Tom.G
...how much would a decent one cost? I've seen some for around $20 but I'm not sure. The most useful iron is one where you know the temperature it is actually operating at. That could be with a control knob and a temperature readout on a display, or one with interchangeable tips for different temperatures. In either case you will want a few different tips of various size and shape. The irons with just a knob pointing to a temperature can be maddening to use; they indicate an approximate temperature without actually regulating the temperature. They can be useful but take a lot of getting used to. The ones with actual temperature control are running around US$150 and up.

I would suggest that even more important than getting a station with temperature control is to get a proper chisel tip. Conical tips can work, and have their place in fiddly situations, but heating a flat surface is best achieved with a hot flat surface.

The drag technique you have apparently seen used with surface mount ICs is not difficult at all if the board is properly prepared with solder mask, which counteracts bridging. Try it out with larger packages like SOIC first.
Good points! They are both important.

Would hot air soldering work for you?

Tom.G
Is solder mask just flux or is it something else?
Solder Mask is the green coating you see on board in @austinuni's post. It keeps solder from the areas that don't need solder. Very handy, it keeps splashes of solder from shorting out traces. When it is also between pads for component soldering it makes the 'drag solder' approach much easier.

Would hot air soldering work for you?
That looks interesting, I'm not too sure what to think of it. It looks like it works nicely but it's also heating everything on the board at the same time, regardless it's good to know this exists as an option.

Baluncore
2019 Award
When surface mounting components by hand, you do not apply the solder with the iron. The component terminals are plated and the board is pre-soldered. Any excess solder must be removed, then position the IC and heat the contacts with the iron to re-flow the solder. Capillary action will fill the space between the PCB pad and the IC contact with solder.

When the iron is run along the row of sequential contacts, it is re-flowing the solder as the iron passes. The iron needs to be tinned as normal but with very little solder. The solder on the iron is a thermal transfer medium that is used to heat the pins more quickly and reliably as the iron passes.

Being able to see well what you are doing is a big deal. Here is a link to A&A in LA. http://www.aajewelry.com/jewelry-supplies-tools-and-equipment/loupes-magnifiers/visors They sell many things of high quality and some very cheap useful tools. I own their best visor and their cheapest one. Both will work.

I use to make gold and silver Jewelry and learned a bunch about heat and how to control it. I also use to own a welding company and did a lot of high quality work in stainless. All of that background helped a lot with soldering. The one thing that stands out is that the cleaner something is the less problem it is to solder. I can not overstate the importance of things being clean and free of oxidation.

Cheers,

Billy

davenn
Gold Member
2019 Award
Yeah they make it look so easy. I'm going to definitely practice on some junk pieces. What kind of flux should I get? Is it easy to find it in stores or do I need to go online?
you don't need to buy separate flux
it is in the solder .... all solder for electronics work has multi cores of flux

rbelli1
Gold Member
With drag soldering you will want a lot more flux than can be supplied from the core of the solder. If you try with just the solder and no supplemental flux you will just bridge across all of the pins.

BoB

Tom.G
Being able to see well what you are doing is a big deal.
Sure is! Here in the U.S., Reading Glasses are readily available in Drug Stores for around US\$10, and you can try them in assorted strengths. I wear them right over my tri-focal glasses when needed.

dlgoff
Gold Member
you don't need to buy separate flux
it is in the solder .... all solder for electronics work has multi cores of flux
True that, but for me, I sometimes use a little extra flux, to get a quicker solder flow which decreases the "iron contact" time. Better to have a little more flux clean-up than having to replace a temperature induced chip failure.

jim hardy
Gold Member
2019 Award
Dearly Missed
You will get frustrated trying to solder tiny things , especially with 63/37 solder or Heaven Forbid , Lead Free !
The end of your solder has to melt before it starts releasing flux. Applying a little bit of flux first helps a lot.

The last place i worked bought liquid flux in 55 gallon drums for wave solder machines.
I filled a couple of 1 oz bottles with dregs from a discarded empty container and it lasted me several years at home. I found i like it much better than the brown goop from Radio Shack. A nail polish brush works great for applying it.

This looks like a handy product
http://www.alliedelec.com/kester-solder-83-1000-0186/70177933/

mouser, digikey, allied, ,,, all the regulars carry several varieties of it .

Baluncore
2019 Award
True that, but for me, I sometimes use a little extra flux, to get a quicker solder flow which decreases the "iron contact" time. Better to have a little more flux clean-up than having to replace a temperature induced chip failure.
The thermal transfer from the iron to the pin is enhanced by the “wet” solder plated on the tip of the iron. Flux is a thermal insulator that keeps air away from the surfaces being soldered. Flux reduces the loss of heat and so results in slower cooling. The use of excess flux will lead to high temperatures for longer which will subject the IC to a greater thermal time envelope.

In production, a solder-flux paste is screen printed onto the PCB pads. The IC is then placed and held with a glue. Heat is then applied to flow the solder. No solder or flux is added.

When soldering SMD by hand, plating the PCB pads with flux cored solder leaves a local coating of solder with a covering of flux. The contacts on a IC are usually pre-coated with tin solder, so only heat is needed to complete the connection. No solder or flux is added.

davenn
Gold Member
2019 Award
The thermal transfer from the iron to the pin is enhanced by the “wet” solder plated on the tip of the iron. Flux is a thermal insulator that keeps air away from the surfaces being soldered. Flux reduces the loss of heat and so results in slower cooling. The use of excess flux will lead to high temperatures for longer which will subject the IC to a greater thermal time envelope.

In production, a solder-flux paste is screen printed onto the PCB pads. The IC is then placed and held with a glue. Heat is then applied to flow the solder. No solder or flux is added.

When soldering SMD by hand, plating the PCB pads with flux cored solder leaves a local coating of solder with a covering of flux. The contacts on a IC are usually pre-coated with tin solder, so only heat is needed to complete the connection. No solder or flux is added.
agree completely, you saved me from making that major point. and the heat is applied using proper soldering heat guns ( hot air re-work stations), NOT soldering irons

eg ....

https://www.sparkfun.com/products/10706

normal soldering irons and SMD components do not mix well

Dave

dlgoff
Gold Member
The use of excess flux will lead to high temperatures for longer which will subject the IC to a greater thermal time envelope.
Depends on who's soldering. Ever had your iron on the side of a pin for a longer than you wanted; ...before the solder starts to flow? That's the additional heat that can blow a junction. Sure, the flux will keep the heat within the solder as you say, that's part of it's job, but the amount of time of this part should be very quick; with proper clean iron tip

Baluncore
2019 Award
Depends on who's soldering.
No, the engineering theory of solder joints is a science. Only competent operators need apply for the job.

Ever had your iron on the side of a pin for a longer than you wanted; ...before the solder starts to flow? That's the additional heat that can blow a junction.
I have personally made over 1000 solder joints in the last 24 hours. That is not surprising as I have a special order low volume electronic instrument production line here. The product is exported, so the cost of a failure would be expensive. In order to maintain product reliability it is necessary to apply a rational engineering analysis to solder connection quality control.

If the “iron” was on the "pin" for longer than I wanted, then that would be because the “iron” was cold, or a layer of oxide on the “iron” prevented the heat transfer.

When you think about it, that could NOT “blow a junction” as you put it, because the heat has not entered the pin.

Integral
Staff Emeritus