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User:WillWare/Homebrew surface-mount construction

< User:WillWare

Lots of people have figured out surface-mount construction techniques and tools that work on a very limited budget. Getting boards assembled by a professional assembly house can be expensive, because they will charge several hundred dollars to make a metal stencil. I've used an ISO-9000 board house in the past, and if I don't bring them paperwork that looks a particular way, they get antsy. So these techniques for soldering teeny fine-pitch parts are cool, especially with the deals one can get these days on small runs of prototype PCBs.

Hints, tips, advice, background info, encouragement from hams, piclist, gEDA mailing list, another ham, and sci.electronics.basics (Usenet).

The "right" way to do surface mount soldering, with the official tools...

You want quiet, lots of light, elbow room, and good magnifiers and tweezers. Good magnifiers can be found at arts-and-crafts stores. A microscope is a good idea - mine has 10x eyepieces and a switchable 2x/4x objective, and it's perfect for SMT work.

Practice is important, and it's a great idea to get a lot of practice without spending a lot of money. Solder, solder paste, irons and wick and toaster ovens are cheap, as are rolls of some 0805 and SOT-23 parts. For fine-pitch ICs with lots of pins, the cheapest I've found is the EPM3064ATC100 ($3.10 from Digikey) in a 100-pin TQFP package with pin spacing of 0.5 mm. I designed a little two-layer board that you can use for practice. Feel free to download the Gerber files.

Techniques edit

Toaster oven reflow soldering edit

A toaster oven can do reflow soldering with solder paste. "Reflow" soldering means that you arrange everything just right, and heat the entire board to the melting temperature of the solder. When the solder melts, the surface tension of the molten solder pulls the ICs and other parts into correct alignment. You can get solder paste from Digikey or JDR or Jameco, or try http://www.smtsolderpaste.com/. I found a toaster oven at Walmart for $20.

 

If you're going to do reflow soldering with a toaster oven, do NOT use the toaster oven that you use to cook food. Get another toaster oven just for soldering. It will fill up with lead and toxic chemicals which can cause cancer. Use a Sharpie marker to draw a SKULL AND CROSSBONES on it as a reminder NOT to use it for food preparation.

Toaster oven discussions on the web talk about the time profile of temperature, and how parts should not be heated more than necessary. You want to run the oven a little below water's boiling temperature for a few minutes to dry out any residual water, and then quickly ramp up the temperature to the maximum temperature. Wait for the solder to reflow and then give it another few seconds, and turn off the oven and open the door so the board will cool quickly.

It's a good idea to place a short length of solder (1/4 inch or 1/2 cm) on a piece of aluminum foil in the oven, and use a flashlight to watch that length through the oven door. It will be very clear when the oven reaches the solder's melting temperature, because it will bead up into a little sphere.

I ordered a stencil from http://www.stencilsunlimited.com and used it for one board, applying way too much solder paste, and my fine-pitch parts were full of shorts. I tried to remove these with solder wick but didn't have much luck. For another board I used a small paintbrush to apply paste directly to pads, and that worked better. One element where practice helps (and unfortunately nothing else seems to) is in knowing how much solder paste is right.

Flood and suck edit

The idea here is that you use normal solder (not solder paste) and apply it liberally to all the pins of a fine-pitch IC, which results in blobs of solder and lots of shorts. Then you clean up the shorts with a solder sucker.

Flood and wick edit

 
Flooding
 
Wicking - oops, lifted a pad off the board

Same idea as "flood and suck" but instead of a solder sucker, you clean up excess solder with solder wick.

My impression is that "flood and wick" is more popular than "flood and suck". I am not so fond of solder suckers myself. The big mechanical shock is a bad idea for fine work, and the suction is hard to position exactly where you want it.

A couple years ago I was quite keen on the toaster oven approach, but now I've tried flood-and-wick and gotten surprisingly easy good results on the first try (one short needs cleaning up). So this is my technique of choice as of November 2006.

Solder wick quality is crucial edit

It's very important to use a roll of solder wick that sucks up the solder well. I have been working with a roll from Radio Shack and another from You-Do-It Electronics, and the Radio Shack roll won't suck up the solder. I heated it a long time to try to make it absorb solder, and as a result one of the pads lifted off the PC board - look at the two pins on the bottom of the "wicking" picture to the right. The GOOD solder wick is made by MG Chemicals, Catalog #425, "Fine braid super wick". Look here to find a vendor.

I don't know the reason solder wick would not suck up solder well. Two thoughts come to mind.

  • Maybe the braid is braided too tightly, and there just isn't room in it for solder.
  • Maybe there is some kind of oil or crud on the bad solder wick that repels the solder.

Looking at the two braids under the microscope, the bad solder wick looks a little dustier, but I think the big difference is that it's braided tighter.

  • Solder wick is sold with flux already applied. After a few years of storage, the flux is no longer effective. In order to use "Radio Shack" solder wick, or any other does not seem to work:
    1. Clean a piece of wick you wish to use with a paper towel and alcohol.
    2. Apply fresh flux to the piece of wick
    3. You will now find the wick to work much better.

Cleaning up the fluxy mess edit

You're going to have sticky brown spots of flux everywhere by the time you're finished. Use a Q-tip and some isopropyl alcohol (the stinky medical kind) to clean up the flux. If you want to be squeaky clean, you can then rinse the board with water, and dry it by baking it just below the boiling temperature of water (200 F or 90 C).

I first tried tissue paper instead of a Q-tip. This left teeny strands of paper all over my nicely soldered pins and it looks kinda ugly. Use the Q-tip.

Post-op reflow edit

I made up a board using flood and wick. The ICs look quite good but the Rs and Cs are all a little off-kilter because of my crummy steadiness of hand. I used a toaster oven to reflow it, hoping the surface tension would pull the Rs and Cs into line and make everything a little prettier. That worked pretty well.

Paste and draw edit

I have seen a very small number of references to this technique, most saying it was better than "flood and suck".

Here is the only description of the technique I can find: the "paste and draw" method, load it up with flux paste and let the pins draw the solder off the iron, that leaves the perfect amount every time.

I have not tried this myself. If I understand this, it works like this. You position the IC on the board, probably soldering down a couple of corner pins to keep it in place. You apply flux (a thick sticky fluid) to the pins; it's nasty to touch so you might want to apply it with a syringe or a teeny paintbrush. You get a molten blob of solder on the end of your soldering iron and draw it past the pins, and then the flux will cause the "perfect amount" of solder to flow onto the pin. You need to be careful to avoid excess flux. I would want to try this under a microscope a few times to make sure I had the procedure right.

Drag Soldering edit

Apply liquid flux, tack opposite pins, bridge two pins opposite the tacked ones, apply more flux, drag iron smoothly from bridged pins to tacked pins. With enough practice, all of the pins will solder correctly on the first pass. If necessary, apply more flux and touch up any bridges and unsoldered pins. Clean and you're done. This is often how rework is done in professional board houses. It's easy to learn, can be done with a cheap iron and there's less chance of burning boards or lifting pads than with solder wick.

A good how to video:

Hot-air soldering pencil edit

I put together the hot air pencil: a desoldering iron $10 with steel wool ($3, Home Depot) wedged into the nozzle, and replace the squeeze bulb with a tube ($2, Walmart) leading to an aquarium pump ($6, Walmart) so that hot air comes out of the nozzle. The trick is to jam as much steel wool into the nozzle as you can, otherwise the breeze will be so strong that it blows the SMT parts around, and it won't get hot enough to melt the paste. The steel wool provides more surface area to transfer more heat to the flowing air.

If you want a hot air pencil that isn't a cost-saving kludge, the Weller WSTA6 is available for about $60. It uses butane. I looked at one and it wasn't obvious where you get butane refills, or how you apply them. Boston-area people can get one at You-Do-It Electronics.

The hot air pencil seems to take considerable practice to use. Also the solder paste seems to be a bit cantankerous; I've read of people using brushes with it. I'll pick up one or two small paintbrushes and see if that makes it any easier.

Videos on the web edit

There are some good videos on Google Video and YouTube showing soldering techniques for teeny parts.

The idea of posting videos for these techniques is brilliant. A picture is worth a thousand words, but a video is worth a whole library. We need LOTS more videos like these. If I get organized enough, I will make some videos of the toaster oven technique, or the flood-and-suck or flood-and-wick technique that appears to be quite successful for a lot of people.

A practice board edit

 
Cheesy little SMT assembly sampler

If you want to practice your SMT assembly, you can use this 4.9-square-inch two-layer board which can be fabbed inexpensively at any of these places.

The board was designed with Cadsoft Eagle and to work with small two-layer boards like this, you can download their free version.

This board is primarily intended as a cheap-as-possible sampler for people honing their homebrew SMT assembly skills, so you can get some practice without killing your wallet when you make a mistake. Where there are layouts for two EMP7064 CPLDs, you can substitute pin-compatible EPM3064s which are three bucks each at Digikey.

There is a TSSOP-28 layout for an A/D converter that I want to use in a SDR receiver design. The A/D receiver board will be a four-layer layout for noise reasons, but this gives me a chance to verify that the design isn't worthless.

I think of this board as one of those samplers people make when they're learning to sew. I would have like to put a picture of a sampler here, but I can't do that without uploading the image and that means hassling with the licensing, so I just linked to an image of a typical sampler.

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