The intention of this tutorial is to provide a low-cost, quick, reliable, and hassle-free method for soldering surface mount parts using the DUE protoshield as an example. This is certainly not the only way to skin this cat, this is what has worked for us and is based upon similar hot-air soldering techniques that have been documented and put into practice.
Things you You will need:
Hot-Air Soldering Iron
Hot-air soldering “re-work” stations can be picked up for ~$100 various places online (try ebay). We recommend a unit with multiple diameter tips for working with different size parts. When using it for soldering components like we plan to do in this tutorial, it can help direct or disperse the heat and increase/decrease the speed of the airflow (when working with really small parts like 0603’s, high speed air can blow a component off the PCB never to be found!). These iron’s are typically Chinese made, and you get what you pay for, but for the typical hobbiest they work just fine. See Zephytronics if you want a high end model.
We recommend zephpaste. A 10cc syringe will last a prototyper/hobbyist for a while (remember to buy a plunger and tips for the syringe!). You will find the Zephytronics website very informative on SMT soldering techniques and equipment. In this tutorial we are using a 22 gauge blue plastic tip (perhaps could have used a size bigger). Your paste gets shipped with a cold-pak and we stick it in the fridge and leave it there between uses (although not required).
SMT Vacuum Placement Wand
You can find manual pump vacuum wands online for ~$5 (again they are typically Chinese made and you get what you pay for). This is an inexpensive option that will work fine for a hobbyist. Note that if you don’t get a perfect seal on the component you have to work quickly, as your part will get dropped. Another option is to go with an electric wand that has pump to create a constant vacuum ~$40 (try ebay for wands or pumps). With either option you will get an assortment of suction cups to accommodate various component sizes, and the bare tip can be used to pick up very small components like chip caps or resistors. See Zephytronics if you want a higher end model.
Magnifying Glass (optional)
This picture shows a 10x magnifying loupe (fancy term for magnifying glass). This is similar to a jewelers magnifying glass. Anything with decent magnification strength will do for looking at the solder joints close-up.
Nothing fancy needed, just something to do a resistance check with.
Lets Get Started:
Step #1: Practice
Practice placing your IC on the PCB with the placement wand. Note the location on the pads where the IC legs land (left, right, middle), this is where you’ll want to try and put your line of paste in the next step.
Step #2: Applying Paste
Squeeze a bit of solder paste out on to a paper towel to fill the tip. Now apply a line of paste across the pads as evenly as possible from start to finish. It won’t look like much paste at all, but don’t worry…. you will not need it. Wipe it up and start over if you think you’ve put down too much paste or if one particular pad has a big blob on it. Excess paste can form solder bridges (legs of the IC’s being soldered together), and can hide cold (poor) solder joints. Too much paste also makes the visual inspection of good solder joint fillets difficult (fillets? what the heck is that?….we’ll get there, just keep reading).
Step #3: Putting Down the Part
Use the placement wand to put the IC down in the paste, get the alignment where you want it. If you smear paste around a bit, don’t worry. Smeared paste may seem like a disaster but everything will soon fall into place once the heat gets going. You can use the bare tip of the wand to nudge the IC into place if you happen to get it miss-aligned or off the pads. Keeping the part aligned on the pads is important.
- Select the tip diameter you want to use for your hot-air iron. If you don’t have any other SMT parts installed around the area you are soldering you can opt for a larger diameter tip. This will give you a wide “patch of heat” that can be applied on the PCB.
- Switch on the station and set the temperature. The paste we are using has a liquidous point of 183C so we set our stations at about 200C. (Warning using very high temps can burn your board and start to melt the solder mask, so don’t crank it up thinking you will speed things up…it’s not worth it)
- If you have an airflow adjustment on your iron try starting out at %25 of the full setting
- Start with your iron blowing down on the IC about 2-5 inches above the leads of the part, pointing the stream of air directly at the leads. The smaller the IC, and the smaller the tip, the further away from the PCB you may want to be. You do not want to the hot-air stream to blow the part off of the pads, the goal is to slowly pre-heat the solder, PCB pads, and IC legs. Move the wand up and down both sides of the part slowly applying heat across the legs and pads.
Step #5: Something Just Melted
Hopefully, that’s your flux. If your solder paste has just turned into what looks like a grey toothpaste puddle and you are beginning to doubt that these leads are not going to be soldered together….that’s actually a good thing, just hang with us. This is the solder flux inside the paste melting, cleaning the way for your solder to make a nice lead to pad bond. Flux application is a vital part of the soldering process, it removes the oxides from the surfaces and good solder joints rely on very clean surfaces. Good things are on the way, now lower your iron by about 1-2 inches and continue to apply heat across the IC leads and pads
Step #6: Reflow Magic
If it hasn’t happened already, your solder will soon melt and magically leap off of the PCB solder mask wicking its way up to the solder pad and IC leads. Continue to move your iron across the leads until each one reflows forms a nice solder joint on the pad. You may find that you are able to move the hot air iron closer to the board once a few joints have cooled a bit and holding the IC in place. Once all pads are soldered, remove the heat and let it cool.
Step #7: Visual Inspection
Using your eyeballs, preferable aided by a magnifying glass….look for any solder bridges or any dull looking patches of solder. What we hope to see are very shiny concave shaped solder joints, these are called “good solder fillets” (‘fill-et-sss’). The fillet is the portion of solder that “fills the gap” between the edges of the “foot” of the IC lead and the pcb pad. Typically a joint is inspected for a fillet at the “toe” (away from IC) and the “heel” (near IC) of the foot. A good fillet should be shiny and concave in shape, this indicates “good wetting” aka a metallurgical bond has been formed between the IC lead, solder, and the pad. If a fillet is in a convex shape (bulging out) could hide potential problems such as a cold solder joint (no bond = no electrical connection, or worse an intermittent electrical connection). If all of your IC’s feet have shiny, concave fillets, you can have a high-level of confidence that you have created a reliable joint. Below you will find some close-up pictures of good fillets covering heels and toes.
In our DUE protoshield photo above (with the aid of a magnifying glass 10x or better) we could see concave fillets on the “toes” and the “heels” of the SOIC feet…however they did seem a bit small. Perhaps this is due to the larger size pads that are on the protoboard vs the width of IC we used? This may have caused the paste to get “wicked out” covering the entire pad (so perhaps we could have used more paste). We also found a few tiny solder balls stuck under the IC that were pulled out with a dental tool (perhaps we need to do better cleaning and preheating beforehand). In this tutorial the assumption is that we are doing prototype/hobby work. As such we are pretty much ignoring the details of properly sizing pads and computing proper paste volumes etc. We provide some professional references for this type of stuff at the end of the tutorial.
Using your DMM you can verify your work by doing an electrical resistance test on your solder joints. First place one DMM lead directly on the PCB pad (NOT directly on the solder joint), and one lead on the IC lead itself, closer to the IC package away from the joint. You should get a low resistance reading (dead short). Tip: if you get an open circuit try doing this test by first lightly scratching the pad and the IC lead with the DMM lead to first to be sure you have a clean surface between your probe and surface. Also do not apply heavy pressure on the IC lead as you may force a bad solder joint to electrically look like a good one. You can do the reverse to check for checking of solder bridges from pad to pad as well. Remember to check every pin! it will save you time in the long run.
Thank you for reading this tutorial, we wish you success.