I have tried loading the board with components on solder paste. The solder paste that I use is intended for stencil application - which I have found, if thined with a solvent, works very well when applied with a blunt round tooth pick. Parts are easy to place and stay in place. The PCB.s that I tried was very dense and because it is HB, I did not have a solder mask to help prevent parts from moving a little when re-flow in a toaster oven. On one occasion, two adjacent parts floated to join at one end, at it turns out the two ends had a small trace between the pads, but it was enough for the solder to flow to each other. A production solder mask would have prevented this movement.
Also, I tied using a small iron with solder pasted parts, because of the density, I have several SMD 1206 parts with a trace between pads. Pressing the parts into the paste squished the paste under the part and that provides a potential short to the trace under the parts. Most of those parts had to be re-fitted. Lesson learned, be extra careful with parts placed over traces - avoid it if you can.
I have tried, and like direct soldered SMD's, which works well for me. I suggest several things;
- Just before installing parts, clean and polish the board as normal, then rub a thin layer of "Flux" over the board, it is a bit messy but it can be cleaned up later. Solder acceptance if much faster and stays cleaner while installing parts. Clean solder does NOT drag when releasing the iron from the pad - resulting in fewer bad joints or shorts.
- I suggest, for a new parts,
- Tin ONLY the first pad.
- Place the part on the pads, one end will be setting on the solder mound from step 1.
- Hold the part down gently with a tooth pick.
- Heat the first pad with a small soldering iron.
- Allow the part to drop onto the pad as the solder melts.
- Do not try to add more solder - that will be done later and only if necessary.
- Do NOT solder the other pad, for now.
- Repeat for a group of SMD parts.
- I like the idea of have a second chance to look at the newly installed parts, one last time before committing to a double ended soldered part.
- Note: Once a part is soldered on both ends, and if it needs to be moved/removed/replaced - it is the same as a board repair, which is never as clean or easy as a new part installation. Removing a part with only one pin soldered is very easy.
- I found that small diameter 62/36/2 Rosin-Core Solder works much better than most other solders that I have used (available from Radio Shack on small spools) - the 2% silver makes for very smooth and shiny joints. Note: I have NOT found another source for this type of solder, a friend, Wayne McFee NB6M put me onto this - thanks Wayne.
- If necessary check adjacent traces for solder and solder paste shorts under the parts, they are much easier to find with fewer installed parts.
- After inspection, solder the other pad(s) of the SMD parts, then touch up the first end only if necessary - the previously rubbed on Flux make the joint touch up just that much easier.
- After soldering is complete, clean the board and remove the Flux as normal - I don't suggest Acetone - I like to use an Isopropyl Alcohol (99% , from the drug store) bath soak and blow dry, and the maybe followed by a "Contact Cleaner" as you get some pressure-washer spray action, and it is easy to use. Rosin is an organic flux and Alcohol is an organic solvent.
- I use the "Carburetor Cleaner" (from the auto store) to pre clean the PCB before Toner Transfer and for Toner removal after etch - Carburetor Cleaner is Great Stuff.
So far, I have successfully produced several HB PCB's, some with very small 8 mil traces, and I am very very pleased with the results.