SMT Soldering 101

For SMT Soldering, this is one of the best SMT Tutorial videos that I have found.

http://store.curiousinventor.com/guides/Surface_Mount_Soldering/101

Near the end of the video, there is an interesting suggestion, use "ChipQuik" for easy component removal - I will have to try it!

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Tin Plating with Solder

Tin Plating with Solder on Home Brew PCB Projects.

On a previous post, a viewer ask via a comment; how did I do a "Solder Wipe"? as shown on one of the example HB PCB Projects.

At the time, I used a Soldering Iron with a little bit of solder and moving back-and-forth over the traces coating each with solder and then while the board is still hot, "Wipe" the solder with a cloth. This produced satisfactory results - but, you have to be quick!

Because I like the looks of bare copper for my projects, "Solder Wipe" is not used very often.

Now, many of my projects are showing their age as the copper tarnishes. I need to start using something more protective on all of my projects.

While removing parts and cleaning-up several old junk board (see below), that are going to be re-used for a SMD solder technique demonstration, I noticed that Solder Wick produced a nice clean dry finish, if it was rubbed over the traces with the heat of the soldering iron.

It looked great! I tried it on the rest of the board - with fantastic results.
 

Without and With Solder Plating

I recommend using a short section of used "Solder Wick" to tin plate (solder plate) new HB PCB projects before parts are loaded.

Used-Wick stays together and provides the small amount of needed solder, New-Wick tends to fall apart (see top left) before it collects enough solder to stay together. Do this task quickly, as you do not want to heat the board more than necessary - it will get HOT.

CAUTION: 
Be Careful, it is HOT

A little bit of liquid "Rosin Flux" helps the plating process (I use MG 835-100ml).

After plating, and Alcohol cleaning,  a quick buff with Scotch Bright removes the shiny surface and avoids the reflections that are so distractive while loading parts.

Note; more Alcohol cleaning is necessary after using Scotch Bright as the LEAD from the solder mars the PCB between the traces.

CAUTION: Remember,

this is "LEAD" that you are touching!

With previous HB PCB (without plating) I always liked to rub a little liquid "Rosin Flux" over the entire board before loading - it gets sticky (rosin is tree sap) and must be dried first, but it provided excellent soldering conditions for SMD parts - I still may continue this practice, even with Solder Plated PCBs. Note: I think rosin will also help seal the edge of the fibreglass PCB.


Now, . . . . if I could only find a way to do a decent Home Brew PCB Solder Resist Mask to seal the board?

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New Soldering Iron

I have been using the same Soldering Iron for the past 30 years, it is a 60W Weller. I have gone through about 4 tips in that time, and now I am due for a tip replacement,  I need a new "smaller tip" for SMD circuits. I have checked with local suppliers, they just laughed! The tips for my old Weller are getting hard to find, if at all. I will continue to look online, the Iron is still good and I like it.

To keep my projects going, I have decided to look for something new. I have checked the local suppliers and did not find anything that I liked.

About 45 years ago, a "rich" friend of mine, lent me his New State-of-the-Art "Cordless Soldering Iron". It was great, very light, heated fast, and with a work light. But, I thought it was a little small for the soldering tasks of the time.

Well, times have changed!

The other day while ordering some SMD components from Mouser, I ran across the Soldering Iron catalogue page, and there on the page was the "Wahl - Cordless Soldering Iron" (557-7800) just like the one that I had used 45 years ago (at least it looks like the same). I am a little richer now, so I added it to the order. I also ordered the Micro HI-E Tip (557-7566-100).

Soldering Iron and Stand

Fine Tip

Today, the order arrived. After unpacking and installing the Micro Tip and doing some initial test SMD soldering joints - I am very please with my new Soldering Iron. It is much better than I remembered, from long ago. From my perspective - parts sizes have caught up with the iron's ability. The Micro Tip is just right for SMD work - and it's not tied to a wall socket!

It is interesting, after all this time, I am still using 45-Year-Old-Technology, on new tech-SMD projects.

Now, if I can just find a tip for my old Weller,  .  .  .  just to keep in going!


UPDATE 1:

• I took it apart (what else would you have expected - it has screws!).  I noticed after sitting on it stand, it indicated that it was fully charged via a little red LED, also I noticed that the handle was warm (I now know that it must have recently finished charging).
  

• How does it know when it is charged?
  

• Simple, there is a "heat latching breaker" that sits on top of, and in contact with the battery. When the battery get hot the breaker contacts opens, and across the breaker contacts is a limit resistor and LED. I assume the small LED current that by-pass the open breaker contacts provides a trickle charge, probably less than 10mA.
  

• To restart the charging cycle, you are required to reset the breaker via a small slide switch. This seams like a cleaver and reasonable approach to a NiCad charging problem.
  

• Interesting, the breaker is not in the tip heating circuit!?

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UPDATE: I think this is a link for new Tips for my old 60W Weller:

http://www.action-electronics.com/wect5.htm

• Three Tips are now on order.

UPDATE 2: Finally, after miss-shipment and lots of delays - I received my order on Nov 6, 2009.

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SMD - Without Solder Mask

The last few days I have been soldering several of my Home Brew (HB) PCB's and playing with several different ideas for the best method of mounting SMD parts. My projects do not have the funds for proper solder mask.

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, 

1. Tin ONLY the first pad.
2. Place the part on the pads, one end will be setting on the solder mound from step 1.
3. Hold the part down gently with a tooth pick.
4. Heat the first pad with a small soldering iron.
5. Allow the part to drop onto the pad as the solder melts.
6. Do not try to add more solder - that will be done later and only if necessary.
7. Do NOT solder the other pad, for now.
8. 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.

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