Tips on Using Silver Solder

Since this topic has come up in recent discussion, here are some helpful tips on the use of silver solder .

Solder Types:

There are two types of solder on the market today, lead-based and silver solder (also called lead-free).  Silver solder has become the mainstream in most manufacturing because it does not contain lead and is non-toxic.  You can look for this with the RoHS logo (Restriction of Hazardous Substances).  This choice of solder really matters as the technical specs for proper melting and flow are different for the two types, silver and lead-based, and also the formulation.  According to Kester, one of the largest solder manufacturers, “Solder iron tip temperatures are most commonly between 315-371°C (600-700°F) for Sn63Pb37 and Sn62Pb36Ag02 alloys and 371-427°C (700-800°F) for lead-free alloys.”  As you can see, silver solder, requires nearly 100 degrees more heat to flow.   Though, silver solder is more electrically conductive and the resulting joint will have less resistance.

Flux:

Flux is a corrosive cleaner, usually embedded in solder, to help clean the metal surfaces to be joined.  There are different types of flux, such as synthetic chemical or organic, and water-soluble or no-clean.  In general, for hobby use, “no-clean” is best as it does not require cleaning the soldered area.  Solder wire is usually embedded with a percentage of flux.  Also, flux can be purchased stand-alone for specific uses.

So which solder should I use?

Considering the above, my personal favorite silver solder with a no-clean flux is the Kester brand “48 Activated Rosin Flux-cored wire”.  In  1.5mm/0.62 thickness, the part number is 24-7068-1400.  This solder is good for most metals and has an “instant-action” wetting characteristic. Also, this particular solder reduces spattering and has a clear residue, compared to the amber colored residue often seen from solder flux spatter.  Though, the one drawback of this solder compared to others is the relatively high temperature requirement to melt and flow. 

http://www.kester.com/products/product/48-Activated-Rosin-Flux-Cored-Wire

Do I need separate flux? 

In general, yes.  Most electronics will have a coating of oil or chemical residue after manufacturing.  When attempting to solder to a commercial PCB with a previously exposed pad, it is often a good idea to clean this residue before attempting to solder to a previously unsoldered area.  Flux paste should be applied to the previously unsoldered area and then melted with the soldering iron right before attempting to solder the pad.

One of the most common flux pastes on the market is the MG Chemical 8341 No Clean Flux Paste:

http://www.mgchemicals.com/products/solder/fluxes/no-clean-flux-paste-8341/

The MG 8341 flux comes in a convenient syringe for application and does not require clean up.

Tip Cleaning:

Historically, cleaning the tip of a soldering iron was done with a wet sponge for immediate cleaning and then an ammonia based compound for deep cleaning or re-tinning.  Today, technology has changed such that wet sponges are no longer used as it is now understood that every time an iron touches a wet sponge, heat is lost.  Instead, modern soldering technique uses wire type cleaning devices, such as this one from Hakko:

http://www.amazon.com/Hakko-599B-02-Solder-Cleaning-Holder/dp/B00FZPGDLA/ref=sr_1_1?ie=UTF8&qid=1445097878&sr=8-1&keywords=wire+solder+cleaner

The wire type cleaners do a much better job of smoothing the melted solder around the tip while also removing oxidization and slag. If your soldering iron did not come with a wire type cleaner, it is highly recommend to pick one up.  They can be purchased for less than $10.

Soldering Irons:

What makes a good soldering iron?:

The critical factors in choosing a soldering iron is that it should heat up quickly and maintain a constant heat while in use.  Back in the days of lead-based solder with lower melting temperatures, low wattage irons were cheap and easy to pick up at the local electronics store.  Though, with the switch to higher temperature silver solder, higher wattage and higher performance soldering irons are now required.  In short, if you want to use silver solder, that Radio Shack pencil iron you have had since the 80s might not be sufficient to do a good job.  The goal when soldering is to apply heat and flow the solder as quickly as possible by ideally apply  the “right” amount of heat to melt solder on a joint in a few seconds. The worst and most grievous mistake in soldering is to use too low of a heat which would require holding the iron in place for a long time before the solder melts as this type of joint will seldom bond well and the solder will often wick up into the wires.  Just remember, the entire solder joint should be completed in no more than a few seconds or you need to turn the heat up and/or change the soldering iron tip.

For RC hobby use, a few guidelines are:

  • Iron temperatures should be adjustable
  • Minimum wattage should be 60watts or greater
  • Recommended wattage would be 25-80watts adjustable
  • Large gauge wire soldering may need up to 100 watts

Electric or Butane?

Electric soldering irons have historically dominated the market due to their convenience.  In fact, you cannot go wrong with an electrics with adjustability and temperature control.  Though, butane irons have really come into the market with much more advanced models as of late.  Some of the new butane irons can run for hours on a single charge, have exceptional heat holding ability, and are also relatively less expensive when compared to a comparable electric.  Best of all, butane irons are portable and can be used at the field.  For the occasional soldering needs of the RC hobbyist, a butane soldering iron can be a great way to go.

 

What are some popular irons brands?

In electrics, two of the top selling soldering iron brands are Weller and Hakko.  Though, there are generics and other brands that make good irons as well. 

In the butane type, some of the top selling irons are made by Portasol, Weller, and Master Appliance.  Here again, there are some generics and other brands available that can be perfectly good.  Just check for adjustable range, good tips, run time, and parts availability.

 

Example: Soldering Connectors to a ESC power wires

  1. Tin the tip of your hot soldering iron by touching solder wire to it.  Make sure to coat the entire tip, top and bottom.
  2. Wipe the tip in a tip cleaner.  In the old days, a wet sponge was used.  Though, modern technique no longer recommends this.  Instead, I really recommend using a wire type soldering iron tip cleaner.  Please see the section above for details.
  3. Place a connector in a helping hands holder, or some other type of jig to hold the connector. Be sure the helping hands are not too close to the point of where you intend to solder or the helping hands can draw off heat and make it more difficult.
  4. Touch a pre-heated tip of a soldering iron to the inside of power connector cup
  5. Touch solder wire to a point between the soldering iron and the cup to leave a very small dab of solder.  The point of this step is that the slight amount of melted solder between the iron and the cup will aid in thermal transfer
  6. Hold the heated iron on the side of the cup for a total of about 3 seconds and then touch the solder wire to the opposite side of the solder cup, being careful that the solder does not touch the iron. 
    1. If the solder melts and flows instantly, then the heat level is perfect
    2. If the solder spatters and begins to drip, the heat level is too high
    3. If the solder does not melt quickly and flow, the heat level is too low.  The heat level can be too low due to iron temperature or also due to something drawing heat away from the connector cup, such as a helping hand being located too closely the point of solder.
  7. Assuming all goes well in the step above, flow in solder to fill about 1 quarter of the volume of the solder cup.  The idea is to “tin” the solder cup and still leave room for the wire without overflowing solder out of the cup.
  8. Wipe the tip of the soldering iron in a tip cleaner to remove any slag build up and re-tin with solder if necessary.
  9. Heat the tip of the power wire to be soldered by touching a soldering iron to it for about 3 seconds and then touch solder wire to a point on the side of the wire, away from the solder tip.
  10. Allow molten solder to flow through the power wire, near the tip of the wire,  on all sides, but this task should be done quickly to avoid solder from wicking up into the wire under the insulation.  Soldering wicking too far up the wire under the insulation can be a terrible thing as this will lead to a brittle connection point that will be prone to breaking. 
  11. Remove the iron from the wire and set down.
  12. Use a helping hands or other jig to position the now tinned wire so that it is touching the opening of the now tinned solder cup.
  13. Clean and re-tin the tip of tip of the soldering iron again, but this time be sure tip is liberally covered in melted solder, but not dripping. 
  14. Touch the tip of the soldering iron to the outside top edge of the connector solder cup and hold for about 3 seconds until the solder in the cup begins to melt. 
  15. Guide the wire into the solder cup of molten solder and wait about 2 seconds, or until you see the solder tinned on the wire also turn shiny and molten.
  16. Remove the soldering iron heat and allow the connector and wire to cool about 30 seconds before touching.  You should see the bright and shiny solder turn a slightly darker color indicating it has set.

How to Solder:

There are many other resources out there that do a fantastic job of explaining how to solder.  Thus, I will redirect to a few of the best:

 

https://learn.sparkfun.com/tutorials/how-to-solder---through-hole-soldering

http://www.howtogeek.com/63630/how-to-use-a-soldering-iron-a-beginners-guide/