Saturday, December 31, 2011

General Hand Soldering Tips

               By Norman Varney

This blog is for the few DIY'ers out there. It seems that hand soldering is becoming a bit of a lost art as technology progresses. Hopefully you'll learn one or two tips and share them with others. These are soldering tips I grabbed from training I did for MIT line workers who were building cable networks, terminating interconnects and speaker cables, etc. It does not cover all soldering techniques for through hole mount, surface mount technology, turret and pierced tab terminals, desoldering or repairs.

Soldering Environment
  • Clean, oil free hands, dust and clutter free work area
  • Good lighting without casting shadows on work
  • Stand-held magnifying glass for small work and inspection
  • "Helping hands" to free both hands and hold work at comfortable position
  • Chair and table heights conducive for allowing arms and/or hands to rest on table for remaining steady while soldering and to keep the soldering iron cord away from the work

Trimming the Wire
Use a mechanical stripper with appropriate gauge die holes. This type of stripper will not cause nicks or scrapes to the conductor. Blades should be replaced as necessary. Be sure that individual conductors are uniform in appearance of twist and grouping with no stray or frayed wires, no "birdcaging", nicks, etc.

Cleaning All Components
  • All leads should be cleaned using an alcohol based, carcinogen free cleaner prior to being soldered.
  • Soldering iron tip should be bright silver and free of flux residue and solder. Any build up of oxide is removed by wiping the tip on a damp sponge ("shocking") prior to applying it to the soldering area. 

RA (Active Rosin) flux type. Flux prevents oxidation, removes the thin layer of oxide and atmospheric gas layer. It permits solder to flow or wet smoothly and evenly. It also improves the flow of heat resulting in faster heating times.

The soldering iron tip mass should be approximately the same size as the conductor being soldered. Heat should be high enough that the work is completed from start to finish between 1-5 seconds. Over heated joints look lumpy, dull, crystalline and/or grainy. Heat sinks (twezzer type) can be used below the soldering location to prevent overheating damage to electronic components.

Solder Iron Wire Tinning
Hold the wire in a downward position with the solder placed underneath the wire and at the center point of the stripped portion of the wire. The solder iron is then applied at the same point and when the solder melts, contact is made with the wire by the iron tip. Both the solder and the iron are moved upwards towards the insulation, and then down and off the end in a continuous motion. The solder should stop one conductor width away from the insulation. The wire is then cleaned of excess oxide and flux. There should be no solder underneath the insulation, as this would degrade the conductivity of the metal.

Solder Pot Wire Tinning
Time in the bath and/or temperature of the bath will need to be adjusted for different gauge wires, even if it is only 18g. to 20g. In every case liquid flux is applied to the stripped wire (allowing the molten solder to strip the insulation contaminates the lead as a conductor). The fluxed wire is inserted into the molten solder to the point where the stripped insulation is about one conductor width away. It is then moved over to one side and removed. The wire should be cleaned to remove any flux so that it doesn't become a problem during termination.

Soldering Procedure
  1. Clean lead wire and pad to be soldered.
  2. Apply liquid flux to lead wire and pad if necessary.
  3. Clean iron tip by wiping off excess residue on a lint free cloth.
  4. "Shock" the iron tip by touching it to a damp sponge.
  5. Place proper amount of solder in contact with the lead wire and the pad to be soldered.
  6. Place iron to the solder without exerting pressure.
  7. Repeat or move along to the other side covering all exposed copper.
  8. The work must remain motionless during the "solidifying" state of cooling.
  9. Clean off any excess flux or burned carbon.
  10. Visually inspect.
  11. Check electronic continuity.
  12. Leave a thin coating of solder on the iron tip when not in use.

Appearance of Good Connections
  1. Smooth 
  2. Bright
  3. Shiny
  4. Clean
  5. Concave solder filet with 0-20 degrees of slope
  6. Good wetting
  7. All of the wire is covered
  8. Contours are visible

Unreliable Solder Joints
  1. Overheating- de-wetting: lumps, dull, crystalline like, looks like sand has been thrown into it. 
  2. Cold- poor wetting: balled up, dull gray.
  3. Fractured- poor wetting: solder has stretch marks between the pad and the lead.
  4. Non-wetting: solder is balled up around the joint.
  5. Excessive solder: lead conductor is not visible and the shape of the solder is convex.
  6. Insufficient solder: hole is not covered, copper is not sealed.
  7. De-wetting- usually excessive heating: solder balls up.
  8. Pinholes or voids- causes can be dust, dirt, flux gas, improper heat, etc.
  9. Lumps and large holes- improper pre-solder cleaning, out-gassing from flux gas.
  10. Damaged wire insulation- excess heat and/or wicking of the solder under the wire insulation.

Gold Pin/Cup Soldering
Tiny pores occur at the connection point and the gold keeps leaching into the tin portion of the solder (within days versus years) resulting in an unreliable, brittle solder joint. The gold contamination must be removed by pre-tinning the gold cup.

Trimming the Wire
  1. Establish the depth of the cup by placing the wire inside and mark it at the top of the cup.
  2. Remove and trim the insulation with a clearance of at least two wire diameters above the mark.
  3. Insert flux cored solder wire (slightly smaller in diameter then the cup) into the cup and cut it flush with the top of the cup.

Tinning the Pin/Cup
  1. Apply clean solder  tip to the back of the cup and below the top edge. Wiat until the flux has bubbled to the surface.
  2. Apply some liquid flux to a solder wick and insert it into the molten solder.
  3. Remove all solder from the cup.
  4. Clean all traces of flux with cleaner.
  5. Insert solder into cup and cut it flush.

Installing the Wire
  1. Apply clean solder tip to the back of the cup and below the top edge. When solder melts, allow all of the flux and flux gas to surface. 
  2. Insert the pre-tinned wire slowly to allow the tinning on the wire and the solder in the cup to mix together. Make sure that the wire goes down to the bottom of the cup. 
  3. Move the wire to the back of the cup, then forward, then back and hold.
  4. Remove iron, while holding still, allow to cool.
  5. Inspection: no solder on the back of the cup, nice solder filet is formed.

                                      The author tinning the wire ends of a speaker cross-over network