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What is the Correct Way to Fill Solder Cups

Question: I noticed a lot of my techs, while soldering cups, are not putting the soldering iron on the back of the cup, filling with solder, then adding the tinned wire. They will tin the wire, set the wire in, press the iron on the wire/solder cup and add solder. I’m not too familiar with soldering cups, but I am thinking that this will cause solder to wick up into the jacket. Is this a fair assumption?

Answer: There are a variety of ways or methods to correctly fill solder cups and it depends upon your application, the wire size or gauge and the application of the cups.

According to J-STD-001, 4.5.1 Gold Removal

  • From at least 96& of the surfaces to be soldered of the through hole component leads with 2.54 um [100uin.} or more of gold thickness.
  • From 95% of all surfaces to be soldered of surface mount components regardless of gold thickness.
  • From the surfaces to be soldered of solder terminals plated with 2.54um [100uin] or more of gold thickness
  • A double tinning process or dynamic solder wave may be used for gold removal prior to mounting the component on the assembly.

The cup should always be placed at an angle, perhaps 45 degrees from the vertical and held in a vise. If the connector has cups, then the connectors should be placed in a vice and held at a 45 degree angle.

The cored solder can be cut and placed into the cup filling the cup with the wire, then the solder iron or heating tool can be applied melting all the solder in the cup. Keep in mind that if this is the first filling of the cup, the solder has to be removed and this process has to be repeated a second time to remove the gold from the cup.

Verify the cut length, or strip length, of the wire is fine for the depth of the cup without violating the insulation spacing requirements. This can be done by placing the tinned wire in the cup and checking its length.

Now the time has come to solder the wire into the cup. If the wire gauge is large enough to support itself in the cup, place the wire in the cup, grasp the solder iron and the cored wire solder. Applying the solder iron to the back of the cup start feeding solder into the cup filling the cup as required by the contract. If the wire is a very small gauge and will not support itself in the cup, fill the cup or cups with solder, then grasp the wire to be installed into the cup, apply the solder iron to the edge of the cup allowing the solder in the cup to become molten and once molten, insert the wire into the solder cup, removed the solder iron and hold the wire until the solder solidifies and you should be all set.

Either of these methods will not impact the amount of solder which will wick up beneath the insulation.