Preparation Before Soldering

1 Eyecheck the appearance of D-sub connector.

Visually inspect the D-sub connector for any visible defects or impurity, such as cracks, breaks, bends or foreign objects in the pins. If any, clean the dust by using a can of compressed air or replace with a sound connector.

2 Strip the wires that will be connected in advance.

Try to trim all the same amount and just enough so that about 0.25-0.3" (6-7mm) gap will exist between end of insulation and end of solder cup. In this step, you can use ruler to mark the stripping length to make sure they are in same size, and then use wire stripper to trim them.

If you have a requirement for consistency of wire length or need to process wires in large quantity, here RS recommend the following machine for better wires stripping.

     
For processing wires: AWG 32-11 (0.03-4.00mm²)
Stripping length: 20mm (max)
Compact design and easy to operate, suitable for applications where space is limited or for simple requirements.

3 Make sure the working area is clean enough for starting your task.
      This can improve your processing effect and guarantee a more reliable electrical connection of the D-sub connector.   

Soldering Process

Everything is ready. Start your soldering journey now!

1 Tinning each pin of the connector

      Do not apply too much solder on it. You're not trying to fill it up, just have some solder coat the inside surface. This operation takes about 2 seconds of heat per pin. The connector should always be placed at an angle and held in a vise, perhaps 45 degrees from the vertical for easier tinning.

2 Pre-tinning the wires

      Apply a small amount of solder to the wires or tin the wires by using solder pot. There may be so many strands to interfere with saturation of solder onto the wires. So you need to give it a little twist (don't twist with force) and that will help hold the strands together as you are applying the solder. After tinning, it will get much greater contact area for the heat transfer so that the heat transfer into the component a lot quicker and more efficiently.

      For the complicated process, RS Wire Stripping, Twisting & Tinning Machine is a best choice for novice less training time.（for wires of 0.8-3.0mm²）

Wire stripping+wire twisting+flux apply+wire tinning, 4 steps in one stop

3 Put on the heat shrink tube if you need

Remember to put the heat shrink tube on before you start to soldering.

You can do whatever fits the application. Using heat shrink tubing on those connectors for demanding applications, since there is a chance that a piece of stray metal can fall between the connectors and short them out while you test the device or when the customer uses the device.

For less demanding applications it’s fine not to apply heat shrink, especially if you might need to probe the connections with a meter or oscilloscope.

4 Waiting soldering iron to heat up and to soldering  

      Heat up the Soldering Iron and use it to touch the pre-tinned wire and pin for about 2 seconds to ensure solder flows evenly between the two surfaces being joined. Once you see a bead of molten solder surrounding the wire, remove iron and let it cool. This should provide you with a clean and strong joint.

There should be no solder leaking out or wetting on the outside of the cup, or touching the iron tip etc. Insufficient solder is not allowed as well.

A good solder joint has the following three elements:

      A: the wire should be in the bottom of the solder cup

      B: there is a insulation gap of the proper dimension

      C: the solder contains no pockets of flux or gas trapped within it

Here are references to judge if your pins are soldered properly:

correct amount of solder/the fillet is lightly concave/there is good wetting of the surface

minimum solder required

the fillets are no longer concave and begin to bulge

not enough solder was used/no fillets to be seen

excess solder at the joint and on the terminal body

5 Repeat the above process for each remaining pin
      Taking care not to rush or apply too much heat. The end result should be clean, strong joints that are electrically sound.

6 Use heat gun to shrink the tube

      OR you can use heat shrinking machine for processing of large batch workpieces.

      RS recommend the following machine due to:

-360° heating/baking to ensure more even and stable shrinking of the tube
Optimal electrically connection and protection for your harnesses!

-Quicker and easier shrinking with high power motor  
-Compact dimension to fit for various scenes
A reliable assistant for your soldering work

Conclusion

If you are a soldering novice, or tired of using hand soldering, you can take RS semi-automatic soldering machine RSM-SSM-200A into account. The machine has 3 wire feeding modes: pre-feed, post-feed and twice feed mode, which can be used at various scenes.

The twice feed mode is especially suitable for the soldering of workpiece that needs pre-tinning and then connect (that means two times of soldering).

The following are the simple comparison between hand soldering and semi-auto soldering machine.

*Literature

-(Basic Soldering Lesson 3 - "Cup Terminals") BY Paceworldwide from YouTube

-(How To Wire A DB25 Connector for Audio) BY Biased Audio from YouTube

-(How to Solder a db9 Connector Correctly) from retrotechlab.com