Is it better to use solder cups with ferrules or pin terminals?

Question

Soldering stranded wire directly onto a PCB pad or hole is bad practice for shock/vibration applications.

Why is soldering directly onto solder cups acceptable, and how is it even reliable for space/mil/aero?

Combining solder cups with either ferrules or pin terminals seems better. What's the nuanced truth here?

For example, with a space grade circular connector with solder cup interface:

1. Strip and crimp each wire onto a pin terminal.
2. Solder each pin terminal into its correct solder cup.
3. Apply heat shrink to each cup/pin/wire assembly

Benefits seem to be:

1. Stronger solder joint (uniformity+)
2. Better strain relief (360deg)

Answer 1

Combining solder cups with either ferrules or pin terminals seems better. What's the nuanced truth here?

I believe that what you're suggesting is:

1. Strip a stranded wire
2. Crimp the wire to a ferrule (or a wire pin)
3. Solder a solder cup to a PCB
4. Place the ferrule/ pin in the solder cup
5. Solder it

What's wrong with it? The following:

• That is 5 steps (compared to two steps when soldering a wire directly to a PCB, two steps when using an IDT board-in terminal, or three steps when using a non-IDT board-in terminal)
• It requires three QA inspections (compared to one inspection when soldering a wire directly using a board-in terminal): 1) crimp; 2) PCB solder; 3) ferrule solder.
• The reliability of soldering a ferrule in a solder cup has not been studied and is unknown.
• The reliability of soldering a wire pin in a solder cup is easily estimated to be poor: the weight of the wire will bend the pin and break it at the mid point.
• Solder will penetrate the ferrule/pin and wick up the wire strands, which weakens the wire and, in the presence of vibrations, may break strands.

Therefore, no, combining solder cups with either ferrules or wire pin terminals is not better, it is worse than using a board-in terminal.

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Board-in terminals include (my site):

Board-in wire terminals: secured to the wire first, then to the PCB

• Board-in wire terminals thru PCB
• Board-in wire terminals to PCB
• Removable board-in wire terminals

Board-in PCB terminals: secured to the PCB first, then to the wire

• Board-in IDT terminals
• Board-in poke-in terminals
• PCB solder cups, slot terminals

Note that ferrules and wire pins are not in that list: they are not board-in terminals.

Answer 2

the solder cup premise is non-negotiable

With that additional information (thank you for adding it), then the only reliable solution is to use the the solder cup as it was intended to be used:

1. Strip the wire
2. Place it in the solder cup
3. Solder it
4. Do a visual QA test

Any other use of the solder cup will be less reliable.

Certainly, adding heat shrink tubing after the QA test would strengthen the connection.