Easy to strip hookup wire which doesn’t shrink back during soldering.
Copyright © 2011 by Stephen H. Lafferty. All rights reserved.
Long ago, I never had any trouble with insulation shrink back when soldering hookup wire. Several years ago, I needed to replenish my stock and also decided to splurge on getting a good selection of colors. (So many times, I had wished for purple to signify the –15V line.)
Much to my dismay, I discovered that some of the modern hookup wire had a problem with shrink back. As seen with the orange PVC wire above, you start with the insulation close to the connection, (upper) and after soldering, you end up with it as seen in the lower picture.
Researching the Problem
After searching the Web, posting questions on forums and asking around, I found that not all vinyl hookup wire has a real problem with this. A buddy says that the hookup wire he buys at RadioShack is fine. I tested the various rolls which I have and found some good ones. Looking at technical documents on the Web, I learned that shrink back is dependent on controlling the relative feed rates of insulation and wire in the manufacturing process. If the metal gets ahead of the insulation, so the plastic is stretched, it leaves a stress in the material which is released when stripped and soldered. Hence, the insulation shrinks back.
The problem was that I needed to order some new hookup wire and there was no way to tell if prospective products would have the issue or not. I couldn’t find meaningful claims or specs that would reassure me. Some people on the forums advocated using wire with Teflon (DuPont’s brand name for PTFE) insulation. While expensive, that would certainly cure the shrink back problem. The downside is that PTFE is difficult to strip. I recalled having trouble with that long ago, when my brother gave me some. Looking around the Web, I found descriptions of slow and tedious procedures for properly stripping PTFE. Geez. All I wanted was to have something like the vinyl wire I had always used.
After additional Web research, I finally discovered Irradiated PVC (IPVC) wire insulation. This is regular PVC wire which has been passed under a powerful electron beam. It cross-links the polymers, making the plastic much more resistant to heat. It also makes it tougher and more resistant to plasticizer loss (which ages many plastics). The only concern I had was the fact that it makes the wire stiffer. Would that make it hard to route in tight corners? I’m pleased to report that it turned out to be a non-issue. The IPVC does not seem at all stiff to me and there is certainly no problem bending the #22 and #18 sizes which I tested. As expected, IPVC is at least as easy to strip as regular PVC wire.
Buying IPVC Wire
Of course, the IPVC is harder to find and more expensive than regular PVC wire but it isn’t usually as expensive as PTFE. The key words to look for are:
- UL1429 (for 150VDC)
- UL1430 (300VDC)
- UL1431 (600VDC)
The 300VDC rating is typical for ordinary hookup wire. However, I was planning a tube analyzer (VTA) project which would need the 600V rating. Also, while I normally use solid hookup wire, the VTA has over 60-wires going to the main PCB. Concern about being able to flip the board over led to stranded wire for flexibility. So the numbers listed may be limited to stranded wire. I was able to find UL1429 and UL1431 at McMaster-Carr. They have 1429 in many colors but 1431 is only carried in black and white:
I ordered UL1431 from them. #22 presently costs $14.06/100ft or
$52.65/500ft. #18 goes for $23.29/100ft or $87.20/500ft.
I also found IPVC on eBay. There I bought 650ft of brown #22 UL1430 for $30 plus shipping.
Testing the IPVC
As mentioned, I was pleased to find that the stiffness is no problem. The wire from mcmaster.com was specified as white but I notice that it is actually off-white (a bit pinkish), which I ascribe to the irradiation process. In the picture above, the brown wire on the right is the IPVC. As you can see, it really nails the shrink back issue, showing no sign of shrinkage or even melting at the point of soldering.
Carrying things one step further, I applied a 700F soldering iron to the #22 wires for 5-seconds. At
right, the brown IPVC shows very little indentation. The orange PVC below it though, has been penetrated to the metal. When you are soldering in close quarters, it is all too easy to brush up against a wire. With regular PVC that usually leaves an unsightly gash. IPVC would barely show a mark from a brief stroke.
It seems clear that IPVC insulation is preferable insulation for hookup wire. It is available in afforable lengths, for certain colors, voltages and strandings. Alas, I have not found it yet in affordable lengths with a #22 solid conductor, 300V rating and in a variety of colors. When that becomes available, I will be ready to replace my reels of common hookup wire!