Wednesday, September 28, 2016

Low Profile Long Lead Thermistors

I put together some thin thermistors as a proof of concept. Aiming to get under 1/16" thick. Very few long lead thermistors on digikey are under this mark. 

I ordered some flat flex cable with connectors, Digikey offers inch increments between 2" and 8" and a variety of termination options, all as value add parts. The flat conductors inside are 0.05" wide on 0.1" pitch.

Digikey stocks 10k NTC thermistors all the way down to 0201 SMD package, but I went 0805 for two reasons. First, they're the same thickness as 0603 chips, but in an easier to handle and more robust package due to the larger area for solder to stick to. Second, 0402 and smaller won't reach across the space between conductors. The manufacturer of this wire makes it in 0.05" pitch as well, but the 2 conductor version isn't stocked at digikey.

 0805 NTC thermistor and flat flex cable

Thermistor soldered directly between the terminations. This requires no modification to the parts as they come, but you're also stuck with the one inch increments that are offered.

 Time for a dip in silicone conformal coating for isolation. I don't think this is the right product for this application. The test leads from my multimeter easily poked through the coating. This coating is great for waterproofing but isn't really mechanically robust. An epoxy would likely be the best choice for potting these.

I tried a second variation where I cut off the connector from one end and then scraped away the polyester insulation. This leaves what looks just like some SMD pads. One side of this wire is flatter than the other. Putting the flat side down makes it possible to only remove insulation on top of the conductors.

 I tinned one pad first, soldered that end of the chip, then soldered the other end. Solder paste and hot air might be easier, but I don't have either of those on hand.

 The insulation is quite heat resistant and didn't show any damage or lifting near the solder joints.

The backside of the cable is totally untouched after soldering. It should still be good to its 300 volt rating.

A piece of Kapton tape is used to insulate the cut end of the wire. I dipped this in conformal like the others, but a potting epoxy right over the chip would be better, like how die on board chips get potted.

Going over the whole thing with Kapton leaves bubbles around the thermistor. I'd be worried the corners of the chip would end up poking through the tape in the long run.

These all measure around 0.042" thick without coating. Two dip coats of the silicone conformal adds about 10 thousandths total thickness,

Until next time.