Yay, another write-up!

Today we'll be looking at Kapton Polyimide Tape, originally produced by DuPont and knocked off shortly afterward. It's essentially a "masking" tape (typically used in electronics) that's able to withstand temperatures between 250-400C depending on the manufacturer and composition.

As techs, we've all seen its sexy translucent glow in most household / industrial devices, though to my surprise, alot of us (including senior techs at work) have never known it by name nor used it on a regular basis. Hopefully that'll change, as it's an extremely useful addition to ones toolbox.

Original pin-modding credit goes out to the LEGENDARY Jeri Ellsworth, quite possibly the greatest female geek in existence. WHY WON'T YOU LOVE ME!?

Kapton as a temporary Solder Mask (homebrew PCBs)

Below you'll find a few examples of Kapton used in conjunction with a soldering iron to hand solder a TQFP64 dsPIC.

The drag-soldering method shown is by no means a new technique (and we do own hot-air rework stations!), however the web is full of nasty examples where solder 'runs' along unmasked tracks, making the whole design look terrible. Kapton to the rescue!

Update : A few of you have asked how we managed to "plate through" the vias underneath the dsPIC. Rest assured, the photos show the design in its un-plated state! Our technique is hidden under the chip during the alignment images.



For "external" vias, we simply use solid core Kynar "modchip" wire, solder and a #87 drillbit to link both sides. #87 perfectly fits the strand, so it's held in-place snuggly while you solder it. For "internal" vias, such as those underneath the TQFP component, we use the same method with thinner stranded wire extracted from cheapie RCA leads.



No magic here folks, you'd be surprised what'll comfortably fit under a dsPIC!

Homebrew PCB : A dsPIC33FJ128GP206 breakout board, freshly etched and silver-plated

Kapton Tape : 5mm polyimide tape that : 5mm polyimide tape that we now stock!

Solder Masking : Cut two strips, and place them directly above and below the pads to be masked

Solder masking : Two of four rows masked (you can do them all if you wish)

Solder Preparation : Apply a liberal amount of Paste Flux to the pads

Tinning : Drag a ball of solder across the pads in the direction shown

*GENTLY* use a blade to score the kapton, then lift the tape to expose the remaining pads

All pads exposed and tinned, using a decent amount of flux avoids bridging of pads/traces while dragging

Align the component to be soldered, mind the location of pin one!

Tack down one pin on each corner using your soldering iron, this will keep the component aligned

Apply more flux in anticipation of the next step

GENTLY run your soldering iron along the edge of all pins, roughly 5 seconds per side

Remove the Kapton tape, and rinse the board with IsoPropanol (or your favourite flux cleaner!)

The chip, now fully soldered! (Due to lighting, the pins look like they've lifted, this really isn't the case)

The headers, now perfectly soldered with no "runs", Kapton rocks!

The fully completed / assembled design

Yes, it DOES work! Testing on our former $5.95 blue 16x2 LCD module

... And that concludes the section dedicated to PCB masking

Using Kapton for "pin-masking" and patching

Let's assume we need to "patch" the soldered TQFP microcontroller by pulling a certain pin to ground, probably due to a faulty design, or you were drunk at the time, or even forced to clean poop and you've had a generally bad day. How does one undertake such a daunting task on such a fine-pitched component?

Locate the pin to be patched and place a small strip of Kapton tape beside (but not covering) it

Place a second strip on the opposite side, so only the pin required is exposed

Tin and flux the "patch lead" (A strand of kynar wire, in this case)

Align it over the designated pin, and reflow the solder with your soldering iron

Remove the tape and voila, no bridged pins! Kapton is the new Duct Tape of awesomeness.

More content goes here. MOARRRRR!

Ready to buy some Kapton goodness? Check out Doug Ford Analog Design's products page or that of OKAY Technologies (whom we're actually not affiliated with) - I'm fairly confident they're the only two companies with aussie stock (ie: next-day shipping) to do it at such a low cost.

...There are also more suppliers with a higher price, if you're curious: Farnell and RS Australia product pages for a price comparison

-PodeCoet

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