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Experiments with MCH mount

A project log for Reflow micro table

USB PD powered micro table to reflow small boards.

VitalyVitaly 10/19/2019 at 04:350 Comments

Since i need to make "popular" project, i use only accessible materials. Those may be not ideal, but that's intended choice.

Don't be confused with photos, those use special test boards for MCH only. With electronics and safety margins size should be ~ 130*70mm.

Attempt 1. Classic insulation.

I started with standard approach - putting insulation between heater and PCB. Found some kind of "aerohel" on aliexpress (not pure), and tried it:

Result was not impressive:

For sure, tried 3 layers of 6mm material with aluminium foil between. Better, but not enough. And with too slow cooldown after power off.

So - don't try to repeat, that's a road to nowhere :)

Attempt 2. Air gap + reflectors

After failed first attempt, i changed approach to air gap + IR reflector.

I did "spot mount" heater on conic bolts and placed acrylic mirror sheet on PCB.

Result become much better, but still needed some improvement. As i found later, my main mistake was to use acrylic mirror in reflector. Probably, aliens stolen my brain when i puchaised those while having normal aluminium adhesive tape.

Let's skip this step and follow to improved attempt, with more details

Attempt 2.1, air gap + right reflector

Here is our hero:

Let me list all condition for best result:

First, i prepared MCH heater: painted top with black temp resistive paint, and bottom with silver paint. Also used second heater with black-top-only, and third unpainted heater to compare. As real hobbist, i powered all 3 heaters, and used hand to feel heat on 1-2cm distance :). Difference was VERY big.

Many of you know, heater must have black top for better work. I can add - it must have reflective bottom to reduce losses and simplify insulation. I'm not sure silver paint is ideal, but have some ideas how to improve.

Now let's see mounting. There are 4 brass inserts, attached to PCB. On top they hold 3mm headless bolts (with inner hex key), which have conic ends (DIN914). Heater lays on cones and has almost zero direct contact. All this is tied with thin SS wire (available for vape)

So:

The last thing we have to do - add PCB reflector, to minimize amount of landed IR waves even more. See image, aluminium adhesive tape (for thermal insulation jobs) will be perfect. After i used it instead of acrylic glass, result become almost ideal:

Note, that's even with reduced bottom mirror, without external border. Also, in theory, bottom reflector should stay between heater and PCB. That's possible with additional intermediate PCB, but will increase height. I did not yet decided, should i use intermediate reflector or not. But you may be sure, current mount is already good enough, if used not 24x7.

What's next?

I have some ideas how to improve result and simplify assembly.

Use Panasonic PGS as top thermal pad

See EYGS091210 (size for 2 heaters ) and EYGS182310 (for 6 heaters). Those have HUGE thermal transfer (2x more than copper), and in theory can solve 2 issues:

Does it worth to pay 20$ more instead of black paint? IMO yes, if my experiment will succeed. With 50*50mm surface you can solder almost any hobby PCB.

Pure PGS works up to 400C. Question is how to glue it to MCH. The most promising idea is to use cements for auto's exhaust system repair. Small amount of cement seems to hold well on MCH body, but i have no PGS to test all assembled with thin cement layer.

Use real foil for MCH bottom

I did quick tests, real alu foil is much better than silver paint. Unfortunately, i used Abro ER-400. Despite of promised 400C, glue layer become liquid on 250C and destroyed very fast. But... until it was alive - very impressive. At 1cm distance it was like almost cold.

So, need better and cheap replacement, or invent how to bond foil on MCH. Problem is, alu & cheramic/cement have different temperature expansion ratios. That may make such joint unsafe after several heating cycles. If anybody has ideas - let me know. I need cheap & accessible glue with operating temp at least 300C (ideally - 350C), able to safely bond aluminium foil to ceramic (MCH body).

PS. I know about 3M enforced alu tape with up to 360C, but it costs 25-50 euro, not nice when we need only 50x50mm piece.

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