small bottom heater for rework and reflow

[exe]

Hi!

I think I need a sort of hot table to work on small boards.  I have two intentions:

1) bottom heating for rework/fixing boards
2) reflowing

I found this device on aliexpress: https://www.aliexpress.com/item/4001255305351.html (picture attached). What do you think of it? Would adding a dimmer make it a cheap reflow "owen"? Provided I solder just a few small boards a year, and I don't really need to follow reflow profile. I also have a thermal camera to manually control the temperature.

[thm_w]

That device looks ok, but:
- its not grounded
- it has a set temperature point (260C) or a set wattage, not clear which. So like you say you'd need to add a 500W+ capable dimmer to that.

Ben K. does this sort of thing with small ceramic heaters. Something like this. I believe he just run them off a DC power supply, to adjust the temperature. Would be a bit safer.

The professional version is this: http://www.miniware.com.cn/product/mhp30-mini-hot-plate-preheater/ or https://www.aliexpress.com/item/1005001586576073.html

[cdev]

Very concentrated heat in a small area getting hotter and colder faster while other nearby areas don't change as fast may put more thermal mechanical stress on a PCB than otherwise. (the stress from expanding and contracting at different rates)

I don't know. My preheater uses a big chunk of aluminum, and I always thought thats why its there. It depends how big the PCB its being done to is.

[exe]

Thank you very much for suggestions. I think I'll go with something more professional and with a built-in thermostat. I didn't know a hotplate can be bought in $30-100 range.

I wonder how much this is different from a reflow owen. As I understand, these devices cannot do thermal profile and there is, probably, more thermal stress on components due to less uniform heating. Also, the board itself acts as a thermal insulator.

[Ian.M]

You'd be surprised how effective a MR16 12V 50W halogen spotlight bulb is at locally heating the underside of a PCB.  At full power you can easily get up to Pb-free reflow temperatures in a couple of minutes.  Its a bit crude and doesn't have temperature control, but a skilled operator will have no problems avoiding board damage.

The setup is relatively simple: Make a 3" tall frame to support a rectangle of drywall board as a heat resistant work table.  In the middle, cut a hole to fit a small (5 oz) tomato puree tin, which are typically a good match to the MR16 bulb diameter and under 3" tall.  Drill the bottom of the tin to mount a high temperature MR16 bulb socket.  Drill or punch the side of the tin at the bottom with a ring of holes for some cooling air flow + to bring out the wires.  Drill four small holes just below the top rim to fasten it to the drywall board with panel pins radially outwards into the core of the board.  Mount the bulb socket on long bolts each with three nuts, one to fasten the bolt to the socket and the other two to sandwich the tin bottom so the bulb height can be adjusted to flush to the top of the tin.   Fasten the wall board to the frame, seal the edges with glue and ideally tape them so it doesn't crumble.  Bond and pin the tin into the hole in the wallboard with its top edge flush with the surface using some sort of heat-resistant cement (e.g. for auto exhaust repair).  Take care not to handle it roughly till the cement has fully hardened.

As MR16 bulbs usually have dichroic reflectors that allow most of the heat to escape behind the bulb, cover the reflector portion of the bulb with aluminum foil, shiny side in, secured with Kapton tape.


Difference between dichroic and aluminum reflector bulbs - get aluminum reflector bulbs if you can.

Connect the bulbholder to a dimmable 12V halogen 'ballast' mounted in the frame, and mount the dimmer control so it is convenient to operate.   'Black' the face glass of the bulb with a marker pen to reduce the glare when it isn't fully covered by the PCB.  Optionally stick aluminum foil over the wallboard, and ground it and the bulb can via a 1 Meg resistor to reduce the ESD risk.

Operation: To avoid excessive thermal expansion stress on larger PCBs, while its coming up to the desired temperature, slide the PCB in an orbiting motion to distribute the heat more evenly at the edges of the area you are spot heating.  If you want to monitor the temperature as you preheat the board, stick a square of kapton tape in the area you want to monitor and use an IR thermometer.  Keep some spare bulbs handy as you will be running it a lot at low power and may also be overrunning it at max with a board over it keeping the heat in, both of which will shorten bulb life.

Sorry, no photos - its at work and we are in beer-bug lockdown here!

[nali]

That's pretty similar to my lash-up, when I needed a preheater and didn't want to spend a lot of money on it, plus needed it quickly. I used a 400W halogen floodlight with a 400W rated lamp dimmer. The face of the lamp was just a bit bigger than the boards I was working on, and a bit of scrap FR4 or some household foil served to blank off the remaining area.

With the lamp on maybe 20% (dim orange) was good for 150C or so preheat. Total cost about £10 

(NB - random board for photo I wasn't reworking cheap eBay dev boards)

[Ian.M]

The drywall board panel round the halogen lamp face is well worth fitting as, round the actual hot area, it provides a safe surface that wont burn you, so you can rest your wrists on it.

[phil from seattle]

You'd be surprised how effective a MR16 12V 50W halogen spotlight bulb is at locally heating the underside of a PCB.  At full power you can easily get up to Pb-free reflow temperatures in a couple of minutes.  Its a bit crude and doesn't have temperature control, but a skilled operator will have no problems avoiding board damage.

And there are off the shelf dimmers that work with the 12V supply for  halogens which will give you some level of control.  There are also off the shelf temperature controllers that could be used for more precise temperature control.

Probably the biggest disadvantage of halogen bulbs is the height of the box to contain them. Maybe 100mm?

[Ian.M]

No.  I just measured the tomato puree tin as 2 7/8" (73 mm) tall,  so with 3/8" thick drywall on top of a 3" frame, the working surface will be 3 3/8" (86 mm) tall with 1/2" under the tin bottom so it doesn't burn your bench. The tin doesn't actually run that hot, and there's quite a bit of spare height inside it, so if you can cut it down evenly you should be able to get the total height down to well under 3".   Add a wrist rest along the front edge of your bench to cushion your forearms and the ergonomics are quite reasonable.

However the minimum height of Nali's 400W halogen setup would be more problematic.

[nali]

You're right it is a bit awkward at 120mm high but served its purpose for the job at the time  and for other occasional use. Coincidentally it was exactly the same width as the board I was working on so heated the entire board which was good as it had internal copper pours.

(Actually it's hard to get domestic halogen lamps over here now because they're mostly banned, although you can get them from some online vendors)

[exe]

I bought mechanic et-10. There is no meaningful reviews or any tear-downs yet, so I might be the first one to do it. The other option I considered was to buy a station on standard rex c100 pid controller, but those do not have stored temperature profiles. I'm not sure if mechanic et-10 is any better in this regard, but it has three indicators (ch1, ch2 and ch3), will see if I can switch between temperatures.

[exe]

I just received the unit. Like many others, it feels too big for what it has inside: there's plenty of free space inside. I guess, they have standard enclosures they use everywhere.

I'm yet to read the manual and to try it, but one thing I noticed is that CH1, CH2 and CH2 are probably to create thermal profile. That's because if I set some temperature on CH1, then CH2 cannot go below that temperature, and CH3 cannot go below CH2. But I'm yet to confirm that.

I hope to test it tonight! May be I'll make an embarrassing video how I do reflow on it. Originally I only needed a bottom heater, but why not "go big" and fully try it

[exe]

I couldn't resist the temptation to try it, so switched it on and tried to set 40C. The plate became unbearably hot. I measured temperature, there is like 30C overshoot or something. I set temperature to 100, and started to monitor plate temperature with flir e4 (I put a piece of black insulation tape). Overshoot was 30C according to flir. I measured temperature with thermal couple, it showed like 24C overshoot. So, overshoot is real.

Also, it seems like readback is very luggy, it takes a lot of time for sensor to update. It also doesn't show overshoot. So, may be there is a problem with temperature sensor, or control circuitry.

I also heard loud relay clicking, I was hoping for SSR control...  So far not happy at all, but let's wait for additional measurements and tear down.

[exe]

I did additional test, soldered a small break-out board at 220C (lead-free solder paste). The board temperature was more tolerable, like +15C to what was on display according to Flir e4. I think flir shows slightly higher temperature that what I get with thermocouple, so it seems at higher temperatures it works more reasonable. Still, I'm annoyed that I cannot set small temperature like 50C. I'd use some 50-60C on a fresh board to easier dispense solder paste from syringe.  I suspect either a firmware issue, or thermal sensor doesn't make a good contact. I'll try to show that on video.

Temperature climbs at about 1C per second. Is this good or not? Sounds a bit slow to me. Although, I measured temperature of the board, not hotplate.

There is also no hardware switch to switch it off completely :/.

I wanted to be able to monitor hotplate temperature to see how things cool down, but this thing has so much thermal inertia that I'm not sure I want this feature anymore. It takes forever to cool down.

On positive side, it seems the enclosure is grounded. At least it passes continuity test from hotplate to ground pin in power socket.