PCB board preheater idea

[rhb]

After reading reviews of low cost board preheaters, I got to wondering about whether it made more sense to just build one.  There seemed to be two main complaints, uneven temperature and inaccurate temperature.

The tentative idea is several sheets of 1/8" perforated aluminum plate with 1/8" holes on 3/16" centers and a cheap paint stripper type hot air gun feeding tangentially into a circular base with a 1/4" - 1/2"  spaces between the perforated plates.  The fundamental concept being that the high conductivity of the aluminum plates will result in a very uniform temperature at the top plate. 

The tangential entry into a circular insulated base chamber will provide upward airflow which should be pretty uniform in temperature, especially if the chamber is made of 1/4" aluminum.  Each of the perforated plates should serve to equalize the temperature of the air passing through it and  the spaces will provide pressure equalization between the perforated plates and thus uniform air flow.

Then use multiple temperature sensors on the top plate to control the heat gun and differential pressure sensors between plates to sense the airflow.  A 1' x 4' sheet of plate is $80-90.  That would make  4 layers with a  1' square heating area and two pieces of  0.0625 sheet would restrict the heat flow for small boards with airflow adjusted appropriately.  That's competitive with the prices of cheap preheaters and kitchen equipment.

Has anyone seen anything like this?  Anyone see any potential problems?

[KL27x]

Problem is power. Each of those plates is a heatsink. You probably want to minimize the aluminum and just get the air redirected and spread out a bit, so's you can stick the heat gun in there sideways and be able to work on the board without a dedicated table/microscope/rework equipment without burning yourself with hot air or starting a fire. It doesn't have to be perfect. You most often will be working on one small area of the board.

The accuracy complaint is a hoot. Doesn't matter, unless you have a cookbook specifically for your board and procedure.

If you want a pro-grade do-it-all tool, but even better than what's out there, it would sure help to start by buying a pro-grade tool and actually using it. Starting in a vacuum, you can guess all you want. If you need something to get a specific job done, and that's it, you would know better than anyone else.

[rhb]

The entire point of the perforated aluminum plates is they *are* heat sinks inside an insulated box.  Once they come up to temperature, the temperature will be very uniform and thus so will the temperature of the air flowing through them. This is a very basic heat flow problem.

If a board is not being heated, the air flow and heat input would cut back to just keep the top plate at the desired output temperature of around 100 C.

Generally speaking, most devices have a heat profile for reflowing in the datasheet.  So you actually do have a very good starting point.  The whole point of the preheater is to offset the effect of ground plane attachments on the device.

I've read reviews of  Chinese heaters that charred the boards set a 100 C.  Obviously there is something wrong with that picture.

I have no interest or justification for buying a $2000 preheater.  And it is very rare that user manuals show any construction details.  The point of the post was to see if someone who had used a pro-grade preheater could comment on the construction details.

[Gregg]

Your basic idea should work and like you know a lot depends on getting a smooth low turbulent flow of evenly heated hot air directed toward the board.  I would think a thick aluminum container wouldn’t add much to the solution and would take a lot longer to heat the whole device to operating temperature.  Some aluminum honeycomb material for the final hot air directing material may be the solution.  The perforated aluminum plate(s) below with temperature sensors may work out well, but the air entering tangentially may or may not help; the air stream needs to be diffused and the possible cyclonic separation may not work out.  If I were trying this, I would go to a thrift store and buy a cheap cooking pot of the appropriate size made of thin aluminum or even stainless and experiment.  My gut feeling is that a taller container would be better than a short one.  Check out some of the YouTube videos about laminar flow.

[rhb]

Thanks!  Honeycomb is obvious, but I didn't think I could get it in small quantity.  But I found 1/8" honeycomb 2" thick on eBay.

The idea of using heavy gauge aluminum for the heating chamber was to provide edge conduction to the perimeter of the first perforated plate.  I'm assuming sufficient insulation that losses would be minimal and the unit could be kept on standby for long periods without consuming a lot of power or heating up the workspace.

However, if the bottom perforated plate were highly polished and the heating element on top of it, muffin fans running at low speed and far enough away might survive and provide more uniform and less turbulent  airflow.

Taller is clearly better than shorter.  I have a 5000+ volume technical library, so I can do much better than YouTube.  I think I should do some basic thermodynamic calculations to determine how much heat and air I need for various scenarios.

[carl0s]

Shirley if it's just to preheat for hot-air work, then it doesn't need to be super accurate and any kind of hot air from below thing should do it? I'm picturing a large glass bowl.

I have a Bibby Sterling hotplate stirrer thing that's been in my garage for years. I tried that recently (having usually not bothered with preheating), and, erm, yeah.. no good. some sort of electromagnetism making every SMD component stand up, and stick to the tweezers like poo to a blanket. Yes the stirrer was off, but there must be something about the heating element.

Yihua make some preheaters. I nearly bought one, but.. it's just gear acquisition syndrome and I just don't need one.

[rhb]

I think the stirrers  spin a permanent magnet.

I was just going to buy one until I read the reviews.  A Hakko is $1600.  I'm certainly not getting one of those.  The alternatives did not look all that attractive once I started reading user reviews.  Lots of complaints of uneven heating, board charring when set to 100 C, etc.  The uneven heating looks to me to be the biggest issue.

I've got a good metal shop, so making one is an easy project.  Right now I'm making sliding trays so that things don't get lost behind my monitors.   There's a shelf above the back of the table, so it's dark back there.  So things like to hide back there.  I just finished modifying some 100% extension Blum Tandem Plus slides.  So the woodwork is next.

[KL27x]

Yeah, i see the basic thermodynamic problem the same way as u have stated. i still think it will be practical problem containing/insulating multiple thick plates and esp the 1/4" chamber walls heating up the room. Unless you use a double insulated wall like a thermos. Or have some asbestos.

The 100C burning a relatively small / 2 layer board, of course a cheap one will do that. Unless that's the minimum setting... so what? If it was a board closer to the size capacity and multiple layers, it might reach only 75C. The 100C is just a number to serve as a reference point. You have to adjust for the board. Unless you have feedback sensor to the actual board, it can't do that for you. You don't get that on a cheap device. Some fairly expensive units, even, just have a scale 1-10.

[Gregg]

My gut feeling is that your idea of an inexpensive heat gun for a heat source and break out the heating element wires to an inexpensive PID temperature control would work quite well; but maybe I'm thinking on the cheapo end of things and you might want to get more sophisticated.  But heating the air before your main chamber may give better control than trying to control the radiant heat from an element within the chamber. 
Trying to do all of the math to engineer a perfect setup before starting this project would drive me crazy; I kind of like to see some quick and dirty results before a project like this becomes a burden. 

[langwadt]

I think the stirrers  spin a permanent magnet.

yep, and if you take it out it works just fine for soldering

[carl0s]

I think the stirrers  spin a permanent magnet.

oh.. yes. of course!  so there's a magnet under the metal plate.

Maybe I'll use it again then.

It's only any good for single sided boards due to it being a flat piece of metal.

I just gave the board lots of gentle all round hot air last time before concentrating on the part I was removing.

[cdev]

A couple of months ago I got a real preheater (basically a digital hot plate). It wasn't that expensive. Actually, let me rephrase that it was cheap. So cheap that I think it would cost more to build my own out of parts, it was maybe $50-60. Its blue. Its a very popular model.  I would be happy to answer any questions about it you have. Its not fancy, its basically just a box whose top is an aluminum slab with a digital readout of temperature. It does what its supposed to do. When its on it also acts as a space heater. It heats the basement area its in up. It doesn't smell which was my main concern with something like that. I have not tested its accuracy but that would not be hard and I'd be happy to do it.

I use it underneath boards I am working on, using an all-metal vise to hold them. I use it on a medium temperature. Not hot enough to melt solder. And then I usually just work on it there. Just be careful not to burn yourself.

It can also be used to reflow entire boards.

[rhb]

A couple of months ago I got a real preheater (basically a digital hot plate). It wasn't that expensive. Actually, let me rephrase that it was cheap. So cheap that I think it would cost more to build my own out of parts, it was maybe $50-60. Its blue. Its a very popular model.  I would be happy to answer any questions about it you have. Its not fancy, its basically just a box whose top is an aluminum slab with a digital readout of temperature. It does what its supposed to do. When its on it also acts as a space heater. It heats the basement area its in up. It doesn't smell which was my main concern with something like that. I have not tested its accuracy but that would not be hard and I'd be happy to do it.

I use it underneath boards I am working on, using an all-metal vise to hold them. I use it on a medium temperature. Not hot enough to melt solder. And then I usually just work on it there. Just be careful not to burn yourself.

It can also be used to reflow entire boards.

So what make is it?  I looked for something like that first.  Google is not letting me find the same things two days in a row.  I looked closely at one from Circuit Specialists and one by Aoyue plus a larger one sold under many names.

[carl0s]

A couple of months ago I got a real preheater (basically a digital hot plate). It wasn't that expensive. Actually, let me rephrase that it was cheap. So cheap that I think it would cost more to build my own out of parts, it was maybe $50-60. Its blue. Its a very popular model.  I would be happy to answer any questions about it you have. Its not fancy, its basically just a box whose top is an aluminum slab with a digital readout of temperature. It does what its supposed to do. When its on it also acts as a space heater. It heats the basement area its in up. It doesn't smell which was my main concern with something like that. I have not tested its accuracy but that would not be hard and I'd be happy to do it.

I use it underneath boards I am working on, using an all-metal vise to hold them. I use it on a medium temperature. Not hot enough to melt solder. And then I usually just work on it there. Just be careful not to burn yourself.

It can also be used to reflow entire boards.

So what make is it?  I looked for something like that first.  Google is not letting me find the same things two days in a row.  I looked closely at one from Circuit Specialists and one by Aoyue plus a larger one sold under many names.

gotta be this one: https://www.aliexpress.com/item/110-220V-BGA-Preheater-BOZAN-946C-Digital-Thermostat-Platform-Heating-Plate-Preheating-Station-PCB-SMD-Heater/32896696219.html?spm=2114.search0104.3.14.750b42d5ugT6kO&ws_ab_test=searchweb0_0,searchweb201602_2_10152_10151_10065_10344_10130_10068_10324_10547_10342_10325_10343_10340_10548_10341_10696_10192_10190_10084_10083_10618_10307_10820_10301_10821_10303_10059_100031_10103_10624_10623_10622_10621_10620,searchweb201603_45,ppcSwitch_5&algo_expid=5c1f6c61-8166-4f9a-a8d5-3d32a26c2340-2&algo_pvid=5c1f6c61-8166-4f9a-a8d5-3d32a26c2340&priceBeautifyAB=0

(BOZAN )

[cdev]

It's a "Persder 946C electronic hot plate" but the brand name is printed quite small.
The 946C is a generic name so there must be some reason for that.

Maybe its a knockoff of some other brand. Anyway, it works.
Its a well built, solid device. it is rated at 800 watts and the temperature can be adjusted up (it says) to 350 degrees C.

There is a 16 page manual but its mostly in Chinese , the first four pages are in English.

EDIT: Yes, it looks almost the same except my unit doesn't use a self contained PID unit - it has a display and the up down buttons, and the blue its painted is a lighter shade. Also mine was I think $46

[carl0s]

A couple of months ago I got a real preheater (basically a digital hot plate). It wasn't that expensive. Actually, let me rephrase that it was cheap. So cheap that I think it would cost more to build my own out of parts, it was maybe $50-60. Its blue. Its a very popular model.  I would be happy to answer any questions about it you have. Its not fancy, its basically just a box whose top is an aluminum slab with a digital readout of temperature. It does what its supposed to do. When its on it also acts as a space heater. It heats the basement area its in up. It doesn't smell which was my main concern with something like that. I have not tested its accuracy but that would not be hard and I'd be happy to do it.

I use it underneath boards I am working on, using an all-metal vise to hold them. I use it on a medium temperature. Not hot enough to melt solder. And then I usually just work on it there. Just be careful not to burn yourself.

It can also be used to reflow entire boards.

Just to clarify, do the boards sit on the heater, or is there an air-gap?

[carl0s]

Having used my lab hotplate, I do think there's an added benefit to either air or infra red, rather than a hot plate.
One of these would seem a better buy to me. It has a bigger brother too, but still fairly affordable.

[cdev]

I put them in a (small) stick vise I have so they can be soldered without moving and the stick vise is made of metal so they are not touching the metal top, but it is. the top is a slab of aluminum that has a concentric circle pattern in it. Solder doesn't stick to it. If I was reflowing something I would just put the board on top of the plate without the stick vise. I need the stick vise to hold the boards in place. Its the only vise I have that doesn't contain plastic or rubber parts. It takes quite a while to cool off once its turned off. Thats why I tend to keep it well below the solder melting point. Still it makes quite a bit of difference.

Edit: I should add here, my boards are all tiny. All the stuff I've been making is small and modular. I'm just fooling around.

[rhb]

At $46, the 946C would be attractive, but at $100 much less so.  It would be better as an IR heater if it were anodized flat black. 

The lack of provision for mounting a board is an issue with the 946C.  The Aoyue 853A has a pretty good arrangement for mounting boards.  The Yihua 853A would need some work to hold a board securely.

It's interesting that so many different designs all have the same model number, even under the same label.

[cdev]

I thought the same thing, It didn't make sense for me for more but at that price it was compelling. My current holder is non-optimal but I also have a VERY old panavise which is lacking its base. Its mostly metal (the parts that would be exposed to heat are all metal) and has a wide grip so can hold a PCB in any position I want once I find the right base for it, (it has a ball clamp) I need to get a tallish base solution for it but that would be an ideal holder. Its an old purpose-built holder for large PCBs. Its much like the holder on the preheater pictured in its construction details.