In the shop -- measuring PCB temperatures in the cooker

Argh -- it was NOT that hot. I'm not sure how I hallucinated a good solder joint (distance and geezer eyeball may have contributed), but I hit on the notion of clipping a bit of solder off of the roll and leaving it on the board -- when it melts, I'll know.

Oh boy -- thrice-baked circuit! I wonder how it'll be?

Updates in real time! The thermocouple read somewhere between 180 and

I need to cobble a fan onto my entirely cobbled-together apparatus -- with the board spaced off of the skillet it seems to heat faster if the air is moving around.

Updates in real time! The thermocouple read somewhere between 180 and

I need to cobble a fan onto my entirely cobbled-together apparatus -- with the board spaced off of the skillet it seems to heat faster if the air is moving around.

The best way I know of to measure temperature on a board is to borrow a diode junction for the sensor:

Transistor base-emitter junctions work fine and the rest of the circuit doesn't need to be powered.

You can use the 183C melting point of 63/37 eutectic lead solder to check the calibration.

Measuring correctly with thermocouples is difficult unless they are embedded within the material. I clamp a thermocouple between two copper washers under the lid knob of my Farberware pots, to know when they reach boiling on the woodstove downstairs. The readout is about

-jsw

So do you think a diode would do better than a thermocouple as far as actually being the temperature of the board?

The thermal response of diodes is *extremely* linear within a range of sensible temperatures. I made a tire thermometer out of one when I was racing, on the recommendation of an electronics engineer. Checked against a precision scientific mercury thermometer, it was +/- about a half-degree C over a two-decade range.

It would likely do better at actually being AT the temperature of the board instead of at some average of the board and the thermocouple's wire leads.

I have better luck measuring surfaces with 30 gauge thermocouples. Try measuring the external temperature of a boiling pot of water with a thermocouple to see how tricky it can be.

-jsw

The single thermocouples I have seen on boards are RTV in a small hole in the board just for that purpose.

The ECD MOLE for use in a convection oven uses many thermocouples that are fastened to board locations to be monitored, with RTV, as I recall. We haven't used ours in 5 years or so, so the procedure is kind of vague!

Get a used toaster oven at a thrift store and use it. Not so much air movement and easier to watch both sides of the board.

Paul

Some photos of my successful attempt to reflow 18(?) HP JetDirect printer cards in a cheap Black and Decker toaster oven: Instrumentation:

What I found was that the temperature varied wildly with the location of the thermocouple sensor in the oven. Correlation between the thermocouple and the IR thermometer was non-existent. I was initially trying to simulate the correct thermal profile, but soon discovered that it was impossible. The temperature varied too much across the boards and across the oven. If you attach thermocouples to the PCB's located in various parts of the oven, you will probably see wide variations in temperature. Sorry, but I didn't record the numbers.

I took my best shot (after a dry run), and found that it worked perfectly. In other words, it does not seem as critical as I initially expected. Since then, I've reflowed a few dozen board with good success using a few more thermocouple sensors just sitting on the BGA chips without silicon grease or any pressure that might cause the chip to move.

This transport company has used a variation:

Seems a bit dubious to me. Same with any variation on "just in time". I'd like the stuff there in plenty of time, thanks.

--sp

I upgraded an underperforming Coleman camp stove oven with an insulating blanket of fiberglass mat within a fiberglass cloth envelope, held together with staples around the folded-in edges. First I stabilized the loose edge threads by melting them with a propane torch.

-jsw

Two decades is e.g. 4.2K to 420K. Pretty good quality mercury, that. ;)

Cheers

Phil Hobbs

Right, that's what Net 30 is for. ;)

Cheers

Phil Hobbs

On Fri, 22 Jan 2016 14:54:12 -0600, Tim Wescott Gave us:

Get an IR imaging device. Hell, they even have units you can attach to your smartphone.

It will be the best few hundred bucks you ever spent.

Otherwise, just use some common sense.

Put some paste into the skillet (direct contact) and observe how long it takes to come up to the point at which the paste reflows, the research what that temp is by the maker of the paste.

Then, you can assume that when you see top side reflow happening on your PCB assembly, that it begins at that temp and should begin at some point after the pan has been on the heat for the pre-determined time period.

Since you need not ramp-up the board for that entire period of time, add the PCB at some point before the pan comes up to reflow temp. A point at which you want your ramp-up dwell time to start at.

How many years are there in a decade and in a century in Phil's world?

On Fri, 22 Jan 2016 17:31:15 -0500, "Carl Ijames" Gave us:

"Heat quicker" doesn't matter. Cast iron in the oven and Al in the oven still come up together because the heat the oven makes gets infused evenly across the entire mass, and the dwell of the solder process itself is longer. Al is good for moving thermal energy, but not retaining it uniformly through a mass. The cast iron is better because it RETAINS the heat more uniformly better. He could get creative by cutting the apron form the skillet and leaving the bottom plate and perhaps a 3/8 inch side apron (1cm or so).

The oven is a good idea because it would heat from the top as well as the bottom. He could thermal epoxy the thermocouple to the skillet plate or even drill a hole in it from the side and insert the TC all the way into the hole. Placing one more in the air above the PCB assembly will likely yield what temp the PCB has been brought to/near.

You can be penalized for early delivery in some contracts. This sometimes gets to be a sales rep's job - holding product in their office and delivering in person. Anything to get the business.

I've seen suppliers renting warehouse space down the road from the client, just to meet their expectations. Just another cost built into the business - but only really paying off at larger scales.

RL

Two decades of Celsius from 1 deg. C, , not of Kelvin.

JIT only works if you run a modern operation with excellent scheduling

- MRP, ERP, SPC, and the works. It doesn't add costs if it's done right -- in fact, the whole point is to save money throughout the supply chain.

The idea is a Japanese import. It works pretty well in a country where the suppliers are an average of 10 - 20 miles from the OEM. It doesn't work nearly as well in big industries in the U.S.

My sister-in-law was a purchasing agent for Caterpillar in Aurora, IL. After a couple of years working with JIT, where they had castings coming in from halfway across the country and regular winter snowstorms, they renamed JIT to OSWO -- "Oh S*it, We're Out."

d8-)