What can I do to keep this board from warping?

I'm trying to make a 10x scope probe that is capable of dealing with slightly higher voltage levels of about +/- 20v yet at a high impedance ~20Meg. This link "

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" has some photo's showing the probe and board warpage. The board was made from double sided

0.020-inch stock etched with a PCB milling machine (perhaps part of the problem). It is (obviously) hand assembled. I would appreciate any tips or techniques that I could use to keep this board from turning into a potato chip during assembly.

Thank you.

al

Reply to
mickgeyver
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dang G00gle.....

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Reply to
mickgeyver

It looks like you used cheap phenolic PCB materials. Fiberglass is a lot more stable.

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Reply to
Michael A. Terrell

Use 0.032" media. Bake under a flat press upon receipt of the bare boards.

Hand assemble... no wave solder, no IR oven.

Those are the only techniques that yield 100% positive results.

Any automated process will certainly have a poorer prime pass yield.

That is, unless the PCB maker was real good at stress relief during the making of the boards.

Reply to
Archimedes' Lever

Is that a Polysulfone board?

With 0.020 inch anything, and all that copper, it's going to warp no matter what you do. Any differential heating, on one side of the board, is going to cause the board to bend.

I ran into the same problem years ago with wave and hand solder in thin boards. Going to thicker boards, with some kind of stiffener was not an option because of weight limitations. However, you apparently don't have that problem. Try 0.032 or 0.0625 boards, keep the heating to a minimum, and it might work.

I think you might be entertained by what happens to just the board, without any components. Clamp one end of the board into a vice. Heat one side of the board with a hot air gun or soldering iron. The copper will expand, causing the board to warp. In other words, the large copper areas are what's causing the warping.

You can somewhat minimize the effect by pre-heating and post-heating the board. We found that freon vapor reflow soldering produced absolutely flat boards. That because the entire board was heated evenly, and cooled down evenly and slowly. Heat one side of the board more than the other (i.e. differential heating) and it's going to warp unless you're very careful on cooling it down evenly. When we went to surface mount devices, small traces, and 0.5 oz Cu traces, the warping was gone.

0.020 is thin enough that the board can be straightened by the mounting arrangement. Instead of the round brass tube, it might be possible to use an aluminum tube, with two grooves milled into the inside diameter. Your current board does not have enough unused area along the lengthwise edges to do this, but it can be expanded somewhat. This also has the advantage of providing a nifty ground connection to the case. However, inside milling is expensive. A square aluminum extrusion is possible, but even more expensive.

Good luck.

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Reply to
Jeff Liebermann

Tin the tracks with hot solder on the side outside the curve, as it cools and contracts it will straighten the board.

Reply to
Mark Harriss

I'll try to find thicker stock. Like Joerg there is no expense money available for this project so I gotta use whatever is available. There is a connector on the other end that I'm using to leech power from the scope so I have limitations here.

Done that. Perhaps if I make a frame to clamp it flat during assembly.

he

I'm the PCB maker all contained in house with a mill machine.

Reply to
mickgeyver

No. Just a scrap of fiberglass board from long ago. I have some Rogers flex board, but the surface appeared too rough to get good results on my PCB mill.

Can't go too thick here as I'm leeching power from a Tek P6204 probe cable.

t

I'll try that just for grins. Never thought of thin PCB material as a bi-metalic thermostat element.

I was concerned that too much bending would crack some of the components giving me problems later on. I don't want this to get too big as it lets me attach this unit to a probe manipulator to use under a microscope to hit those tiny nodes. Like I've stated before.... I'm leeching power from the business end of a TEK P6204 probe. The 3/8 brass tube mates up nicely with the Tek probe cable strain relief at the probe end. I'll post more photo's of the other end for the groups viewing pleasure.

Reply to
mickgeyver

Then your dwell time on each solder joint is way too high.

Try 1/20th gram per joint, not the half gram per joint the picture appears to show. You know... NASA soldering specs. As little solder as is possible and perform the operation quickly. What I saw looked like several time the amount needed in the dip chip. Even the connector was huge and blobby on the solder joints. The builder does not need to flood those pins with solder. The solder only needs to be between the pin and the board ala SMD soldering. FILLET not BLOB.

The soldering iron should also be at 600° F, NOT 800° F.

If you are having wetting problems due to age of the leads of your parts or such, you may need to try different solders to see if a different flux choice might yield a good solder joint faster than your current choice.

RMA is harder to clean (hot alcohol bath) but it does make a good, well wetted solder joint faster in some instances. I am not sure which flux choices are the most active these days. It has been a long time since I did production level soldering, and we currently use odd technology since we get exempted from RoHS compliance.

Reply to
Archimedes' Lever

Ok. I just noticed that the backlighting LCD inverter in a typical laptop is constructed similarly to your prototype. Very thin G10 board, 10mm wide, and all the parts on one side. To make matters worse, a big routed hole in the middle for the xformer. Huge copper ground plane on one side of the board.

Yet, all the boards in my collection are quite straight. That demonstrates that it can be done. I'm not sure what you're doing that's wrong, but I agree with others that suggest you're using far to much solder and heat. Smaller parts, less solder, and less heat will help.

Incidentally, when I say less heat, I don't mean less temperature. I use the hottest iron tip I can get away with, and work fast. That minimizes the energy transfered to the solder joint and therefore reduces the heat affected zone.

PCB warping will certainly cause problems. It really depends on the physical size of the components. Parts with flexible leads (xsistors, IC's) are not much of a problem because the leads absorb the flex. Parts that are soldered directly to the board, will either crack, or rip the traces off the board. If you can't fix the flexing problem, switch to a flexible PCB design and apply an insulating stiffener board on the circuit side. It can't be prototyped with at PCB router, but at some point, you're going to have to commit to a PCB layout. Might as well do it early.

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Reply to
Jeff Liebermann

I think one problem is that this is a home made board. The holes are likely too large for the components (because most people don't have very small drills on hand or even the drill chuck to hold them) and obviously they aren't plated through. That can make it necessary to glob on the solder.

As far as temperature, I agree with you. I've never found a need to solder anything at less than 800F. Smaller parts just require shorter dwell time.

I have no idea why the OP is using such thin stock, though. It looks like he'd have room for something as robust as .050".

Reply to
Smitty Two

Good observation. The lead size, and hole size and even the annular ring width all inter-relate if one is to produce minimalist 'NASA' style soldering results.

This one is off the mark. Perhaps you meant "at MORE than 800°F" That would certainly be an upper max for me, and it is quite easy to solder at much lower temperatures. Well over a 100 degree differential, and that is significant.

He could make little SMD circuits that he places on Silica wafers, and then place those wafers on the substrate with epoxy, and wire the sections together.

I used to use 0.020 wafers up to 2 x 2 inches to make little smd proto circuits on. I have glued smd parts on, and hand wired point-to-point the nodes between the parts. I have also 'dead bug' glued thru-hole 8 pin dips onto it and wired those nodes with hook-up wire. The leads were shortened, of course.

That sounds like a good proto build solution, but to make a production run would require a different decision.

Reply to
Archimedes' Lever

Pardon my ambiguity. What I meant to say was "anything other than 800F." That's the upper limit on most fixed-temperature tips, or adjustable irons.

Reply to
Smitty Two

Yes, and despite most folks cranking the damned things up that high, basic soldering operations (pre-RoHS) were NEVER meant to be performed at that high a temperature. Even on big 0.092" stock with lots of Copper. In that case, one should pre-heat the whole assembly a couple hundred °F, which will insure that all the solder joints wet well and flow through properly.

Reply to
Archimedes' Lever

Uh-huh. I've personally hand soldered hundreds of thousands of joints at

800F, and overseen the soldering of millions. Turning down the flame has only one result: It takes too damn long to build the board. It's sort of like the clock speed on a computer. It doesn't matter much if something takes 1 second instead of 1/10 second, but it makes a big difference if it takes an hour instead of 6 minutes. When I was a kid I built some Heathkit stuff and spent 30 seconds making every solder connection, because the iron was underpowered and I didn't know enough to know it. Machine soldering is different, of course, but for hand work, 800 is perfect for everything and anything.
Reply to
Smitty Two

I concur - 700F tips are great - no problems in the last 15 years. The only time I go for a 700F is terminating some cables where the insulation is super sensitive to the heat and shrinks back or melts through.

Reply to
Nik Rim

Typo - should have said 800F tips......

Reply to
Nik Rim

The sinking capacity of a soldering iron has little to do with the temperature it sits at and claims to be maintaining. The ability of the iron to keep a tip at a given temperature is what makes a good iron, and no, those shit dial types with 800° at the upper end of the dial are NOT good at maintaining a set point, and there is NO spec ANYWHERE that uses

800° as an iron tip temp far assembly, even by hand.

And even at 650° or 700°, a good soldering tech can perform a good solder joint in the same amount of time with a proper iron that holds its set point properly.

Proof that you do not really know anything about it. So despite all the talk about dwell times, and claim of "oversight" of others, your decisions about ignoring the facts and standards the industry has known about for decades means that your credibility for actually knowing what is going on fell right through the floor.

Reply to
Archimedes' Lever

You are both idiots that would never sit very long at one of my benches.

Not to mention that neither of you would make it more than a day in a NASA cert course either. Not with a stupid mind set like that.

Reply to
Archimedes' Lever

Obviously you have the engineering mindset, while I have the practical, get-it-done mindset. Even NASA gave up their low-temperature bullshit when they figured out that components are subjected to *less* thermal stress with high temps and short dwell times. Sure I know the difference between power and temp, but as far as production soldering, a lack of either has the identical result, and that's a lack of production, which translates to a lack of income, which translates to going out of business for a job shop. Many industries are, or were, profitable enough to have people working along at a snail's pace, but contract manufacturing doesn't have that false luxury.

How long does it take you to solder a standard thru-hole 14 pin DIP IC at 650? Anything more than about 1 second per leg, and you'd be fired from *my* production line. The same IC in a surface mount package, and you'd be expected to skate down each side and make seven perfect solder joints in a total of about 2 seconds, once the IC was tacked at opposite corners and fluxed.

I'll make the implicit explicit here, and let you have the last word. But it won't make you right.

Reply to
Smitty Two

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