Author Topic: EEVblog #317 - PCB Tinning Myth Busting (Read 50543 times)

EEVblog · « **on:** July 20, 2012, 06:36:58 am »

Dave.

David_AVD · « **Reply #1 on:** July 20, 2012, 06:53:27 am »

I wonder if the standard HASL process during bare board manufacture would measurably decrease the track resistance compared to masked?

george graves · « **Reply #2 on:** July 20, 2012, 06:54:07 am »

So basically, you're adding a resistor(the solder, lets call that R2) in parallel to the copper (lets call that R1) - and so that will always *decreases* resistance. No matter how big R2 is.

Duh! I'm a noob and even I knew that

Now the question I have is that if you could magically separate that solder from the copper, and measure it's resistance, would it add up? Or is there an interaction between each little segment of copper, and each little segment of solder happening to further lower the resistance?

huseyinkara028 · « **Reply #3 on:** July 20, 2012, 07:56:46 am »

I am wondering,
Is it possible for either copper or solder can crack due to heat and expansion by the length?
More clearly we have two different materials on top of each other and they have different thermal properties. The amount of current they carry probably different and the heat be produced by this current will be different. Due to this heat they will expand ( I know small amount) and apply opposite forces to each other. Is that possible to be the trace crack due to this reason? Have you folks see this kind of stuff before?

Thanks.

Electr0nicus · « **Reply #4 on:** July 20, 2012, 08:48:56 am »

That would be very unlikely. You are right that different materials expand differently, but solder is a very soft alloy, so cracks would be relatively rare. Further reading about thermal expansion http://en.wikipedia.org/wiki/Thermal_expansion

arcoshpl · « **Reply #5 on:** July 20, 2012, 09:38:33 am »

I thought about the same Problem like huseyinkara028.

I think in long term there can be an issue. Tin has a two times bigger expansion coefficient. When a device heat up thousand times in his live the resisters will increase because of minimal cracks in the material. I don't think that you will find something like this in high quality devices.

Arcosh

robrenz · « **Reply #6 on:** July 20, 2012, 10:00:49 am »

Wtih Tin/Lead alloy and Copper all being very ductile/malleable I don't think you will see cracks from difference in themal coeficient of expansion ever.

ductility http://en.wikipedia.org/wiki/Ductility

David_AVD · « **Reply #7 on:** July 20, 2012, 10:05:49 am »

Over many years and many many circuit boards in equipment seen for service, I can't say I've ever seen cracking in the solder mass areas.

The cracking is always on joints that had insufficient solder to start with or were subjected to high heat stresses. Never on thickened traces though.

arcoshpl · « **Reply #8 on:** July 20, 2012, 10:16:24 am »

I mean no visual cracks.

More something like this.

http://en.wikipedia.org/wiki/Grain_boundary

But I think it is a normal process that also in normal soldering point happens.

Arcosh

digsys · « **Reply #9 on:** July 20, 2012, 10:25:00 am »

Same here, rarely ever seen the solder separate from the Cu.
The junction where the 2 metals meet (1 metal and 1 "compound") forms a sort of new valence bonding.
ie at that point, you have a new alloy, after all, the metals chosen in the solder are selected EXACTLY for that
purpose. Alloy junctions can have weird properties, DIFFERENT to the base metals. It's kinda a voodoo science.
SO to answer a previous question, IF you separated the Cu and Solder and calculated resistance separately,
it would differ slightly to them combined because that junction has different properties.
Mixing different metals together to find new and often strange properties is an awesome field. Some of my
colleges have spent 20-30 yrs playing with that shit.

Stephen Hill · « **Reply #10 on:** July 20, 2012, 10:30:52 am »

Dave,

Would you be willing to test the actual current capacity of a soldered and unsoldered trace by slowly increasing the current until the trace lifts/breaks?

In theory ohm's law should hold true that if you half the resistance you can double the current. I'm curious if the real world matches the theory.

Cheers
Stephen

robrenz · « **Reply #11 on:** July 20, 2012, 10:32:24 am »

Daves measurements from before (1.001A & .05249V = .05096 Ohm) and after solder removal (1.004A & .05249V = .05228 Ohm) give a 2.59% increase in resistance with the minimal solder film still on. Is there an alloying effect going on increasing the resistance or is this just measurement variations? (or did I make some math errors?

)

dda · « **Reply #12 on:** July 20, 2012, 11:15:07 am »

Quote from: robrenz on July 20, 2012, 10:32:24 am

Daves measurements from before (1.001A & .05249V = .05096 Ohm) and after solder removal (1.004A & .05249V = .05228 Ohm) give a 2.59% increase in resistance with the minimal solder film still on. Is there an alloying effect going on increasing the resistance or is this just measurement variations? (or did I make some math errors? )

It would have been good to weigh the Cu board and Cu+solder boards to see how much was still there after removal. Need a 5 or 6 figure balance.

EEVblog · « **Reply #13 on:** July 20, 2012, 11:19:03 am »

Quote from: robrenz on July 20, 2012, 10:32:24 am

Daves measurements from before (1.001A & .05249V = .05096 Ohm) and after solder removal (1.004A & .05249V = .05228 Ohm) give a 2.59% increase in resistance with the minimal solder film still on. Is there an alloying effect going on increasing the resistance or is this just measurement variations? (or did I make some math errors? )

I can only presume that some copper leeches out during the wicking process.
The setup seemed too stable and repeatable for that to be within the margin of error.

Dave.

Monkeh · « **Reply #14 on:** July 20, 2012, 01:06:48 pm »

I think it's pretty clear that you can rely on getting 15% or so improvement in current capacity with this method. Which is pretty good as a safety margin. Just helps find that little extra breathing room on a PCB.

Psi · « **Reply #15 on:** July 20, 2012, 01:15:36 pm »

It's really only good for adding extra safety margin to your existing margins.

But that's still quite useful, especially when free.

mikeselectricstuff · « **Reply #16 on:** July 20, 2012, 01:16:30 pm »

Quote from: robrenz on July 20, 2012, 10:32:24 am

Daves measurements from before (1.001A & .05249V = .05096 Ohm) and after solder removal (1.004A & .05249V = .05228 Ohm) give a 2.59% increase in resistance with the minimal solder film still on. Is there an alloying effect going on increasing the resistance or is this just measurement variations? (or did I make some math errors? )

The solder does form an amalgam with copper at the surface - my metallurgical knowledge isn't up to knowing how the conductivity of amalgams relates to the constituent metals. As Dave mentioned, scraping of copper during unsoldering may also be an issue - whouldn't be hard to test that

Rufus · « **Reply #17 on:** July 20, 2012, 01:54:36 pm »

Quote from: EEVblog on July 20, 2012, 11:19:03 am

I can only presume that some copper leeches out during the wicking process.

If solder didn't dissolve copper they wouldn't need to iron plate soldering iron bits and Muliticore would never have made Savbit solder which already contains 2% copper.

On the vid shame you didn't attempt to measure the thickness of added solder - didn't you get some callipers free the other week? If whichever solder has 10 times the bulk resistance of copper then adding 10 times the thickness of solder will halve the track resistance.

On Dave vs. Mike competition I get the impression Mike wants to know the answer and the video is a bonus, Dave wants to make a video and the answer is a bonus

GK · « **Reply #18 on:** July 20, 2012, 02:25:56 pm »

Just for general trivia, never tin plate tracks carring heavy RF current with the idea of reducing track resistance. The skin effect will only ensure that the current largely flows in the lesser-conducting tin plating, thus actually raising the track resistance. This actually becomes significant at frequencies lower than what some might imagine. But then again if you are designing high power RF amps (just for example) you probably know this anyway.

nitro2k01 · « **Reply #19 on:** July 21, 2012, 02:38:44 am »

GK, for the same reason, would it make sense to route multiple parallel traces instead of a big solid single one, similar in spirit to Litz wire, or do you run into other problems if you do that?

EEVblog · « **Reply #20 on:** July 21, 2012, 03:00:26 am »

Quote from: Rufus on July 20, 2012, 01:54:36 pm

On Dave vs. Mike competition I get the impression Mike wants to know the answer and the video is a bonus, Dave wants to make a video and the answer is a bonus

How do you figure?
I went to more trouble to get additional results.
If I just wanted a video I could have just wicked off some solder quick and dirty like Mike did

Quick lead free video shot this morning. Sorry, no measuring of actual thicknesses etc.

Dave.

croyleje · « **Reply #21 on:** July 21, 2012, 03:05:05 am »

I am not exactly sure how the flux is applied in wave soldering if anyone knows I would like to know but I think that using rosin core solder like Dave did would have some marginal effect on the copper itself. Rosin being a organic acid is going to attack the copper just as it does any oxidation on the board and at only 35 microns it wouldnt take much to change the trace.

amspire · « **Reply #22 on:** July 21, 2012, 03:49:26 am »

Quote from: croyleje on July 21, 2012, 03:05:05 am

I am not exactly sure how the flux is applied in wave soldering if anyone knows I would like to know but I think that using rosin core solder like Dave did would have some marginal effect on the copper itself. Rosin being a organic acid is going to attack the copper just as it does any oxidation on the board and at only 35 microns it wouldnt take much to change the trace.

The board first gets flux applied, then passes on to the wave.

The wave is just solder - nothing else. To stop oxidation of the solder, there is a thin film of oil over the top of the solder, but it is inert - it does not act as a flux. Any solder oxides float on top of the solder and they get scraped off from the wave.

IanB · « **Reply #23 on:** July 21, 2012, 04:24:35 am »

Would be interesting to see a comparison with a length of copper wire laid on the trace and soldered down, or a length of copper wire soldered through hole as a jumper instead of the trace.

Rufus · « **Reply #24 on:** July 21, 2012, 04:33:50 am »

Quote from: EEVblog on July 21, 2012, 03:00:26 am

Quote from: Rufus on July 20, 2012, 01:54:36 pm
On Dave vs. Mike competition I get the impression Mike wants to know the answer and the video is a bonus, Dave wants to make a video and the answer is a bonus

How do you figure?

For example you spend about 1:30 talking about 4 wire measurement and more a bit later, Mike covered it in about 20 seconds.

Mike documents what he did and the results with the video. You spend more time explaining why and how you did which is for the sake of the video.

Showing what you did is required to understand the results, talking about why and how doesn't get better or faster results.

I am not critical of either approach just commenting on the difference.


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Author Topic: EEVblog #317 - PCB Tinning Myth Busting (Read 50543 times)

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