Author Topic: EEVblog #319 - Lead Free PCB Tinning  (Read 13379 times)

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Offline EEVblog

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EEVblog #319 - Lead Free PCB Tinning
« on: July 22, 2012, 02:30:33 am »


Dave.
 

Offline Psi

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Re: EEVblog #319 - Lead Free PCB Tinning
« Reply #1 on: July 22, 2012, 02:45:59 am »
Next up... Silver solder :D
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Offline naimis

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Re: EEVblog #319 - Lead Free PCB Tinning
« Reply #2 on: July 22, 2012, 03:01:34 am »
Based on my fairly limited education in chemistry I would think the lead would actually be more conductive.  Lead and tin have the same number of electrons in their outermost shell but lead, being one period down on the table has a "less tight grip" on its outer shell of electrons... and as such the electrons in a lump of lead ought to move more freely than in a lump of tin (or to take the same argument even further, in a lump of germanium or silicon or carbon).

But I'm probably talking out of my ass.
 

Offline PeteInTexas

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Re: EEVblog #319 - Lead Free PCB Tinning
« Reply #3 on: July 22, 2012, 04:02:02 am »
Do wave solder machines use solder with some other composition Dave/Mike tested?  I'm thinking it might be different because the process is very different from hand soldering.  And if its different, somebody will need to test that to satisfy the forum.  ;D
 

Offline amspire

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Re: EEVblog #319 - Lead Free PCB Tinning
« Reply #4 on: July 22, 2012, 04:27:36 am »
Based on my fairly limited education in chemistry I would think the lead would actually be more conductive.
Lead has over double the resistance of tin, and the 60/40 solder has about 25% more resistance then the Tin solder.

I do not think the 0.7% copper content in Dave's lead-free solder is anything to do with lowering the resistance, as Dave suggested. I think it helps prevent copper ions diffusing into the solder which can cause the solder to become brittle.  This makes it liable to fracture and turn into an intermittent joint. That is something I heard decades ago, so if the explanation is wrong, I hope someone can correct me.

Richard.
 

Offline EEVblog

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Re: EEVblog #319 - Lead Free PCB Tinning
« Reply #5 on: July 22, 2012, 06:24:48 am »
I do not think the 0.7% copper content in Dave's lead-free solder is anything to do with lowering the resistance, as Dave suggested. I think it helps prevent copper ions diffusing into the solder which can cause the solder to become brittle.  This makes it liable to fracture and turn into an intermittent joint. That is something I heard decades ago, so if the explanation is wrong, I hope someone can correct me.

I was implying that it would only have a smidgen difference, in proportion to it's ratio.
It's not in there to lower the resistance, but for metallurgy reasons of course.

Dave.
 

Offline ftransform

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Re: EEVblog #319 - Lead Free PCB Tinning
« Reply #6 on: July 22, 2012, 06:44:56 am »
is this a response to the thermal imager video made by mike?

the arms race goes crazy, simply way out of control.
Whats next???? Where will this competition lead to????????

tinning tactical smart missiles? phase-plasma pulse rifles? sonic electronic ball breakers? sharp sticks?
« Last Edit: July 22, 2012, 06:47:54 am by ftransform »
 

Offline free_electron

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Re: EEVblog #319 - Lead Free PCB Tinning
« Reply #7 on: July 22, 2012, 07:58:16 am »
The reason for the copper is the intermetallic layer that is need for a bond to be created.


The bond in regular leaded solder is copper To tin. An top of the copper pad a layer of CuSn is grown ( dont remember exactly how many of each. Believe it's cu6sn4 , i'm no chemist or metallurgist)

Depending on temperature there may be a layer fo a different CuSn allow between Cu and Cu6Sn4. This layer is brittle and cracks easy under mechanical stress.
As the solder joint ages this layer becomes thicker. After half a year a few extra micron of CuSn has formed. The higher ambient temperature the faster this goes. That is why hot solder joints fail over time. The CuSn keeps growing and breaks off under mechanical stress.

Now, on top of the CuSn layer there is now a shortage of tin ( it has bonded to copper. ) so there we get an almost pure lead layer that then diffuses back to SnPb.

The more iron is thrown in the mix the faster  this CuSn is formed ( were talking fractions of a second during solder being liquid. )

The more tin a solder alloy contains the faster it steals iron.In classical tin lead soldering the tin strips  part of the iron plating of the soldertip ( this is necessary otherwise no bonding to the pins of parts). This is why lead free soldertips have a thicker iron plating. So they last the same amount of time.

lead free solders run at higher temperature. The higher the temperature the faster you steal copper and the thicker the CuSn layer would be. So in SAC 305 they pre saturate the alloy with copper and the silver takes the place of lead to form the crossoover from CuSn to Sn ( just like you had a pure Pb layer in leaded solder you have a pure Ag layer in SAC soldering.

Now it gets more complicated. If you have surface that have nickel , old or palladium ( enig or enepig plating ) then additional intermetallic layers are formed. Nickel bonds to tin, and the layer does not grow post soldering..

Tin bonds better to nickel than to copper , but nickel corredes like the pest so we need to throw gold on it to protect it , which lower shearing strenght so we add palladium in the mix ...

The reason for all this creazy metal in the mix stuff is to increase tensile strenght and shearing resistance of the joint.
Now, we only looked at copper to solder bonding. What about solder to pin of the component ? Most ic pins are an alloy called kovar and thats where the iron from the solder tip comes into play.

Its' all very complicated stuff...

SAC305 is now widely considered the 'garbage' of the lead free solders.. Snib , Snag , Sn99c Sn100c are other alloys that work better. Especially Sn99c and  sn100 are really good . Less microcracks because the CuSn layer is thinner.

Snxxc alloys are good for wave soldering as there is almost no dross formation ( dross is a kind of scum that forms on top of the wave and doesnt flow and causes brittle joints. Its like bad solder.
Sac305 is horrible for that.

Picking solder is a science in its own. You need to know what is the board surface , is it wave where the wave is hot for hours and we dont want solder to go bad , or reflow
What flux is in the mix ? What is final application ? Hot ? Cold. .some solders work better than others for certain post production ambient operation.

I got some documents on all that stuff. Pretty complicated, lots of research and a bunch of contradictions as well..
Professional Electron Wrangler.
Any comments, or points of view expressed, are my own and not endorsed , induced or compensated by my employer(s).
 

Offline SeanB

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Re: EEVblog #319 - Lead Free PCB Tinning
« Reply #8 on: July 22, 2012, 08:15:53 am »
Silver is a much better conductor.............. I use a 5% silver solder to do piping, or a phosphorous containing copper rod for fluxless brazing. Does not work on electronics, unless they are silicon carbide chips that can handle the heat.

The Manhattan project needed tons of wire for the electromagnets used to separate the isotopes for enrichment. They used silver from the US Mint as copper was not available, and remelted it afterwards back to ingots.
 

Offline Psi

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Re: EEVblog #319 - Lead Free PCB Tinning
« Reply #9 on: July 22, 2012, 08:27:58 am »
wilfred,  two irons means you don't have to keep changing the tips.

You can have a large general tip and a smaller SMD tip available all the time.
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Offline EEVblog

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Re: EEVblog #319 - Lead Free PCB Tinning
« Reply #10 on: July 22, 2012, 09:23:43 am »
is this a response to the thermal imager video made by mike?

No, I shot this a few days ago, before Mike did his video.

Dave.
 

Offline EEVblog

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Re: EEVblog #319 - Lead Free PCB Tinning
« Reply #11 on: July 22, 2012, 09:29:34 am »
And, just out of curiosity, why do you have two soldering irons? I noticed you were not using the one nearest the work you were doing.

I've mentioned that in the soldering tutorial, so I can lift components etc.
The old Hakko had a bigger tip fitted, so I used that instead of the FX-888.

Quote
Isn't it about time for another power supply video?

Maybe, but I have so many others to do as well.
Two new Rigol products coming in the next few days, the Mantis, the BK load, the Agilent scope and PFANG, ad the LCR meters all hanging around for reviews or more talk. That's just on the equipment front!

Dave.
 

Offline Bored@Work

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Re: EEVblog #319 - Lead Free PCB Tinning
« Reply #12 on: July 22, 2012, 10:17:58 am »
Hire an apprentice or send some stuff to Mike so he can break it :)
I delete PMs unread. If you have something to say, say it in public.
For all else: Profile->[Modify Profile]Buddies/Ignore List->Edit Ignore List
 

Offline ejeffrey

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Re: EEVblog #319 - Lead Free PCB Tinning
« Reply #13 on: July 22, 2012, 01:32:46 pm »
Based on my fairly limited education in chemistry I would think the lead would actually be more conductive.  Lead and tin have the same number of electrons in their outermost shell but lead, being one period down on the table has a "less tight grip" on its outer shell of electrons...

It is quite a bit more complicated than that.  You can't really extrapolate the atomic binding energy to the behavior of electrons in a metal lattice.  In fact, despite the strong potential of the nucleii, conduction electrons behave nearly free in a metal, any picture that has them bound even loosely to individual nuclei is not going to give you the right intuition.  The carrier density is certainly a big factor, but remember that while tin and lead have the same outer shell structure, lead has a larger atomic radius and therefore has fewer electrons / cm^3.  The other big factors are dependent on the crystal lattice structure: the effective mass and phonon scattering.  Basically all of this comes to mean that while elements from the same groups often have similar conductivity, it is very hard from just looking at the periodic table to predict quantitative differences in conductivity.
 

Offline rusle

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Re: EEVblog #319 - Lead Free PCB Tinning
« Reply #14 on: July 22, 2012, 03:44:19 pm »
I have done a quick joggling with the rho value of the different metals in a spreadsheet and if I have done it correctly there should have been a decrease of almost 19% in resistance when going from 60/40 soldering tin to Pb free.


I have of course  calculated that Dave got an extremely steady hand and the amount of soldering tin is evenly distributed along the PCB.
The temperature is exact the same, etcetera.

A lot of factors which should give Dave's cows plenty of time to get home if he should take them all in the practice of his experiment. :)

After looking at the values, I would say the best ting would be to solder a copper wire to the PCB, but it would not be popular in the production. (yes I have done it because I did not trust the soldering trick alone would be enough)
 

Online ciccio

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Re: EEVblog #319 - Lead Free PCB Tinning
« Reply #15 on: July 22, 2012, 05:41:34 pm »
I use this technique ("reinforcing" copper areas whit solder by leaving them  free from solder mask and having the solder bath cover them with solder) a lot, as do many designer of analog circuits,  expecially on single-sided boards.
The decrease of resistance is not very high:  may be calculated (I'll do some fast, inaccurate calculations)
- copper has a specific resistivity (ro) of about 1.7 x 10^-8 ohm per meter,
- tin and lead are about 1.8 - 2.2 x 10^-7 ohm per meter (one order of magnitude higher) , I don't know the specific resistivity of the solder alloy, but it must be in this order of magnitude.
-in order  to get the same resistance of the copper track (resulting in half the total resistance) the solder thickness   must  be 10 times the copper's.
This means that a 2 oz copper (70 micron) mus be reinforced with a 700 micron (0.7 mm). This is really thick, but it is possible to get it.

What I get with this "free" trick  is another bonus: lowering the resistance (even in the order of 10-20%) will reduce the temperature increase, and the risk of track de-lamination, especially when the track size  used is smaller that the optimal one for the current value involved.
Another bonus: the impedance of the ground ("common") track will decrease, reducing hum in unregulated power supplies (you can measure this, I've done this many times).

This is my humble opinion.
Best regards.


Ciccio

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Offline Stephen Hill

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Re: EEVblog #319 - Lead Free PCB Tinning
« Reply #16 on: July 23, 2012, 10:48:02 am »
Shouldn't we be measuring the resistance of the solder on it own? Providing that it's the same length and diameter, you should be able to see the difference in resistance.

 

Offline dcel

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Re: EEVblog #319 - Lead Free PCB Tinning
« Reply #17 on: July 23, 2012, 03:33:14 pm »
Shouldn't we be measuring the resistance of the solder on it own? Providing that it's the same length and diameter, you should be able to see the difference in resistance.

Not necessarily accurate due to the variations in the flux core of the solder wire. Some manufactures employ multi-core and others have a very lax specification on the flux content percentage, IE 3.3%~5-10 tolerance.

Chris
 

Offline eV1Te

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Re: EEVblog #319 - Lead Free PCB Tinning
« Reply #18 on: July 29, 2012, 06:03:23 pm »
Just wanted to add that if you add Copper to Tin you would reduce the conductivity (increase resistivity) since you would introduce more "defects" (on atomic scale) in the material. You would need to have somewhere around 70-80% Copper and 20-30% Tin in order to achieve a resistivity lower then pure Tin). (Same reason why you try to remove even the last ppm of oxygen in Copper sometimes (OFC), since it has a large impact on the resistance, much larger than 1 ppm)

If you work at low temperature (like liquid nitrogen) an 0.7% addition of Copper to Tin would worsen the conductivity by more than an order of magnitude.

 

Offline eV1Te

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Re: EEVblog #319 - Lead Free PCB Tinning
« Reply #19 on: July 29, 2012, 06:14:33 pm »
One more thing I noticed, why would you want a mains-earth referenced soldering iron tip?

My soldering iron and most others that I have seen was either ESD protected from earth by at least 1 Mohm or the tip was entirely floating.
 

Offline T4P

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Re: EEVblog #319 - Lead Free PCB Tinning
« Reply #20 on: July 29, 2012, 06:22:04 pm »
One more thing I noticed, why would you want a mains-earth referenced soldering iron tip?

My soldering iron and most others that I have seen was either ESD protected from earth by at least 1 Mohm or the tip was entirely floating.

The classic FX-888 and the 936 AFAIK is mains earth referenced ... everything's on the same plane so it's pretty safe
 


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