Andrew:
This was a reasonably civil/respectful forum until about 24 hours ago. I even recall Gordon
Davy apologizing in this forum for some perceived insensitivity in one of the more polite
objections ever written.
Which brings us to your wording, particularly the literary flailing of Harvey. As the person
who raised the matter of pasty range in the first place, I guess it's my place to expand on
the issue that you've so cavalierly dismissed.
The issue of a pasty (or "plastic") range relates entirely to wave soldering. Specifically, it
concerns what happens as the circuitry leaves the solder pot. If the pasty range falls within
temperatures that the solder may reach during the assembly's exit from the solder wave, the
probability of bridging and other forms of excess solder increases.
After decades of use, we know that tin/lead alloys in the range of (not precisely at) 63/37
produce no to very narrow pasty temperature ranges. Probably more significant, most companies
run their wave solder pots at excessively high temperatures, which helps explain why
deviations in purity of the tin/lead alloy generally do not cause havoc. On the other hand,
there are limits to how much deviation is acceptable. There is a reason why 60/40 solder was
employed for wire solder but not in solder pots. And the reason is bridging.
The problem becomes more serious as leads are placed closer together and through-hole
components fade into extinction.
So much for the history lesson. Now to the point I (and, I believe, Harvey) was trying to make
with respect to lead-free solders in wave soldering.
Unlike tin/lead alloys, there is little experience with the behavior of lead-free alloys.
With sufficiently high process temperatures, there's little doubt that the pasty range can be
avoided. The question is: how much extra heat above the eutectic point is needed to avoid
bridging and other forms of excess solder? After all, we're already talking about melting
temperatures approximately 50 degrees C higher than for the old standby.
Those who have referenced existing products assembled with high-temperature solder are missing
my point. Those products are assembled in manners other than wave soldering and, therefore, do
not run into the pasty range issue. There's nothing mysterious about those lead-free products;
the industry has been turning out products with high-temperature solders for decades.
To sum up:
1. Pasty range is a concern almost exclusively limited to wave soldering
2. We have vast experience using tin/lead alloys in wave soldering
3. Our experience shows that excess solder problems (most notably bridging) increase as the
deviations from eutectic tin/lead increase
4. There's little information (that I, at least, have seen) about the forgiveness of lead-free
alloys in wave solder uses
5. Until more work is done with lead-free solders in wave soldering, there's cause for concern
6. The more vitriolic the wording, the less confidence I have in the message
7. Unrestrained sarcasm demeans the writer and the forum
So endeth the lesson.
Sincerely,
Jim Smith
Managing Director
Cambridge Management Sciences, Inc.
4285 45th St. S.
St. Petersburg, FL 33711-4431
Tel: (727)866-6502 ext. 21
Fax: (727)867-7890
eMail: [log in to unmask]
Andrew Hoggan wrote:
> Thanks Harvey,
>
> nicely put.
>
> So to phase diagrams - lies, damn lies, statistics and phase diagrams!
>
> OK I'll admit it I'm an organic chemist not a metallurgist (this'll make
> some friends happy), I thought the 63/36 phase diagram was a map of
> Bourgogne till I followed it and ended up in New Jersey.
>
> I don't know, I've never taken the decade off paying the mortgage to carry
> out the work (wife wouldn't let me) to confirm the results of 63/37 as
> manufactured by .... sorry I lost count I don't have enough fingers to count
> up the number of suppliers out there. Never mind their all using virgin tin
> so I'm sure the analysis will be spot on.
>
> In operation, lets see the 63/37 is contaminated with copper, silver,
> palladium, gold (not sure of the rest - wait a minute forgot nickel and good
> old pasty 60/40)- take account of throughput rate, drag out and
> replenishment, we get equilibrium. Then someone changes the board cycle
> (lucky buggers got a new account) so that screws that up , but then gosh
> (with a little bit of luck) equilibrium is achieved again, but wait, isn't
> the composition different from the previous run - could be! How does that
> effect the performance, but hehe, we've 4000 years of using tin lead behind
> us with absolutely noooooo problems, so that's OK. Anyway we analyse the pot
> every month, 3 months, 6 months, 12 months, pot analysis?
>
> Heh, that's good old 73/36, that could never be the case with other alloys
> could it?
>
> Tell you Harvey, you have some points but you're pointing in the wrong
> direction. The issue won't be the pasty range of the alloys, I've seen some
> nasty, nasty differentially cooling 62/36/2, I don't see planes trains or
> auto's falling out of the skies any more than they do now - for that reason.
>
> What really worries me is the research (yeah, these guys weren't married,
> didn't have mortgages) that indicates the so called improvement in strength
> by utilising lead-free alloys ShOuTeD quietly from the rooftops by other
> researchers (no names, no embarrassment, no litigation....) is conditional,
> not absolute.
>
> In other words, the initial testing carried out was limited it indicated
> (specific) lead-free alloys gave improvements in physical performance over
> tin lead (63/37) alloy. Unfortunately (and this is the really worrying part)
> if you were to take the same alloys and run the testing past the 1000 hours
> or change the cycle rate or change the stress and/or the frequency applied,
> the results don't indicate performance improvements........!
>
> Now that scares me when I'm driving my Merc 230K at 140mph, worst still it
> scares when someone else is driving by my kid at 30mph......
>
> But look Ford have approved lead-free alloys after a three year programme so
> it must be OK.
>
> Seriously though, do some research, get surfing. Check out a couple of the
> links on my website (I built it, it's mine!!!!).
>
> Best regards,
>
> Andrew Hoggan
> BBA Associates Ltd
> www.bba-associates.com
>
> -----Original Message-----
> From: [log in to unmask] [mailto:[log in to unmask]]
> Sent: 01 August 2001 04:29
> To: [log in to unmask]; [log in to unmask]
> Cc: [log in to unmask]
> Subject: Re: [LF] Eutectic alloys?????
>
> Andrew Hoggan
> I am sure that you were never even on a banana boat on the River Clyde.
> And I thank you for pointing out that under- hood and airframe electronics
> are important niches where high temp alloys do fit. Also, it occurs to me
> that they use ceramic packages/packaging substrates that start out with
> Level
> 1 moisture sensitivity. So maybe they degrade a level or two, they are
> probably reliable enough. But all this would not apply to the passenger
> compartment electronics. I do want to check with friends at Rockwell
> Collins
> before conceding.
>
> Re pure ____ for 63-37, I note from phase diagrams that the pasty zone
> is
> much smaller and well-contained than it is for the SnAg, SnCu, and SnAgCu
> alloys when composition deviates from eutectic. I am still trying to
> understand the effects on production throughput, disturbed joints, and ____
>
> Also will the higher surface tension of lead-free alloys affect lead,
> solder ball, and flip chip package self-location. Tin lead works so well.
> Anyone?
>
> I am certainly willing to agree that lead-free has a place, not because
> itis free of lead, but because of other characteristics conferred. But, when
> all the shouting is over, in 2010, SnPb will dominate.
>
> By the way, would you want to be defibrillated lead-free?
>
> Harvey Miller
>
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