RoHS Combined with non-RoHS...

[Rickster C]

What happens if RoHS, no Pb processes are mixed with leaded assembly? Someone is telling me this is a real problem where the solder joint can solidify with different compositions, creating stresses that result in joint failures.

The context was specifically BGA balls where there would be a significant amount of solder in the ball combined with a significant amount of solder paste on the board. When assembling SM QFP or QFN type packages (and various passives) will this be much of a factor? If the solder paste has Pb and the pins are tinned only, I can\'t see it resulting in much of a problem. But then I\'m not a metallurgist. Any metallurgists in the group? Anyone with experience in this?

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Rick C.

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[server]

On Thursday, 12 November 2020 21:13:12 UTC, Rickster C wrote:

What happens if RoHS, no Pb processes are mixed with leaded assembly? Someone is telling me this is a real problem where the solder joint can solidify with different compositions, creating stresses that result in joint failures.

The context was specifically BGA balls where there would be a significant amount of solder in the ball combined with a significant amount of solder paste on the board. When assembling SM QFP or QFN type packages (and various passives) will this be much of a factor? If the solder paste has Pb and the pins are tinned only, I can\'t see it resulting in much of a problem. But then I\'m not a metallurgist. Any metallurgists in the group? Anyone with experience in this?

There are companies that specialise in reballing BGA packages with tin/lead
solder for military applications, so there probably is a real issue when
systems have to work over very wide temperature ranges. For QFP and QFN
packages the relative amounts of the two types of solder don\'t seem to cause
any problems. The issue is likely to be that mixtures of the two are non-
eutectic and solidify over a wide temperature range giving low-strength
joints.

John

 

[Joe Gwinn]

On Thu, 12 Nov 2020 14:03:40 -0800 (PST), jrwalliker@gmail.com wrote:

On Thursday, 12 November 2020 21:13:12 UTC, Rickster C wrote:
What happens if RoHS, no Pb processes are mixed with leaded assembly? Someone is telling me this is a real problem where the solder joint can solidify with different compositions, creating stresses that result in joint failures.

The context was specifically BGA balls where there would be a significant amount of solder in the ball combined with a significant amount of solder paste on the board. When assembling SM QFP or QFN type packages (and various passives) will this be much of a factor? If the solder paste has Pb and the pins are tinned only, I can\'t see it resulting in much of a problem. But then I\'m not a metallurgist. Any metallurgists in the group? Anyone with experience in this?

There are companies that specialise in reballing BGA packages with tin/lead
solder for military applications, so there probably is a real issue when
systems have to work over very wide temperature ranges. For QFP and QFN
packages the relative amounts of the two types of solder don\'t seem to cause
any problems. The issue is likely to be that mixtures of the two are non-
eutectic and solidify over a wide temperature range giving low-strength
joints.

The reason that the liitary insist on lead-bearing solder is tin
whiskers, which will short random things out.

It\'s *very* exciting when a 50-kilowatt 300 Vdc power panel with
tin-coated busbars shorts out due to a tin whisker starting an arc
between the bars. Nobody was hurt, but the power panel was destroyed.
Probably was some collateral damage too.

Joe Gwinn

 

[TTman]

On 12/11/2020 21:13, Rickster C wrote:

What happens if RoHS, no Pb processes are mixed with leaded assembly? Someone is telling me this is a real problem where the solder joint can solidify with different compositions, creating stresses that result in joint failures.

The context was specifically BGA balls where there would be a significant amount of solder in the ball combined with a significant amount of solder paste on the board. When assembling SM QFP or QFN type packages (and various passives) will this be much of a factor? If the solder paste has Pb and the pins are tinned only, I can\'t see it resulting in much of a problem. But then I\'m not a metallurgist. Any metallurgists in the group? Anyone with experience in this?

We never mixed lead free/ leaded. For military etc, everything that was
only available as Pb free was dipped, cleaned and Pb dipped. BGA
reballing was common. Not cheap...

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