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paste Sn42/Bi58 and Lead Finish

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#67582

paste Sn42/Bi58 and Lead Finish | 2 November, 2012

Hello! We want to try Lead Free paste Sn42/Bi58. Can we use this paste with Lead fihish such as HASL. I am worry about low melting temperature of 138C.

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#67583

paste Sn42/Bi58 and Lead Finish | 2 November, 2012

Sure you can use it it with leaded HASL. Recognize that ... * Your Sn42/Bi58 will no linger be Sn42/Bi58 * Your leaded HASL will no longer be the same leaded HASL that walked in the door to your plant * Lead from HASL coatings can diffuse through the grain boundaries of the alloy. * Some people argue that you'll end-up with a eutectic composition of Bi52Pb32Sn16 in the grain boundaries. Bi52Pb32Sn16 has a melting point of 95*C. [Other people say don't worry.] * Peak of your thermal recipe will be ~170*C * And although bismuth-containing solders may appear new to the average user, they have been in use for a long, long time. Bismuth has a well established history in both tin-lead and lead-free soldering. * Obviously, I don't know how your product will perform for your customer.

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#67586

paste Sn42/Bi58 and Lead Finish | 2 November, 2012

So how does this (lead free paste Sn42/Bi58) solder compare with the Sn-Ag-Cu (Tin-Silver-Copper) combination which does not flow as well (in the oven), needs higher oven temperature, (that can be adjusted by reducing the oven chain speed) and cost more.

*** By the way, since we have used the Tin-Silver-Copper lead free solder there has been a strange liquid type chemical leek from the oven exhausted pipes.

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#67590

paste Sn42/Bi58 and Lead Finish | 3 November, 2012

EricR: Regarding "*** By the way, since we have used the Tin-Silver-Copper lead free solder there has been a strange liquid type chemical leek from the oven exhausted pipes."

... I don't know anything about your process, but the first thing I'd investigate is the potential that you are collecting flux residue in your exhaust air vent. Further, your collection is substantial enough to have essentially filled the pipe and is now running out of the pipe and slubbering on your boards, machine and floor.

Some people set their machine offset from the exhaust pipe and then install a flexible pipe between the machine and the up-pipe that is so long that it touches the floor. [Picture of this like the trap in the plumbing of your sink drain] When the flex pipe gets heavy with flux residue, they dispose of it in accordance with local regulations.

Or you can take down your piping, rent a pressure washer, clean the piping, and dispose of the residue in accordance with local regulations.

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#67591

paste Sn42/Bi58 and Lead Finish | 3 November, 2012

We have experience with this alloy, but with enig and we did start with PB HASL, none of our PCBs would of course pass thermal cycling at normal temperatures with the HASL so we went to ENIG. we could thermal cycle at 100-115C with no issues. We found that with enig, the 42/58 has about the same reliability as 63/37 solder in our applications.

Jb

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#67592

paste Sn42/Bi58 and Lead Finish | 3 November, 2012

EricR: There's substantial information about the properties of solder alloys on the web. Here's an example of something that I clipped from one site

Alloy ||Solidus (°C)||Liquidus (C°) ||Tensile Strength (psi / MPa) Sn42 Bi58||-E-||138||8000 / 55.2 Sn96.5 Ag3.0 Cu0.5 ||217||219||7340 / 50.6

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#67598

paste Sn42/Bi58 and Lead Finish | 6 November, 2012

Hi Popov,

can you tell us the reason for the use of Sn-Bi ?

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#67599

paste Sn42/Bi58 and Lead Finish | 6 November, 2012

Ryan ...

"As for bismuth-based lead-free alloys, a lower melting temperature than that of tin-lead is offered together with a cost similar to that of tin [in the area of $3/lb]. Unfortunately, bismuth in soldering alloys tends to create embrittlement, and if a bismuth-based alloy picks up any lead, the melting temperature will drop again, causing joint embrittlement. At first glance, the bismuth bearing alloys appear to offer high tense strength, but in peel strength test they are prone to failure due to poor fatigue resistance. The same can be said for indium-based lead-free alternatives." [Understanding Lead-Free Alloys, Karl Seelig 1/98]

More detailed information can be found in "Advantages of Bismuth-based Alloys for Low Temperature Pb-Free Soldering and Rework" Brook Sandy, Edward Briggs and Ronald Lasky, Ph.D., Indium Corporation, First Published at SMTA's International Conference on Soldering and Reliability in Toronto, May 2011

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#67601

paste Sn42/Bi58 and Lead Finish | 7 November, 2012

Thanks Dave ,

my question is about Spopov want to try "Lead Free" SnBi (but uses it on Leaded finishes), but is worry about its low melting point (at this point, no longer 138 degrees as he wrote, but rather in an unknown and unpredictable range between 95 and 138 degress); therefore, it seems to me that he did not need any of LEAD-FREE, not even the low melting point. And then, therefore, what is the purpose of the use of Sn-Bi?

(About the statement you mentioned, showing the embrittlement of the alloy, and dated 1998, it has been today overcome with the introduction of a small percentage of Ag, usually 0.4-1% )

Anyway, I see a sudden and increasingly widespread interest, a year to date, of Indium as well as other U.S. producers, in the Sn-Bi .

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#67603

paste Sn42/Bi58 and Lead Finish | 7 November, 2012

Multiple process and rework thermal cycles are brutal on boards and components. Improving reliability of products through reducing thermal stress ends-up being a major driver to the interest in lower MP LF solders. Binary LF solders require the addition of silver in order to improve reliability, but silver is expensive. In SAC305, that 0.5% silver costs more than everything else. * Henkel is talking about tin-zinc alloys [Sn91/Zn9] for higher volume products. * Indium likes 57Bi/42Sn/1Ag

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#67643

paste Sn42/Bi58 and Lead Finish | 14 November, 2012

Ryan, in some cases we use 2 pastes with different melting temperature (LF SAC and PbSn)for complex boards with components on two sides. And we want to try to replace LF SAC with low melting paste.

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#67647

paste Sn42/Bi58 and Lead Finish | 15 November, 2012

Thanks for your reply.

As already said by Anvil1021, is not a good idea to mix Lead and Bismuth! Stay away from Bismuth if you have even the slightest traces of Lead on yours PCB, stencil, squeegee, spatula, blade, cleaning paper or cleaning cloths, stencil cleaning machine etc …

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#67648

paste Sn42/Bi58 and Lead Finish | 15 November, 2012

Maybe this piece of information will help. In China (just returned from visit) there are many factories that are using a dual temperature process using SAC305 or Sn100C and 42/58. Apparently this allows them to run their through hole parts through re-flow instead of wave solder and at a lower temperature. It is apparent that this lower temperature solder is a cost saving move, as re-flow ovens are cheaper than wave solder systems loaded with SAC alloy. Of course this is just my guess as to the reasoning. Regards, Anvil

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#67877

Indium Technical Paper in Newsletter | 26 December, 2012

"Advantages of Bismuth-based Alloys for Low Temperature Pb-Free Soldering and Rework"

Author: Brook Sandy, Edward Briggs and Ronald Lasky, Ph.D.

Abstract: The increased function of personal electronic devices, such as mobile phones and personal music devices, has driven the need for smaller and smaller active and passive components. This trend toward miniaturization, occurring at the same time as the conversion to RoHS-compliant lead-free assembly, has been a considerable challenge to the electronics assembly industry. The main reason for this is the higher reflow process temperatures required for Pb-free assembly. These higher temperatures can thermally damage the PCB and the components. In addition, the higher reflow temperatures can negatively affect the solder joint quality, especially when coupled with the smaller paste deposits required for these smaller components. If additional thermal processing is required, the risk increases even more. First Published at SMTA's International Conference on Soldering and Reliability in Toronto, May 2011...

Link: http://www.smtnet.com/library/index.cfm?fuseaction=view_article&article_id=1848

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