14
\$\begingroup\$

I am about to try my first "reflow skillet" soldering job, and as I look at the available types of solder paste I see there are lead-free pastes with much lower melting temperatures than others.

For example, this one from ChipQuik.

The advantages seem obvious, but somehow the marketing literature does not mention any drawbacks to this type of solder paste. In the quantities I would order the price seems about the same. Is there a reason this Sn42Bi58 formula hasn't become standard?

\$\endgroup\$
4
  • 1
    \$\begingroup\$ Too expensive for production perhaps? \$\endgroup\$
    – Olin Lathrop
    Commented Jan 7, 2012 at 14:04
  • \$\begingroup\$ @OlinLathrop FYI 7 years on (due to a reader's recent 'questions'). The alloy is sold mainly as a desoldering solution because, when used to resolder an existing lead based solder joint, the resultant alloy can then be mechanically picked apart easily with a sharp tool. The solder CAN be used in very controlled situations by competent assemblers when lead poisoning can be guaranteed not to occur - IBM used it at one stage. \$\endgroup\$
    – Russell McMahon ♦
    Commented Jan 7, 2019 at 4:24
  • \$\begingroup\$ @OlinLathrop Many examples of sale for desoldering use here \$\endgroup\$
    – Russell McMahon ♦
    Commented Jan 7, 2019 at 4:26
  • \$\begingroup\$ @RussellMcMahon the OPs link is a dead link, but they mention "solder paste", not "desoldering". Your link pulls up a desoldering product, not solder paste. "Chip Quik� Surface Mount Desoldering Kit" \$\endgroup\$
    – johny why
    Commented Jan 8, 2019 at 19:06

2 Answers 2

Reset to default
22
\$\begingroup\$

42/58 Tin / Bismuth is not unknown as a low temperature solder but has issues.

While widely used for some very serious applications (see below) it is not a mainstream industry contender for general use. It is not obvious why not given its substantial use by eg IBM.

Identical to the Bi58Sn42 solder you cite is:

  • Indalloy 281, Indalloy 138, Cerrothru.

    Reasonable shear strength and fatigue properties.

    Combination with lead-tin solder may dramatically lower melting point and lead to joint failure.

    Low-temperature eutectic solder with high strength.

    Particularly strong, very brittle.

    Used extensively in through-hole technology assemblies in IBM mainframe computers where low soldering temperature was required.

    Can be used as a coating of copper particles to facilitate their bonding under pressure/heat and creating a conductive metallurgical joint.

    Sensitive to shear rate.

    Good for electronics. Used in thermoelectric applications.

    Good thermal fatigue performance.

    Established history of use.

    Expands slightly on casting, then undergoes very low further shrinkage or expansion, unlike many other low-temperature alloys which continue changing dimensions for some hours after solidification.

Above attributes from the fabulous Wikipedia - link below.

According to other references it has low thermal conductivity, low electrical conductivity, thermal embrittlement issues and potential for mechanical embrittlement.

SO - it MAY work for you, but I'd be very very very cautious about relying on it without very substantial testing in a wide range of applications.

It is well enough known, has obvious low temperature advantages, has been widely used in some niche applications (eg IBM mainframes) and yet has not been welcomed with open arms by industry in general, suggesting that it's disadvantages outweigh advantages except perhaps in areas where the low temperature aspect is overwhelmingly valuable.

Note that the chart below suggests that flux cored versions seem to be specifically unavailable either as wire or as preforms.

Comparison chart:

enter image description here

The above chart is from this superb report which however does not provide detailed comment on the above issues.

Wikipedia notes

  • Bismuth significantly lowers the melting point and improves wettability. In presence of sufficient lead and tin, bismuth forms crystals of Sn16Pb32Bi52 with melting point of only 95 °C, which diffuses along the grain boundaries and may cause a joint failure at relatively low temperatures. A high-power part pre-tinned with an alloy of lead can therefore desolder under load when soldered with a bismuth-containing solder. Such joints are also prone to cracking. Alloys with more than 47% Bi expand upon cooling, which may be used to offset thermal expansion mismatch stresses. Retards growth of tin whiskers. Relatively expensive, limited availability.

Motorola's patented Indalloy 282 is Bi57Sn42Ag1 . Wikipedia says

  • Indalloy 282. Addition of silver improves mechanical strength. Established history of use. Good thermal fatigue performance. Patented by Motorola.

Useful lead free solder report - 1995 - nothing to add on above subject.

\$\endgroup\$
16
  • \$\begingroup\$ This answer is hard for me to read, cuz the OP asked about lead-free, and you keep mentioning leaded alloys. Created confusion for me. \$\endgroup\$
    – johny why
    Commented Dec 27, 2018 at 21:08
  • 2
    \$\begingroup\$ @johnywhy Maybe your comment was placed on the wrong answer. It does not match my answer in any way. My ONLY references to lead solder are 1. A warning that the solder asked about can cause joint failures when used on joints that have already been soldered with lead-based solders. An important warning. 2. The same warning in more detail in a note. Note that this relates to parts that have been PRE-TINNED with lead solder. If you use such with this solder the joint will probably fail, ... \$\endgroup\$
    – Russell McMahon ♦
    Commented Jan 6, 2019 at 18:16
  • 1
    \$\begingroup\$ @johnywhy ... viz "A high-power part pre-tinned with an alloy of lead can therefore desolder under load when soldered with a bismuth-containing solder." \$\endgroup\$
    – Russell McMahon ♦
    Commented Jan 6, 2019 at 18:17
  • \$\begingroup\$ your warnings are only "important" if the original poster asked about that scenario. Which they didn't and no reason to assume they plan on doing that. Otherwise just creates confusion. \$\endgroup\$
    – johny why
    Commented Jan 7, 2019 at 1:23
  • 1
    \$\begingroup\$ @johnywhy It seems unlikely that we'll be able to bridge our communication gap :-). - The OP says: "... the marketing literature does not mention any drawbacks ... Is there a reason this Sn42Bi58 formula hasn't become standard?..." -> My answer directly addresses this core question with an extremely important answer. This may be summarised as "In tightly controlled environments under expert management the product may be useful (eg IBM use). However, if lead "poisoning" is possible, as is commonly and semi randomly the real world case, then the joints are likely to fail mechanically". \$\endgroup\$
    – Russell McMahon ♦
    Commented Jan 7, 2019 at 4:10
5
\$\begingroup\$

The only thing that springs to mind is that some components may get hotter than the solder and melt it?

It'd be quite rare for that to happen, but supposing you had a component which used some pins as a heatsink (some use ground pins as this), and it got hotter than the solder could cope with - the solder would melt, the connection would break down, the heat sink would fail, and the component would fry.

- This is just my thoughts, so is probably completely wrong ;)

\$\endgroup\$
1
  • \$\begingroup\$ Actually that sums up the most important point. \$\endgroup\$
    – needfulthing
    Commented Feb 26, 2019 at 19:00

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.