2
\$\begingroup\$

Does regular 60/40 solder have a practical lower bound on temperature that the solder joint can withstand? That is, will solder joints become cracked below a certain temperature?

\$\endgroup\$

2 Answers 2

3
\$\begingroup\$

The issue isn't typically low temperature, it is the temperature cycling between hot/cold. This causes a thermal stress crack in some solder joints.

This means that there isn't a "lower bound" at which the solder will crack, but it depends on how hot the joint gets during operation, then cycling back down. For example if you have a circuit inside the engine compartment of a car, you could go from 0°C to ~80°C pretty quick, and then back again as the car speeds up and slows down or stops. These thermal stresses cause cracking.

\$\endgroup\$
6
  • \$\begingroup\$ Any way to mitigate that? What do they do for spacecraft and satellites? \$\endgroup\$
    – Mike
    Commented Mar 23, 2020 at 16:42
  • 2
    \$\begingroup\$ In space they use heating/cooling systems to help keep electronics from being exposed to direct/extreme thermal changes. According to NASA-STD-8739-2-2 they use SN60 or SN63 flux-cored solder. For higher temp applications, they use something like SN96AG04A. \$\endgroup\$
    – Ron Beyer
    Commented Mar 23, 2020 at 16:55
  • 2
    \$\begingroup\$ They also support the wires to limit movement \$\endgroup\$
    – Solar Mike
    Commented Mar 23, 2020 at 16:56
  • 1
    \$\begingroup\$ 60/40 (tin/lead ratio) is not accidental and is the so-called "eutecticum" in which both metals are in the same phases and this is the lowest melting point of a two-component alloy. \$\endgroup\$
    – Peter MP
    Commented Mar 23, 2020 at 17:20
  • 1
    \$\begingroup\$ That's not quite right. It's the difference in CTE (coefficient of thermal expansion) between the component and it's attachment (leads, balls, etc) and the printed circuit board that stresses the solder joint when the temperature changes. If you take a component lead or ball by itself and solder/attach it to a board, then temperature cycle the assembly many, many times, you'll find the solder joint remains intact. \$\endgroup\$
    – SteveSh
    Commented Mar 23, 2020 at 18:25
0
\$\begingroup\$

I wrote this up for another discussion, but never posted it

Temperature swings are bad from a reliability standpoint in that it is temperature changes that stress interconnects, particularly those involving IC’s or even discrete semiconductors. These temperature changes induce a stress on the interconnect between the component and the board due to the differences in Coefficients of Thermal Expansion (CTEs) between the component and the board. For example a typical FR4 PCB has a CTE of ~15 ppm, while a BGA package might have a CTE closer to 6 ppm. These differences in CTE cause a stress to be exerted on the solder joints that attaches the part to the board as the temperature changes. These stresses are proportional to the changes in temperature and the size of the package and over time, given enough temperature cycles, can lead to a fracture of the solder joint or attachment to the board.

Leaded parts, such as the old 14/16/20 pin flat packs are much more forgiving in this environment than are rigidly attached packages such as Ball Grid Arrays (BGAs) because the leads of the former provide a significant amount of compliance that reduces the stress on the solder joint.

Mechanical engineers have developed equations to quantify this failure mechanism, and this goes into the structural analysis for the design. If that analysis shows that a component's attachment lacks sufficient structural margin, then steps have to be taken to improve that margin. These steps may include:

1) Improving the thermal environment so as to reduce the temperature swings

2) Adding structural enhancements such as underfill to BGA types of parts

3) Replacing a direct attach part with a leaded part

\$\endgroup\$

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.