Low-Temperature Solder Paste: A Practical Sn-Bi Guide

Some parts and some boards simply cannot survive a standard lead-free reflow. SAC305 peaks somewhere around 230 to 250 C, and that is enough to cook heat-sensitive LEDs, melt plastic connectors, drift sensors, or warp a large thin board until the corners lift.

Low-temperature solder paste exists for exactly those cases. It is a lead-free paste, usually tin-bismuth, that melts at 138 C and reflows with a peak near 165 to 180 C, roughly 70 to 80 C cooler than a SAC305 profile.

This guide covers what the alloys actually are, the real temperatures, when low-temp paste is the right call and when it is a bad one, how the reflow profile is shaped, and how to print it.

For the full range of solder alloys and melting points, see our solder alloy guide. Here we focus on Sn-Bi.

Quick Answer: What Is Low-Temperature Solder Paste?

Low-temperature solder paste is a lead-free paste, usually tin-bismuth (Sn-Bi), that melts at about 138 C and reflows with a peak near 165 to 180 C, far below the 230 to 250 C peak a SAC305 profile needs. It protects heat-sensitive parts and thin boards and enables step soldering, but bismuth joints are more brittle.

Low-Temp Alloys and Temperatures

The table below sets the low-temperature alloys against the two baselines most engineers already know. Note that SAC305 is near-eutectic rather than eutectic, so it melts across a range, and note that not every Sn-Bi-Ag alloy is a low-temperature paste.

Two rows in that table are worth pausing on, because they are where most confusion starts.

First, silver is not a simple upgrade. A small addition of around 0.5% Ag improves wetting and refines the microstructure, but published work shows that above roughly 1 wt% Ag the Ag3Sn phase grows and ductility actually falls. More silver is not better.

Second, the bismuth content decides everything. Sn-Bi is eutectic near 57 to 58% Bi, which is why Sn42Bi58 melts cleanly at a single temperature. An alloy like Sn64Bi35Ag1 sits far off that eutectic, so it starts softening near 138 C but is not fully liquid until roughly 176 C. That wide pasty range makes it a different tool, not a cooler one.

All of these are lead-free. Bismuth is not a restricted substance under RoHS, so Sn-Bi pastes are RoHS and REACH compatible in the same way SAC305 is.

Why Use Low-Temperature Solder Paste?

It protects heat-sensitive components. LEDs, plastic-bodied connectors, some sensors, cameras, displays and certain electrolytic capacitors have maximum body temperatures well under a 245 C peak. Dropping the peak to 175 C keeps them inside their rating instead of hoping they survive.

It reduces warpage and delamination. Large, thin, or high-density boards bow when heated, because copper, laminate and components expand at different rates. Lower peak temperature means less thermal stress, less bow and twist, and fewer opens on the corners of big BGAs.

It enables step soldering and package-on-package. Solder the first side with SAC305, then run the second side with Sn-Bi at 165 to 180 C. Because that peak sits nearly 40 C below the 217 C point where SAC305 even begins to melt, the first-side joints stay solid and do not slump.

It costs less energy and eases nearby rework. A cooler oven draws less power, and lower heat around a repair site is gentler on neighbouring parts. Low-temp paste is common on LED boards and flex assemblies, and it pairs naturally with the thermal behaviour discussed in our metal-core PCB guide.

The Trade-offs: When NOT to Use It

Low-temperature paste is a real engineering trade, not a free upgrade. These are the reasons to walk away from it.

Never mix Sn-Bi with leaded (Sn-Pb) solder. This is the single most important line in this article. Bismuth, tin and lead together form a ternary eutectic measured by NIST at 95.3 +/- 0.5 C. A joint contaminated that way can soften or fail at temperatures a product might genuinely reach in service. Keep leaded and Sn-Bi pastes, stencils, squeegees and rework tools strictly separate.

That hazard is not only about leaded paste. A leaded HASL surface finish is also a lead source, so check the board finish and component terminations before committing to a low-temp build.

Bismuth joints are more brittle. Eutectic Sn-Bi has a microstructure of ductile beta-Sn mixed with a genuinely brittle bismuth phase, and that shows up as poorer drop, shock and mechanical-stress performance than SAC305. A small silver addition helps, but it does not turn Sn-Bi into a rugged alloy. Avoid it on handhelds, wearables, automotive and anything drop-critical unless the design has been qualified for it.

Compatibility is not automatic. Not every component finish or board finish behaves well with a low-temp alloy, and Sn-Bi can be oxidation-prone, which puts more weight on flux chemistry and on paste storage and handling.

Voiding and material control. Large low-standoff parts such as DPAK can trap outgassing under the tab and void. And because low-temp paste is not the default material in most shops, it needs real inventory discipline so it never gets mixed up with the standard reel of SAC305.

The short decision rule: excellent for heat-sensitive assemblies, thin warping boards and step soldering. Avoid it where mechanical robustness is the priority.

The Low-Temperature Reflow Profile (Sn-Bi)

The shape of a low-temp profile is the same as any reflow curve. Only the numbers move down. Treat the following as typical and always follow the profile on your paste datasheet.

Preheat runs from room temperature to roughly 90 to 110 C, ramping at about 1 to 3 C per second. A soak follows, holding long enough to activate the flux and even out the board temperature before the alloy melts.

The peak sits around 165 to 180 C. The useful way to think about that number is that a reflow peak generally lands 25 to 30 C above the alloy's melting point, and 138 plus 25 to 30 puts you in exactly that window. Time above liquidus is short, usually well under a minute, and cooling is controlled rather than abrupt.

Set against a SAC305 curve, the whole profile drops by roughly 70 to 80 C at the peak. That gap is the entire point of the alloy, and it is what keeps first-side SAC305 joints solid during a second-side low-temp pass.

Do not overcorrect downward. Too low a peak, or too short a time above liquidus, leaves incomplete reflow and dull, grainy joints, because the flux never fully activates in the lower window. Our reflow soldering guide covers profile setup in more depth.

Printing Low-Temp Paste: Stencil Considerations

The stencil rules do not change with the alloy. Match foil thickness and aperture size to the finest pitch on the board and keep the area ratio at or above 0.66, exactly as you would for SAC305. What changes is the tolerance for error, because a low-temp joint has less thermal headroom to pull a marginal deposit into shape.

Sn-Bi paste can also be oxidation-prone, so clean release and crisp print definition matter. Electropolished and nano-coated apertures improve paste release, and our comparison of nano-coating and electropolishing explains when each is worth specifying.

Handle the material properly too. Paste storage, warm-up and working life all affect printability, and our guide on how to use solder paste covers the handling side.

Stencil selection itself is covered in how to choose a solder paste stencil, and the foils themselves come off our precision stencil manufacturingline.

Common Problems With Low-Temp Solder and Fixes

• Brittle or cracked joints - bismuth brittleness under mechanical stress - move to an Ag-bearing Sn-Bi, reduce mechanical stress at the joint, and avoid drop-critical use.
• Voiding under large low-standoff parts - trapped outgassing with nowhere to escape - tune the soak, and use window-pane apertures instead of one large opening.
• Poor wetting or dull, grainy joints - oxidation or weak flux activation - use fresh paste, the right flux chemistry, and confirm the peak is genuinely reaching target.
• Very low remelt or unexpectedly soft joints - accidental mixing with leaded solder - never mix, and segregate materials and tools.
• Incomplete reflow - peak too low or time above liquidus too short - raise the peak toward about 175 C and extend time above liquidus slightly.

Our guide to preventing solder defects during reflow works through the diagnosis process in more detail.

FAQ about Low-Temperature Solder Paste

Conclusion: Use Low-Temp Paste Where It Fits

Sn-Bi low-temperature paste reflows roughly 70 to 80 C cooler than SAC305, which makes it a genuinely good answer for heat-sensitive components, thin boards that warp, and step or sequential soldering.

The conditions attached to it are just as real. Bismuth joints are more brittle, so keep it away from drop-critical products, and never let it meet leaded solder, because the resulting ternary eutectic melts near 95 C.

Match the alloy to the board, print it through a clean and consistent stencil, and profile it properly. Or hand the whole job to an assembly partner and let them run it.