Low Temperature Solder: Low Temperature Lead Free Soldering on Flexible Circuits

Soldering a flexible circuit with a lead free alloy is a compromise between the process temperature and the materials of the cable. A conventional lead free alloy melts near 217 degrees Celsius, which means a peak above 240, and that is above the limit of some coverlay adhesives and of the connectors commonly used on flex. Lowering the temperature protects the cable, but it changes the alloy and therefore the properties of the joint.

The alternative is a low temperature lead free alloy, usually based on tin with bismuth or with indium. These alloys melt between about 138 and 180 degrees Celsius, which brings the process back into the range that a polyimide flex and a plastic connector can tolerate. The question is whether the joint they produce is good enough for the application.

Why Temperature Is the Constraint

Polyimide itself withstands soldering temperatures, but the adhesive that bonds the coverlay and the connector housing does not. A coverlay adhesive that softens at 200 degrees will lift at the edges of a pad when the flex is reflowed, and the lift is often invisible at first and grows during subsequent thermal cycles. A connector moulded from a liquid crystal polymer tolerates the temperature better than one moulded from a lower grade plastic, but the lower grade is cheaper and is often used.

The second reason is dimensional. A flex cable expands when it is heated and does not return exactly to its original dimensions, so a high temperature process introduces a shrinkage that affects the position of the pads at the connector. On a fine pitch cable where the registration between the flex pads and the board pads is already tight, that movement can be enough to cause an open or a bridging joint. A lower process temperature reduces the movement and preserves the alignment.

The Low Temperature Alloys

Tin bismuth alloys melt around 138 degrees Celsius, which is low enough to be attractive for temperature sensitive assemblies. The joint they form is harder and more brittle than a tin silver copper joint, and bismuth is not compatible with lead in the sense that a small amount of lead contamination changes the melting behaviour. A board that carries a leaded solder finish or a leaded component must therefore be assessed before a bismuth alloy is used.

Tin silver bismuth alloys with a small amount of copper melt in the region of 180 to 200 degrees Celsius. They are less brittle than the binary tin bismuth alloy and are more tolerant of contamination, at the cost of a higher process temperature. The choice between them follows from the sensitivity of the cable, from the mechanical requirement of the joint and from the thermal cycling the product will see.

Joint Quality and Its Limits

A low temperature joint is not automatically weaker than a higher temperature one, but its failure modes differ. The bismuth containing alloys are more prone to brittle fracture under mechanical shock, which matters for a flex cable that is handled during assembly and that may be bent in use. They are also more sensitive to the cooling rate, because a fast cool produces a finer structure and a slower cool a coarser one.

The thermal fatigue behaviour is the other consideration. Under repeated temperature cycling the joint must accommodate the expansion mismatch between the flex and the board, and a brittle alloy accommodates less before cracking. This is why a low temperature alloy is usually selected for a joint that is mechanically supported, either by an adhesive fillet or by a stiffener, rather than for one that has to carry the whole strain.

Process Control for a Low Temperature Profile

A lower peak temperature does not mean a less demanding process. The window between the melting point and the temperature at which the coverlay or the connector degrades is narrower in absolute terms, so the profile has to be controlled more tightly. The soak and the ramp rate matter as much as the peak, because the assembly spends a larger fraction of the cycle close to the limit.

Hot bar soldering and laser soldering are both used for these assemblies because they deliver heat locally rather than to the whole part. A hot bar applies the temperature through a tool that contacts the joint, and the rest of the cable stays cool; a laser delivers energy to the joint without contact. Both reduce the exposure of the cable, and both allow the temperature to be measured at the joint rather than inferred from the oven.

Verification and Reliability Testing

Inspection of a low temperature joint follows the same rules as any other: a fillet with a defined shape, no bridging and no voids. Because the alloys are more brittle, a peel or a bend test on a sample is more informative than a visual check, and the fracture surface shows whether the failure was in the alloy, at the interface or in the flex itself. A joint that fails in the flex indicates that the pad or the stiffener is the weak point rather than the solder.

Reliability testing should include thermal cycling and the mechanical stress the cable will see in the product. Where the joint is on a cable that folds, the bend test should include the joint, because that is where the strain concentrates. Recording the resistance of a daisy chain through the joints before and after each test gives a quantitative measure that a visual inspection cannot, and it is the evidence that supports the choice of a lower temperature alloy. The general approach follows the same logic as any lead free decision, with the additional constraint that the substrate is the limiting component.

When a Low Temperature Alloy Is the Right Answer

The alloy is justified when the substrate or the connector cannot tolerate the standard profile and when the joint can be mechanically supported. It is also justified for a repair, where the risk of damaging the assembly with a second high temperature cycle outweighs the mechanical advantage of a stronger alloy. In those cases, the lower temperature is not a compromise but the correct engineering choice.

It is the wrong answer when the joint has to survive mechanical shock on its own, when the product will see wide thermal excursions, or when the assembly can comfortably take a standard profile. In those cases the additional process control and the narrower window of the low temperature alloy add cost without adding reliability. The decision should be made from the thermal budget of the assembly, and it should be documented so that a later change of connector or coverlay does not invalidate it silently.

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

FAQ

Is a low temperature alloy as strong as a standard lead free alloy? Not in shear or in shock. It is adequate where the joint is supported and where the mechanical loads are modest.

Can a bismuth alloy be used on a board with leaded finish? Only after assessment, because lead and bismuth interact and change the melting behaviour of the joint.

Why not simply lower the peak of a standard profile? Because the alloy has to melt completely for the joint to form. Below its melting point the paste does not reflow, however slowly it is heated.