Low Temperature Solder Reflow Process Control for Assemblies

Reducing the peak temperature of a reflow process sounds like a simple saving, but it changes the alloy, the flux, the joint appearance and the reliability of the finished product. Low temperature solder is used because thin boards warp, because some components cannot survive a conventional lead free profile, and because the energy saving is real. The trade is a narrower process window and a joint that behaves differently from the one most inspection criteria were written for.

Why Lower Temperatures Are Used

A conventional lead free alloy melts at around 217 degrees, and the process has to reach roughly 235 to 245 degrees to form a reliable joint. That temperature is high enough to warp a thin board, to stress a large package and to damage components that were designed for a lower ceiling, such as some connectors, sensors and electrolytic capacitors.

Lowering the peak temperature addresses all of those at once. It reduces the thermal load on the assembly, allows thinner and larger panels to stay flat, and makes it possible to use materials and components that would otherwise need a special process or a different assembly sequence. The energy saving is a secondary benefit, and one that is usually smaller than the reduction in scrap and rework.

Alloy Families and Melting Points

Low temperature alloys are usually based on tin with bismuth, sometimes with additions of silver or copper. The melting point falls as the bismuth content rises, and the common alloys melt in a range between roughly 138 and 170 degrees, which allows a peak temperature below 200 degrees.

The alloy chosen has to match the flux and the application. A lower melting point generally means softer solder, lower creep resistance and a joint that is less tolerant of high service temperature. Where the product operates warm, the alloy has to be selected with that in mind rather than purely on the assembly temperature.

Bismuth Behaviour and Alloy Mixing Risk

Bismuth has one behaviour that dominates the whole subject: it lowers the melting point of any tin based alloy it is mixed into. That is useful inside a single joint, but it is dangerous when a bismuth containing solder comes into contact with a conventional lead free alloy, because the mixture can melt well below the intended temperature.

The practical risk is contamination. Solder left in a pot, paste left on a stencil, tools shared between lines and even a splash of the wrong alloy can lower the melting point of a subsequent joint. Where both alloys are used in the same factory, the two processes should be separated physically and the separation should be written into the procedure rather than trusted to memory. Colour coded containers, dedicated stencils and a controlled list of approved alloys make that separation visible on the floor.

Thermal Budget of Components and Board

Every component has a maximum temperature and a maximum time at that temperature, and the assembly profile has to satisfy the most sensitive part on the board. A low temperature process enlarges the margin for those parts, which is often the reason the alloy is chosen in the first place.

The board contributes to the same budget. The laminate, the solder mask and the surface finish all see the peak temperature, and a finish or mask that is marginal at 240 degrees may be comfortable at 190. The thermal management of the whole assembly is discussed in the guide to thermal management design.

Warpage and Thin Board Benefits

Warpage is driven by the difference in expansion between the materials in the stack and by the temperature the assembly reaches. Lowering the peak temperature reduces that difference and reduces the permanent deformation that remains after the board cools.

For a large, thin panel or for a board with an asymmetric build, the improvement can be the difference between passing a flatness requirement and failing it. The benefit is largest for boards that are already at the limit of the process, and it should be verified by measurement rather than assumed. A flatness check before and after assembly, on the same panel design with both profiles, gives the evidence in a single experiment.

Reflow Profile Design

A low temperature profile is shorter and cooler, but it still needs a controlled ramp, a soak that activates the flux and a peak that is high enough above the melting point to wet properly. Because the difference between the peak and the melting point is smaller, the relative tolerance on the peak temperature is tighter than in a conventional process.

That tightness is the main engineering challenge. The same oven that delivers plus or minus five degrees at 240 degrees may deliver an unacceptable spread at 190, so the profile has to be measured on the real board and the oven zones adjusted accordingly. Thermocouple placement should include the coldest and the hottest points of the assembly, and the profile should be re-measured whenever the board revision, the copper distribution or the component mix changes, because all three alter the way heat is absorbed.

Joint Appearance and Inspection

Low temperature joints look different from conventional ones. They are often duller, they have a different fillet shape and they can appear grainy, and an inspector who is used to bright, shiny lead free joints may reject a perfectly sound one. The acceptance criteria used for visual inspection may need to be redefined for the alloy.

That means the criteria should be agreed with the customer or the responsible engineer before production, with photographs of acceptable joints. Where the criteria are imported from a different alloy, the process will be adjusting to a specification that does not describe it, which is a common source of unnecessary rework.

Reliability and Thermal Cycling

The mechanical properties of a low temperature joint differ from those of a standard lead free alloy: lower melting point generally means lower strength at temperature and greater creep under continuous load. For many consumer products that is acceptable, while for automotive, industrial and high reliability applications it has to be evaluated rather than assumed.

Thermal cycling and, where relevant, high temperature storage are the tests that reveal the difference. The failure mode is usually a crack through the bulk of the joint rather than at the interface, and the joint’s behaviour should be qualified on the actual component mix rather than on a test vehicle that does not represent the product. Common failure patterns are described in the guide to solder defects and board failures.

Process Control and Documentation

The alloy has to be identified and controlled at every step: incoming inspection, paste or wire storage, the printer, the oven and any rework station. A single unidentified spool of wire or a pot of solder without a label is enough to introduce contamination that will not be noticed until a joint fails, and by then the affected assemblies have been built and often shipped.

Documentation should record the alloy, the profile, the peak temperature and the inspection criteria, together with the separation rules that apply on the shop floor. Because the process is less forgiving than a conventional one, the records are what make the difference between a controlled low temperature process and one that happens to work at the moment. The verification methods used to support it are described in the guide to solderability testing.

FAQ

Is low temperature solder weaker than standard lead free solder? Generally yes at elevated temperature, because the melting point is lower and creep resistance is reduced. Whether that matters depends on the service conditions. For a product that runs cool and is not heavily loaded the difference is often irrelevant, while for a hot or highly stressed application it has to be qualified.

Can the two alloy systems share a production line? They can, but only with strict separation of paste, stencils, tools, pots and rework stations. Cross contamination lowers the melting point of the conventional alloy, and the resulting joints can fail at temperatures the design assumed were safe.

Does the joint need a different inspection standard? Often it does, because the appearance of the alloy differs from what inspectors expect. The criteria should be agreed in advance with photographs of acceptable and unacceptable joints so that sound joints are not rejected on appearance alone.