Protecting Temperature Sensitive Components

Not every component can survive the reflow temperature that the solder alloy requires. Electrolytic capacitors dry out, plastic connectors deform, batteries vent, sensors drift and some optical parts lose calibration. The design team usually discovers this after the schematic is fixed, and the manufacturing engineer is then asked to find a way to build the product without cooking the parts. There are several legitimate answers, and the right one depends on how far the component is from the limit.

<img src="https://www.gopcba.com/wp-content/uploads/2024/10/HDI-PCB-min.jpg" alt="Heat shield clipped over a delicate component” />

Establishing the Damage Threshold

The first step is to find out what the component can actually tolerate. Datasheets frequently quote a peak temperature, a maximum time at a specific temperature and a maximum number of reflow cycles, and those three numbers together define the exposure budget. Where the datasheet is unclear, the supplier should be asked directly, because the difference between a part rated for one reflow and one rated for three determines how the product can be processed.

The budget should be compared with what the profile delivers at that specific location on the board. A component near the edge of a panel may see a shorter time above liquidus than one in the centre next to a heavy plane, so the assessment has to use measured data rather than the profile’s nominal values. Profile development produces exactly that data, and it should be recorded per location rather than as a single curve.

Profile Shaping

The cheapest protection is often a change to the profile. Reducing the peak by moving towards the low end of the alloy’s window, shortening the time above liquidus and cooling more quickly all reduce the thermal dose. The limits are set by the paste chemistry, which needs enough time to wet, and by the other components, which may need the longer profile for their own joints. Where both requirements are present, the profile becomes a compromise that has to be verified at both extremes.

Shaping is also possible locally. Adding thermal mass near a sensitive part – a copper pad, a small heatsink or a strip of aluminium tape – reduces the temperature that part reaches, at the cost of a longer time to reach temperature for the joints nearby. This is the reverse of the usual thermal management approach, and it works because the objective is to keep one area cool rather than to heat everything evenly. Reflow profile practice covers the measurement that confirms it worked.

Heat Shields and Fixtures

A heat shield is a small metal or polymer cover that clips over the component and blocks the convective air stream during the hot part of the profile. Shields work well in convection ovens because the dominant heat transfer mechanism is air, and they work poorly in vapour phase or in infrared ovens where other mechanisms dominate. Their effectiveness depends on the fit, and a shield that leaves a gap around the component allows hot air to circulate underneath.

Shielding also has a cost in process control: the shield must be fitted and removed, it must not touch the component, and a missing or misplaced shield produces a failure that appears much later. Where a shield is used, the work instruction must show which component and how it is fitted, and the assembly should be inspected for shield presence before the board enters the oven. Protection methods in general follow the same rules.

Second Side and Process Order

Sequencing is often the most elegant solution. A component can be placed on the second reflow if it can tolerate one cycle instead of two, or it can be added after reflow by hand soldering, selective soldering or a conductive adhesive process. Moving a part to a later operation removes it from the thermal budget entirely, at the cost of an additional process step and its own quality risks.

Second side processing brings its own constraint: the board passes through the oven twice, so the components on the first side experience two cycles. Sequencing therefore has to consider the whole assembly rather than the single problem part, and the component with the tightest budget should be the one that determines the order. Second side assembly practice and the thermal budget should be documented together so that the logic is not lost.

Lower Melting Alloys and Selective Processes

Where the component simply cannot take the primary reflow temperature, a lower melting alloy can be used for its joints. This introduces the possibility of remelting during a later process step, and the design has to ensure that the low temperature joints are not exposed to a temperature above their melting point afterwards. Where the sequence is controlled, the approach is reliable and is widely used for parts such as connectors and displays.

Selective soldering and laser soldering are the other common routes. Both apply heat locally, so the surrounding board stays cool, and both can be used to attach a part after the main reflow. Their drawback is throughput, since each joint is made individually, and the process has to be developed and verified like any other. Mixed alloy practice describes the compatibility considerations when two alloys end up in the same assembly.

Verification and Control

Whichever method is chosen, the result has to be verified on the actual product. That means measuring the temperature at the protected component during a production profile, inspecting the joints around it and, where the part is critical, confirming its function after assembly. A protection method that is not verified is an assumption, and assumptions about thermal damage usually surface as field returns rather than as line defects.

Control also means making the protection visible in the process documents. The work instruction should identify the component, the method and the check; the profile should record the temperature at that location; and the inspection should confirm that the method was applied. Where a shield or a fixture is used, it should be listed in the tooling register so that it is available on every build, and its condition should be checked as part of the changeover.

Additional Considerations for This Build

Practical attention to temperature sensitive pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating temperature sensitive explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

Process Control and Verification

On a design of this kind, second side is the item that decides how the rest of the board is arranged. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

FAQ

How do I know if a component can take the reflow temperature? From the datasheet’s peak, dwell and cycle limits, confirmed with the supplier where the data is unclear, compared with measured temperatures at that location.

Do heat shields work in any oven? They work best in forced convection, where blocking airflow is effective. They are much less predictable in vapour phase and infrared processes.

Can a part be added after reflow? Yes, by hand or selective soldering, at the cost of an extra operation. This is often the simplest solution when the thermal budget cannot be met.

Is a lower melting alloy safe? It is, provided that no later process step exceeds its melting temperature and the sequence is documented.