FAQ Reflow Soldering of Spring-Loaded SMD Battery Contacts

Reflow Soldering of Spring-Loaded SMD Battery Contacts (UEBK)

Introduction

The spring-loaded SMD battery contacts of the UEBK series from uwe electronic can be soldered onto the PCB using standard lead-free reflow processes. Because the contact pin contains a precision spring made of spring steel, the reflow profile should be chosen to minimize the thermal load on the spring without compromising solder joint quality. This application note explains the material background and provides a concrete profile recommendation.

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Material Background: Why the Thermal Load on the Spring Should Be Limited

Depending on the alloy, spring steels can only be used with low relaxation up to certain continuous service temperatures – e.g. standard spring steel per EN 10270-1 (DH/SH) up to approx. 80 °C, stainless spring steel 1.4310 up to approx. 160 °C, and 1.4568 up to approx. 200 °C. For spring-loaded contact pins, a limit of approx. 150 °C has become an established industry rule of thumb for the continuously non-critical service temperature – above this temperature, the spring can lose preload/spring force due to relaxation (a thermally activated creep process).

Relaxation is a cumulative, temperature-activated process: it increases exponentially with rising temperature, but also with increasing dwell time at a given temperature. For the reflow process, this means:

• Not only the peak temperature matters, but the total time the contact pin spends above approx. 150 °C – including the preheat, soak, and ramp phases, not just the short peak segment.
• Brief temperature spikes of only a few seconds are, based on experience, tolerated considerably better by spring materials than sustained exposure at the same temperature. For the specific alloy used in your contact pin, we recommend confirming the permissible seconds-level exposure with the relevant datasheet or with our technical team.

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Reference: Standard Reflow Temperature Profile (Lead-Free, e.g. SAC305)

For context, here is a typical industry-standard profile for lead-free reflow soldering, as commonly used for mixed SMD assemblies (reference values based on common practice/IPC recommendations; the actual profile always needs to be determined for the specific oven and assembly):

Phase	Temperature Range	Duration / Rate
Preheat	Ambient → approx. 150 °C	Ramp 1–3 °C/s
Soak	approx. 150–200 °C	60–120 s
Reflow / Peak (TAL)	Peak 230–250 °C, liquidus from approx. 217 °C	Time Above Liquidus (TAL): 45–90 s (30–90 s depending on source)
Cooling	Peak → ambient	–2 to –4 °C/s

In this standard profile, the assembly spends a total of 90–180 seconds above 150 °C (soak zone plus ramp times), of which 45–90 seconds are above the liquidus temperature of approx. 217 °C. This time is required to achieve complete wetting and intermetallic layer formation at all solder joints on the assembly.

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Recommended Reflow Profile for Spring-Loaded SMD Battery Contacts

To significantly reduce the total time above 150 °C compared with the standard profile, we recommend a “ramp-to-spike” profile with no or only a minimal soak plateau for UEBK SMD battery contacts. The peak is held only as long as necessary to reliably melt and wet the solder:

Phase	Temperature Range	Duration / Rate
Preheat	Ambient → approx. 140–150 °C	Ramp 1–3 °C/s (standard)
Soak	omitted or minimal (no plateau)	if technically required: as short and as cool as possible, target < 30–45 s
Reflow ramp to peak (“spike”)	150 °C → approx. 250 °C in one continuous ramp	as steep as the oven allows, no dwell below the peak
Peak dwell time	≥ 217 °C (liquidus), peak approx. 250 °C	only 2–3 seconds at the absolute peak
Cooling (active)	Peak → < 150 °C as fast as possible	≥ 4 °C/s, forced cooling (cooling zone/air) if possible

With this profile, the total time above 150 °C can be reduced to approx. 60–90 seconds (compared with 90–180 seconds in the standard profile), while the short, targeted peak phase still reliably forms the solder joint.

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Important Note on Protecting the Spring

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Recommendations for Process Implementation

• Measure the thermal profile directly on the component: place the thermocouple as close as possible to the contact pin to verify the actual peak dwell time and the total time above 150 °C.
• Set the oven zone allocation and conveyor speed for a continuous, steep ramp to the peak rather than an extended soak plateau.
• For assemblies populated on both sides: place the contact pin in the second (final) reflow pass if possible, or choose its position so that the cumulative thermal load stays minimal.
• A lower-melting solder paste system (e.g. SnBi alloys with a peak of approx. 180–210 °C) can further reduce the thermal load, provided it is compatible with the other components on your assembly.
• Before production release, we recommend checking spring force and travel on a sample assembly before and after reflow to verify the effect of the profile used on the spring.

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Quality Control After Reflow

• Visually inspect all solder joints on the contact pin: shiny, fully wetted, no cold joints
• No visible discoloration or deformation on the contact pin housing (an indicator of overheating)
• Check spring travel and return force after cooling

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Summary

Phase	Temperature Range	Duration / Rate
Critical spring limit	> 150 °C	common industry standard for spring contacts
Peak temperature	approx. 250 °C	for reliable solder melting
Peak dwell time	≥ 217 °C, peak approx. 250 °C	2–3 seconds
Total time above 150 °C	150–250 °C	target approx. 60–90 s instead of 90–180 s in the standard profile
Cooling after peak	250 °C → < 150 °C	immediately, ≥ 4 °C/s or forced cooling

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Questions About Your Application?

Our technical team is happy to support you in designing your reflow profile for UEBK battery contacts. An overview of our contact strip range is available at uweelectronic.de – Spring Contact Strips.