FAQ Soldering Spring-Loaded THT Battery Contacts

Soldering Spring-Loaded THT Battery Contacts (UEBK) into the PCB

Introduction

The spring-loaded THT battery contacts of the UEBK series from uwe electronic are inserted with their terminal pins through holes in the PCB and soldered on the solder side – typically by wave soldering, selective soldering, or, for small batches and prototypes, by hand. Because the contact pin contains a precision spring made of spring steel, the thermal load on the spring should also be limited during THT soldering. This application note explains the material background and provides specific process recommendations for the common THT soldering methods.

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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. 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.

One effect works in favour of THT components: on populated assemblies, the contact body containing the spring sits on the component side (top) of the PCB, while the soldering process takes place on the opposite solder side. The spring is therefore heated primarily by thermal conduction through the terminal pins and the PCB, and generally reaches a lower temperature than the solder joint itself.

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3. Reference: Common Soldering Methods for THT Components (Lead-Free)

For context, here are the soldering methods commonly used in electronics manufacturing for leaded (THT) components, with typical process parameters (reference values based on common practice; the actual profile always needs to be determined for the specific equipment and assembly):

Method	Typical Parameters	Contact Time with Solder
Wave soldering	Topside preheat 100–150 °C (ramp 2–4 °C/s); solder wave approx. 250–260 °C	approx. 2–4 seconds per terminal
Selective soldering (mini-wave/nozzle)	local preheat approx. 100–130 °C; solder pot/nozzle approx. 260–270 °C	approx. 2–5 seconds per joint, local heating only
Hand soldering (single units/rework)	no assembly preheat required; soldering iron approx. 330–350 °C (lead-free)	approx. 2–3 seconds per terminal

All three methods inherently involve only a short contact time with the molten solder (approx. 2–5 seconds) – unlike reflow soldering, there is no soak plateau at 150–200 °C lasting 60–120 seconds. The preheat zone generally stays well below 150 °C.

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The Soldering Methods in Detail

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Wave Soldering

In wave soldering, the populated PCB is carried through the soldering machine on a conveyor and passed with its solder side over one or more waves of molten solder. Two waves are often used: a turbulent first wave that reliably fills even hard-to-reach joints and plated through-holes with solder, and a calm, laminar main wave that produces a clean, bridge-free solder finish. Because all populated terminals on an assembly pass the wave at the same time, they are soldered in a single pass.

1. Fluxing: applying flux to the solder side (spray or foam fluxer) to remove oxides and improve wetting.
2. Preheating: heating the topside of the assembly to approx. 100–150 °C (IR emitters or hot air) to activate the flux, drive off solvents, and avoid thermal shock on wave contact.
3. Wave contact: the solder side passes the solder wave(s) at approx. 250–260 °C for approx. 2–4 seconds, soldering all leaded terminals simultaneously.
4. Cooling: after leaving the wave, the assembly cools in the open machine section or in a cooling zone.

Wave soldering is the most economical method for series production where all THT components on an assembly are to be soldered in a single pass. Since every joint passes the wave for the same short period, the contact time is inherently limited by the process and readily reproducible.

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Selective Soldering

In selective soldering, the PCB is not passed over a continuous wave; instead, each individual joint is contacted in a targeted manner with a small solder wave (mini-nozzle, often only a few millimetres in diameter) or by a dip process. A machine with X-Y-Z axes positions the PCB or the nozzle so that only the terminals to be soldered come into contact with the solder.

1. Selective fluxing: applying flux only to the points to be soldered, e.g. using a micro spray valve (jet fluxer).
2. Local preheating: targeted heating of only the relevant assembly area to approx. 100–130 °C, e.g. using a focused IR emitter or hot-air nozzle – adjacent heat-sensitive components remain largely unaffected.
3. Solder contact: the mini-nozzle (solder pot approx. 260–270 °C) or dip nozzle contacts each joint individually or in small groups for approx. 2–5 seconds.
4. Cooling: rapid, locally confined cooling, since only a small area of the assembly was heated.

Selective soldering is particularly suitable for assemblies with mixed technology (SMD + THT) or with several temperature-sensitive components in the immediate vicinity, since only the actual joint is heated. For UEBK battery contacts, this is advantageous when other heat-sensitive components are located nearby, or when even finer control of the thermal load is desired.

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Hand Soldering (Manual Soldering / Rework)

In hand soldering, a temperature-controlled soldering iron with a fine tip is guided to the joint by hand; the solder is also fed manually as wire. This is a purely manual, non-automated process.

1. Set the soldering iron to approx. 330–350 °C (lead-free) and wet the tip with a little fresh solder to ensure good thermal contact.
2. Apply the tip for approx. 2–3 seconds to the connection point (solder pad and terminal pin) on the solder side – not to the contact body or spring housing.
3. Feed solder until the joint is fully and brightly wetted.
4. Remove the soldering iron and let the joint cool undisturbed.

Hand soldering is suitable for prototypes, small batches, repair/rework, and for assemblies that cannot be machine-soldered for geometric reasons. Because temperature and contact time depend directly on the operator, trained personnel and a clear work instruction are important.

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Recommendation for Spring-Loaded THT Battery Contacts

For UEBK THT battery contacts, we recommend making consistent use of the inherently short, process-determined contact time of the methods described, and keeping the preheat zone at the lower end of the usual range:

Phase	Recommendation	Objective
Preheat (topside)	approx. 100–130 °C, ramp 2–3 °C/s	lower end of the standard band, no unnecessary extension of preheat time
Soldering method	prefer wave or selective soldering	short, process-determined contact time instead of prolonged heat exposure
Solder temperature (wave/pot)	approx. 250–260 °C	standard range for lead-free SAC solder, do not set unnecessarily higher
Contact time with solder	approx. 2–3 seconds per terminal pin	minimum for complete through-hole tinning
Cooling	promptly after solder contact	active cooling zone or ambient air, no reheating

The same logic applies to hand soldering/rework as to soldering a connection wire: set the soldering iron to approx. 330–350 °C and apply it only briefly (2–3 seconds) to the terminal pin on the solder side – never to the contact body or spring housing.

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Important Note on Protecting the Spring

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Recommendations for Process Implementation

• Measure the thermal profile on the component: place the thermocouple as close as possible to the contact pin or spring housing to verify the actual temperature and dwell time above 150 °C.
• For wave soldering, choose conveyor speed and wave width so that the contact time stays in the lower range that is still sufficient for complete through-hole tinning.
• For assemblies with other temperature-sensitive components: consider selective soldering, since only the actual joint is heated locally.
• Before production release, we recommend checking spring force and travel on a sample assembly before and after the soldering process to verify the effect of the profile used on the spring.

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Quality Control After Soldering

• Visually inspect all solder joints: complete through-hole tinning, shiny, 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
Preheat	approx. 100–130 °C	keep at the lower end of the standard band
Solder temperature (wave/pot/iron)	approx. 250–350 °C depending on method	standard values, do not raise unnecessarily
Contact time with solder	—	2–3 seconds per terminal
Cooling	after solder contact	promptly, active cooling or ambient air

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Questions About Your Application?

Our technical team is happy to support you in designing your soldering process for UEBK THT battery contacts. An overview of our THT contact range is available at uweelectronic.de – Battery Probes THT.