# Soft soldering

Joining with low-liquidus filler: alloy/flux selection, wetting, thermal profiles, residue compatibility, and finished-joint acceptance.

Reviewed: 2026-11-01

Tags: processes, joining, soldering, electronics

Using Indium's **220°C liquidus** for the listed **SAC305**, the example trace rises from **200°C at 50 s to 240°C at 90 s**, and cools from **250°C at 100 s to 200°C at 150 s**. Linear interpolation gives crossings at **70 s** and **130 s**, or **60 s above liquidus**. Verify real joint traces and paste and component limits. [Indium alloy values](https://www.indium.com/products/alloys/solder-alloys/)

Lucas Milhaupt distinguishes soldering by a filler liquidus below **450°C (840°F)** and below the base-metal solidus. Identify the filler alloy rather than inferring the process from silver content or tool temperature. [Lucas Milhaupt, Frequently Asked Questions, undated](https://www.lucasmilhaupt.com/Resources-Library/FAQ)

The process serves electrical/electronic assembly and other suitably designed metal joints. It does not automatically provide the strength, creep resistance, service-temperature capability, or pressure-system approval required by a particular application. Select the complete joint and process against its intended environment.

## Specify alloy, surface, and flux separately

Identify the exact filler composition, product form, and flux system. Wire can be solid or flux-cored; paste contains metal powder and a flux vehicle; preforms provide a controlled shape and quantity. A product family name may identify the flux rather than the alloy, so record both.

As bounded alloy examples, Indium Corporation's alloy table lists **Indalloy 106, Sn63/Pb37**, as eutectic at **183°C**, and **Indalloy 256, SAC305 (Sn96.5/Ag3/Cu0.5)** with **217°C solidus and 220°C liquidus**. These are the supplier's listed melting values, not recommended reflow peaks, continuous-service temperatures, or a reason to mix alloys indiscriminately. [Indium, Solder Alloys, undated product table](https://www.indium.com/products/alloys/solder-alloys/)

Specify the actual substrate/metallization and its condition: bare copper, plated copper, nickel/gold finishes, tin coatings, or other surfaces can behave differently. Oxides, contamination, storage, and prior heat exposure influence solderability. Flux promotes the intended surface reaction and wetting; it is not a substitute for every cleaning operation or a remedy for an inherently incompatible joint.

Indium's *Intermetallics in Soldering* article explains that visible spreading and metallurgical bonding are not identical. Its examples distinguish copper and nickel interfaces and show why time, temperature, dissolution, and intermetallic formation depend on the surface system. More heating is not automatically better, because excessive dissolution or brittle reaction layers can also be undesirable. [Indium technical article, undated in retrieved text](https://www.indium.com/blog/intermetallics-in-soldering/)

## Match the heating route to the assembly

| Route | Key controls to establish |
| --- | --- |
| Hand soldering | Tip geometry and condition, actual heat transfer, contact time, filler feed, and component limits. |
| Reflow | Paste deposition, placement, measured assembly profile, atmosphere, and cooling. |
| Wave or selective soldering | Flux application, preheat, contact conditions, drainage, and assembly compatibility. |
| Torch or other local heating | Substrate and flux compatibility, joint gap, heating distribution, and protection of adjacent materials. |

These routes share filler melting and bonding principles but do not share a universal temperature recipe. Heating a joint with an iron differs from heating an entire board in an oven. Define how both mating surfaces reach the required condition without damaging insulation, components, laminates, coatings, or earlier joints.

Hakko's tip-life guidance explains that an appropriate tip shape and size improve heat transfer and can avoid the common response of simply raising the station temperature. Oxidation and poor wetting of the tip impede performance; maintain the tip using the equipment instructions. A station setpoint is not direct measurement of the joint temperature under load. [Hakko USA, How to Maximize Soldering Iron Tip Life, undated](https://kb.hakkousa.com/Knowledgebase/10322/How-to-Maximize-Soldering-Iron-Tip-Life)

## Bounded flux-cored-wire example

The retrieved **Kester 44 Flux-Cored Wire TDS, issue May 17, 2021**, identifies an activated-rosin formulation classified **ROM1 under J-STD-004**, offered with leaded and lead-free alloys. This designation describes the flux system; it does not identify a particular tin/lead or SAC composition. The document's printed issue date is distinct from dates in the download library. [Kester 44 technical data sheet, page 1](https://www.kester.com/DesktopModules/Bring2mind/DMX/API/Entries/Download?Command=Core_Download&EntryId=61959&PortalId=0&TabId=96&language=en-US)

The same sheet provides different hand-soldering guidance for leaded and lead-free alloys and instructs heating the land and component lead before feeding solder. Treat its ranges as product guidance, not proof that every component will tolerate the same tip temperature or dwell. Record the selected wire alloy, core percentage, diameter, and process rather than specifying only “Kester 44.” [Kester 44 TDS, processing guidance](https://www.kester.com/DesktopModules/Bring2mind/DMX/API/Entries/Download?Command=Core_Download&EntryId=61959&PortalId=0&TabId=96&language=en-US)

## Reflow profiles are measured on the assembly

Establish ramp, soak where used, peak, time above the selected liquidus, and cooling against both solder-paste guidance and component/assembly limits. Oven zone settings and conveyor speed are inputs; they are not the temperature-time history at every joint. Indium's profile-matching example evaluates a specific solder-paste specification against a temperature trace, including time above liquidus and cooling rate. Its example is a method demonstration, not a generic SAC profile. [Indium, Matching a Reflow Profile to a Solder Paste Spec](https://www.indium.com/blog/matching-a-reflow-profile-to-a-solder-paste-spec/)

### Worked example: interpolate time above liquidus

This original arithmetic example adopts the Indium table's **220°C liquidus** for its listed SAC305 alloy and assumes a measured joint trace containing these points:

- Rising from **200°C at 50 s** to **240°C at 90 s**.
- Cooling from **250°C at 100 s** to **200°C at 150 s**.

Assuming linear temperature change within each stated interval, the upward crossing is 50 + (220−200)/(240−200) × 40 = **70 s**. The downward crossing is 100 + (250−220)/(250−200) × 50 = **130 s**. Calculated time above liquidus is therefore **60 s**.

This does not establish an acceptable profile: temperatures between the points, measurement uncertainty, thermal variation across the assembly, and the selected paste/component limits remain to be assessed. An alloy melting table alone cannot approve the peak or the duration. [Indium alloy values](https://www.indium.com/products/alloys/solder-alloys/)

## Residues and cleaning need a qualified decision

“No-clean” does not mean residue-free or automatically compatible with every coating, high-impedance circuit, connector, or service environment. Specify whether residues remain, the required cleanliness test, and compatibility with subsequent operations. If cleaning is needed, qualify chemistry and access to hidden regions as well as external appearance.

MicroCare discusses cleaning no-clean residues and matching the cleaner to both residue and assembly materials. A solvent that removes visible flux can still damage plastics, markings, or coatings; partial cleaning is not evidence of the required electrical reliability. [MicroCare, Why Clean a No Clean Flux, undated](https://www.microcare.com/en-US/Resources/Resource-Center/Tech-Articles/Why-Clean-a-No-Clean-Flux)

## Inspect the joint and preserve traceability

Define the required fill, wetting, fillet geometry, alignment, bridging limits, disturbed-joint criteria, and inspection method under the applicable product specification. Electrical continuity alone cannot establish joint strength, internal void condition, or long-term reliability. Where needed, use appropriate mechanical, electrical, thermal-cycle, or sectional evidence representative of the actual assembly.

Keep mechanical support and strain relief separate from the solder bond. Inspect after the agreed cleaning/finishing stage and document rework cycles. A shiny surface is not a universal acceptance criterion across different alloys and processes.

Follow the exact solder, flux, and cleaner SDS and equipment instructions. UK HSE identifies rosin-based flux fume as an occupational-asthma risk; ventilation and exposure controls matter even when a solder is lead-free. Lead-containing alloys additionally need the applicable handling and contamination controls. [HSE, Solderer](https://www.hse.gov.uk/asthma/solderers.htm)

A useful handoff records substrate/finish, filler alloy/product/lot, flux and cleaning route, joint geometry, heating/profile evidence, component limits, acceptance criteria, and rework history. Retain supplier examples as bounded guidance rather than universal production settings.

## Related reference

[Welding methods](index.md)