2.2 Termination & Soldering
When solder makes sense, heat-shrink use, and strain-relief best practices.
Soldering in wire harness workassembly is aless targetedabout skill—usedroutine whenand more about precision where mechanical crimps cannot reach. It bridges the gap in legacy connectors, splices,PCB orinterfaces, PCBsand requireshield it,terminations, but avoideddemands wherecareful crimpscontrol orof ferrulesheat, giveflux, betterand durability.support Goodto avoid creating brittle weak points. When paired with strain relief techniques like boots, clamps, and heat-shrink, solder jointsbecomes startnot withjust intacta strands,bond, properbut support,a durable safeguard against vibration, flex, and controlledlong-term heatfatigue. soSelectively wickingapplied, staysit shortturns and jackets don’t scorch. Whether it’s a solder-cup, PCB lead, or shield sleeve, the finish should be smooth, well-wetted, and followed by heat-shrink or boots for strain relief. Short pigtails on shields, clamps before the first bend, and service loops keepvulnerable joints from fatiguing in service. When soldering is applied selectively and supported mechanically, the result isinto compact, reliablesealed terminationsconnections that stay solid overendure years of use.service.
2.2.1 Should you even solder? (quick decision)
Use crimps by default. Choose solder only when it brings a clear advantage.
Solder is a good choice when…
- Solder-cups/turrets are the only interface (legacy/mil connectors, some sensors).
- You need a sealed, inline splice or shield bond using a solder sleeve (controlled, repeatable).
- You’re making flying leads to a PCB and can anchor the wire mechanically nearby.
- Field repair must be compact and you can add strain relief.
Avoid solder (choose crimp/ferrule) when…
- The joint will flex or vibrate (robot arms, engine bays). Solder creates a stiff wick zone that breaks strands.
- The conductor terminates under a screw/pressure clamp. Do not tin stranded wire for screw terminals—use a bootlace ferrule.
- High current lugs or ring terminals are needed—use the manufacturer’s crimp.
2.2.2 Common soldered terminations (how to do them right)
A) Solder-cup connectors (plugs, circulars)
- Prep: strip to the drawing (no nicked strands), lightly pre-tin the cup (not the wire).
- Support: clamp the cable/backshell so the cup sees no cable weight.
- Heat & feed: tip sized to the cup; heat the cup, not the insulation. Feed solder until the cup is full and a small meniscus forms; no solder balls.
- Inspect: smooth, concave fillet; no exposed bare strands; wicking under insulation ≤ 2 mm.
- Strain relief: install the backshell boot or adhesive heat-shrink after cooling.
B) Wire to PCB (flying lead, THT pad)
- Anchor first: tie-down, clamp, or adhesive within 50–80 mm of the pad.
- Form: route so the first bend is after the anchor, not at the pad.
- Solder: wet both pad and wire; 95% fill on plated-through holes; trim flush; clean flux if required.
- Relief: finish with heat-shrink over the exit or a dab of approved adhesive on the wire body, not the joint.
C) Solder sleeves (splices, shield bonds)
- Pick ID & shrink ratio for the jacket OD; for shields, use sleeves with preform ring + flux.
- Hot-air tool, correct nozzle; heat until solder ring fully flows and adhesive extrudes at the ends; no burning of jacket.
- Shield 360°: for braided shields, fan evenly under the sleeve; for foil, ensure drain contact.
- Inspect: continuous fillet around 360°, clear adhesive bead both sides, wires centered (no skew).
D) Lap/inline solder splices (only if spec requires; sleeves preferred)
- Stagger splices by ≥ 30 mm across a loom.
- Twist lightly, solder through capillary action (no blobs), cover with adhesive heat-shrink.
2.2.3 Heat-shrink: selection & application
Choose
- Material: polyolefin is default; dual-wall (adhesive-lined) for sealing/strain relief; clear when you need to see the joint.
- Ratio: 2:1 for snug fits; 3:1 or 4:1 to bridge boots or mixed diameters.
- Length rule: cover the joint + 1–2× wire OD beyond each side (sleeves: per spec).
Apply
- Pre-cut lengths; mark position from a datum (backshell/boot).
- Use hot air, not an open flame; move the nozzle to avoid scorching.
- Shrink from middle outward to push adhesive to the ends; let cool undisturbed.
2.2.4 Strain-relief best practices (make the joint live long)
- First clamp before first bend: place a cushioned clamp 50–80 mm from the backshell/joint.
- Service loop: leave 80–120 mm where the product needs movement; gauge it on the nailboard.
- Boots: use molded or adhesive heat-shrink boots on backshells/branches; avoid zip-ties over bare insulation.
- Keep the stiff zone short: limit solder wicking under insulation to ≤ 2 mm; if longer, reject or re-terminate.
- Route away from edges: add grommets/edge guards and sleeves (19.4) anywhere the cable can rub.
- Potting (only if spec’d): pot over the relief, not over the solder fillet alone; allow drip-free cure before handling.
2.2.5 What “good” looks like (acceptance cues)
Tug test: after cool-down, a firm pull inline shows no movement. For quantitative checks, use the pull-force table from 20.1.7 as the minimum baseline for that AWG.
2.2.6 Flux, alloy, and heat (keep it predictable)
- Flux: no-clean or RMA for harness work; never acid flux. Clean if customer/spec requires.
- Alloy: match plant standard (SnPb vs SAC). Use the same alloy as the mating hardware when practical.
- Heat: temperature-controlled iron/hot air; tips matched to the joint mass. Too hot = burnt jacket; too cold = grainy joints.
2.2.7 Common traps → quickest reliable fix
2.2.8 Pocket checklists
Before you solder
- Right termination method (crimp vs solder) chosen for the environment
- Wire strip per drawing; strands intact; parts supported
- Sleeves/boots sized & pre-cut; hot-air/iron temps set
During
- Heat the metal, not the insulation; fillet forms smoothly
- Wicking checked (stop ≤ 2 mm under jacket)
- Install heat-shrink from middle out; adhesive visible (dual-wall)
After
- Tug test pass; no bends at the joint; clamp within 50–80 mm
- Visuals: no spikes/balls; sleeve fully recovered; labels undamaged
- Log lot/tool/operator if required; stamp module PASS