Cable and Wire Harness Assembly in Vietnam: What to Specify and Test

Almost every box build contains cables. A power lead, a display flex, a sensor pigtail, an internal harness linking the main board to switches, fans and an antenna. They are often the last items designed and the first items to delay a production start, because connectors are on allocation, a drawing is missing a length tolerance or the harness supplier and the PCBA supplier never checked that their parts mate.

This article covers what a buyer or hardware engineer needs to specify for cable and wire harness assembly: how the two differ, how IPC/WHMA-A-620 classes work, what belongs on the drawing, how harnesses are tested, what tooling is involved and why it pays to keep the harness inside the same program as the PCBA. Vietnam is used as the sourcing context, but the specification points apply anywhere.

Vietnam has an established wire harness industry. Japanese and Korean automotive harness groups have run large plants there for many years, and wire harness is a meaningful export category. That base has produced a skilled operator pool and local tooling support, which electronics EMS factories in the north and south draw on for cable and harness work.

Cable assembly vs wire harness

The terms are used loosely, but the construction and the testing are different enough that your drawing should make the choice explicit.

Aspect	Cable assembly	Wire harness
Construction	One cable or several conductors inside a common jacket, connectors at the ends	Several individual wires or cables bundled to a layout, with branches
Typical location	Often external: power leads, I/O cables, sensor cables	Usually internal: inside the enclosure, between boards and components
Protection	Outer jacket, overmolded strain relief	Ties, sleeving, tape, convoluted tubing, grommets
Built on	Cutting and stripping machines, crimp presses, overmold presses	Harness boards (form boards) with pegs and routing marks
Key drawing content	Pin-out, length, jacket spec, overmold	Layout with branch lengths, pin-outs per connector, securing points
Common failure points	Strain relief, shield termination, crimp quality	Miswires, wrong branch length, chafing at edges

Many products need both. A kiosk might have an external mains cord (bought as a certified cord set), a USB cable assembly and an internal harness to the display, touch panel, speakers and fans.

IPC/WHMA-A-620 and its classes

IPC/WHMA-A-620, ‘Requirements and Acceptance for Cable and Wire Harness Assemblies’, is published jointly by IPC and the Wire Harness Manufacturers Association. It is to cables what IPC-A-610 is to circuit board assemblies: an acceptance standard with pictures and criteria for crimp terminations, insulation displacement connections, soldered terminations, splices, connector assembly, overmolding, marking, coaxial and shielded cable terminations, harness securing and electrical testing.

It uses the same three classes as IPC-A-610:

• Class 1: general products where function is the main requirement.
• Class 2: dedicated service products; most commercial and industrial equipment.
• Class 3: high performance or harsh environment products where failure cannot be tolerated.

The class of a harness should normally match the class of the assemblies it connects. The previous article, IPC-A-610 Class 1, 2 and 3, explains how to choose a class and how the standards fit together. The short version for cables: most products are Class 2; call out Class 3 only where the consequence of failure justifies it, and put the class on the harness drawing.

What to put on a harness or cable drawing

A harness drawing that a factory can quote and build without a round of questions typically contains the items in this table.

Item	What to specify	Why it matters
Wire gauge	AWG per circuit, conductor material (bare or tinned copper), stranding	Current capacity, voltage drop, terminal compatibility
Insulation	Material (PVC, XLPE, PTFE, silicone), temperature and voltage rating, color per circuit	Environment, safety approvals, assembly error prevention
Wire style	UL style (for example UL 1007, UL 1015) or equivalent recognized wire	Needed for UL and other safety listings of the end product
Connectors and terminals	Manufacturer part numbers for housings, terminals, seals; plating	Mating, retention, current rating
Approved alternates	Alternate parts you have checked, or a clear ‘no substitution’ note	Protects lead time without risking mating problems
Crimp requirements	Crimp height and pull force per terminal manufacturer specification, or A-620 values	Primary quality control for every crimp
Pin-out	From-to table for every circuit, including unused cavities and plugs	Basis of the continuity test program
Lengths and tolerances	Overall and branch lengths, reference points, tolerance per length band	Fit in the enclosure, service loops
Labels	Content, format, material (heat shrink, wrap-around), position from connector	Traceability and field service
Shielding	Braid or foil, coverage, drain wire, how the shield is terminated at each end	EMC performance, pre-compliance results
Protection and securing	Sleeving, tape, tubing, tie spacing, grommets	Chafing and vibration resistance
Overmolding	Material, hardness, color, strain relief geometry, bend test if required	Durability of external cables
Compliance	RoHS, REACH, halogen-free if required	Market access
Class and tests	IPC/WHMA-A-620 class, electrical test list and values	Sets inspection and test scope

Two details cause more quote questions than any others. First, length tolerances: without them the factory will apply its own, and a harness that comes out 15 mm too long in a tight enclosure can pinch or kink. Second, shield termination: ‘shielded cable’ on a BOM line says nothing about whether the shield is grounded at one end, both ends or 360 degrees through the backshell, and that choice often decides EMC results.

Testing: continuity, hipot and pull test

Harness testing is split into tests done on every unit and tests done on samples.

On every unit:

• Continuity: a cable tester checks every circuit for opens, shorts and miswires against the pin-out table. For harnesses, 100 percent continuity testing is the normal expectation.
• Insulation resistance and hipot: for mains-voltage or high-voltage harnesses, a dielectric withstanding voltage test confirms insulation between circuits and to shield. Test voltage and duration come from the product safety standard, not from the harness factory.
• Visual inspection: labels, routing, securing, connector seating and locking, to the agreed A-620 class.

On samples, at setup and at intervals:

• Pull test: a crimped terminal is pulled to a minimum force on a calibrated tester. It is destructive, so it is done at the start of each run, at each applicator or wire change and at defined intervals.
• Crimp cross-section: the crimp is cut, polished and photographed to check wire strand compaction, bellmouth and symmetry. Common during tooling qualification and first article.
• Crimp height measurement: a quick, non-destructive check against the terminal specification, taken at setup and periodically.

Many automated crimp presses also have crimp force monitoring, which flags individual bad crimps in real time. It is worth asking whether the factory uses it on your terminals.

Tooling: applicators, harness boards and molds

Harness work involves tooling that buyers often do not budget for:

• Crimp applicators: terminal-specific dies fitted to crimp presses. Each terminal family needs its own applicator. Factories often own applicators for common terminals; unusual terminals may need a new one.
• Hand crimp tools: used for low volumes and prototypes, ideally the terminal manufacturer’s own tool for that terminal.
• Harness boards: plywood or aluminium boards with pegs and a printed layout, one per harness design. Cheap, but needed per design.
• Overmold tools: steel or aluminium molds for overmolded strain reliefs and connectors. These are the most expensive and slowest items.
• Test fixtures: mating connectors and adapters that let the cable tester plug into your harness.

Tooling is usually quoted as NRE. Clarify ownership and whether the tooling stays at the factory. The box build cost article shows how NRE is typically separated from unit price.

Why the harness belongs in the same box-build program

It is common to buy PCBAs from one supplier and harnesses from another, then assemble in a third place. It works, but it moves interface risk onto you. Keeping the harness inside the same box build assembly program has practical advantages:

• Mating is checked once. The factory that assembles the product finds a connector mismatch at first article, not after two separate shipments arrive.
• Fit is verified in the real enclosure. Lengths, bend radii and routing are checked during mechanical fit verification before scale-up.
• Testing covers the system. A harness that passes continuity on its own can still fail when installed; functional test of the complete unit catches it.
• Change control is simpler. A pin-out change on the PCB and on the harness goes through one engineering change process.
• Logistics are shorter. One production plan, one set of long-lead parts, one shipment.

The vetting questions in how to vet an EMS provider in Vietnam apply here: ask whether cable work is done in house or subcontracted, and if subcontracted, who inspects it.

Lead times and sourcing

Indicative lead times for custom harnesses, which must be confirmed by factory quotes for your specific parts:

• Samples: 1 to 3 weeks when connectors, terminals and wire are in stock.
• Production: 2 to 6 weeks after sample approval, depending on volume and complexity.
• Overmold tooling: 3 to 6 weeks for a new mold.
• Branded connectors on allocation: 8 to 20 weeks or more, which is why approved alternates matter.

To get comparable quotes, send the same package to every factory: harness drawings with the items above, the A-620 class, test requirements, annual volume and release quantities. The box build RFQ checklist lists what else to include, and the box build assembly guide explains the typical project flow from brief to shipment.

boxbuildvn.com is a matching service, not a factory. Send your harness and PCBA drawings through the box-build RFQ form and get matched with up to 3 Vietnamese EMS factories within 24 business hours. Free for buyers, NDA on request. If you only need cables or PCBA without full box build, see EMS contract manufacturing in Vietnam or use the same RFQ form and say so in the brief.