Key takeaways
- PBT's low equilibrium moisture uptake (typically under 0.5% at saturation vs 2.5–3.5% for unconditioned PA6) is the core reason connectors favour it — dimensions and dielectric properties hold steady through humid service, whereas nylon swells and softens.
- Both resins trade almost exclusively glass-filled and flame-retardant for connectors: GF30 FR (UL94 V-0) is the workhorse, with glass fibre controlling warpage and CTI/GWIT governing tracking and ignition behaviour at live contacts.
- Comparative Tracking Index (CTI, IEC 60112) and Glow-Wire Ignition Temperature (GWIT, IEC 60695-2-13) are the two safety numbers connector buyers specify alongside V-0 — high-CTI grades (CTI 600, PLC 0) command a premium and are not interchangeable with standard FR grades.
- Nylon's edge is impact toughness and a higher melt/continuous-use temperature (PA66 ~265 °C melt vs PBT ~225 °C), which matters for SMT reflow and rugged housings; PBT wins on as-moulded precision, low warpage and stable insulation resistance.
Walk any connector tooling shop and the resin drums tell the story: glass-filled PBT dominates, with flame-retardant glass-filled nylon (PA66) as the standing alternative. Both are workhorse engineering thermoplastics, both arrive at the moulder as GF30 FR pellets, and both can hit UL94 V-0. The reason designers reach for PBT first comes down to a handful of properties that matter specifically at a live electrical contact: moisture uptake, dimensional stability, and the speed at which the polymer crystallises in the cavity.
This is a grade-selection decision, not a generic materials comparison. The numbers that decide it — equilibrium water absorption, CTI, GWIT, glass loading and melt temperature — are the same numbers that price the resin. Here is how a connector buyer actually weighs PBT against nylon, and where each one wins. For the broader nylon picture see our PA6 vs PA66 buyer's guide, and for the full engineering-resin map, engineering plastics compared.
Polyamide is hygroscopic by nature — the amide groups in the backbone hydrogen-bond with water. An unconditioned PA6 part can take up 2.5–3.5% water at saturation; PA66 a little less. As it conditions, the part grows dimensionally and the polymer plasticises — impact toughness improves, but stiffness, heat-deflection temperature and, critically, insulation resistance drop. For a connector with sub-millimetre pin pitch and a published creepage distance, that swing is unacceptable.
PBT is a polyester. It absorbs typically under 0.5% water at saturation — roughly an order of magnitude less than nylon. Contact spacing holds, the dielectric stays predictable, and the part you measured at first-off looks the same after a year in a humid engine bay or a bathroom appliance. That single property is the headline reason connectors favour PBT.
Nylon's water uptake is a feature in a gear and a liability in a connector — PBT trades a little toughness for dimensions that simply don't move.
PBT crystallises fast — far faster than its polyester cousin PET, which is why PBT, not PET, became the connector resin. Quick crystallisation means short cycle times and, more importantly, parts that are fully dimensionally stable as they leave the cavity, with low and predictable mould shrinkage. Glass fibre is added (commonly 30%) to suppress warpage from anisotropic shrinkage and to lift the heat-deflection temperature so the part survives reflow-adjacent processes and under-bonnet heat. The same fast-crystallising behaviour shows up in PET resin, where it has to be managed in the opposite direction.
Nylon also moulds well and glass-filled PA66 is dimensionally fine as moulded — the problem is what happens afterwards as it picks up moisture. For tight-tolerance connector bodies, the designer wants the part to stop changing the moment it is ejected. PBT delivers that; nylon delivers it only once you account for conditioning, which is an extra step and an extra source of error.
A connector grade is defined by three flammability and tracking ratings, and buyers should treat them as separate line items — a V-0 rating does not automatically deliver a high CTI or GWIT.
- UL94 flammability — the V-0 / V-1 / V-2 self-extinguishing classification at a stated wall thickness. Connectors are almost always V-0; always confirm the thickness the rating was tested at, because a grade can be V-0 at one wall and only V-1 thinner.
- CTI (Comparative Tracking Index, IEC 60112) — resistance to surface tracking, the conductive carbonised path that forms between live contacts under voltage plus surface contamination. Rated in volts up to 600; PLC 0 (CTI ≥ 600 V) is the top band. High-voltage and fine-pitch connectors specify high CTI, and these grades carry a premium.
- GWIT (Glow-Wire Ignition Temperature, IEC 60695-2-13) and the related GWFI — resistance to ignition from a hot wire, the key test for unattended mains-powered equipment under IEC 60335. A high GWIT lets a part sit close to live conductors in white goods and electronics without an ignition risk.
Standard FR PBT and FR PA66 both reach V-0 readily. Where they differ is in tracking: glass-filled FR grades can have a lower CTI than the unfilled base polymer, so a high-CTI connector grade is a deliberate, separately-priced formulation — not a default. If the drawing calls for CTI 600 / PLC 0, that is a specific grade, and it is not freely interchangeable with a standard V-0 grade.
| Property | PBT GF30 FR | PA66 GF30 FR |
|---|---|---|
| Water absorption (saturation) | ~0.4–0.5% | ~2.5–3.5% (unconditioned) |
| Melt temperature | ~225 °C | ~265 °C |
| Dimensional stability in humidity | Excellent — very low uptake | Moderate — swells, plasticises |
| As-moulded precision / low warpage | Excellent (fast crystallisation) | Good |
| Impact toughness | Good | Higher, esp. once conditioned |
| SMT reflow survival | Limited (lower melt point) | Better (higher melt point) |
| CTI ceiling | Up to 600 V (specific grades) | Up to 600 V (specific grades) |
| Typical use | Automotive & appliance connectors, sensor housings | High-temp, rugged or reflow connectors, terminals |
PBT is the default, not the only answer. Flame-retardant glass-filled PA66 earns its place wherever the connector has to take a beating or take the heat. Its higher melt point (~265 °C vs ~225 °C for PBT) and higher continuous-use temperature make it the choice for parts that must survive surface-mount reflow soldering, and its impact toughness — which actually improves as it conditions — suits rugged housings and field connectors. High-temperature nylons (PA46, PPA) push this envelope further for engine-compartment and LED applications.
The price of that toughness is the moisture sensitivity discussed above: PA66 parts must be measured after conditioning, and the design has to budget for dimensional growth. Where the trade-off tips toward PBT is any part where as-moulded precision and stable insulation resistance outrank peak toughness — which describes most signal and low-voltage connectors. For high-clarity or high-impact housings around a connector assembly, designers sometimes reach instead for polycarbonate grades.
Connector resins carry a compliance file as heavy as their datasheet. Flame-retardant packages in particular sit under scrutiny — halogenated FR systems must clear RoHS (restricted substances) and REACH (ECHA SVHC) screening, and many OEMs now demand halogen-free FR grades. Verify the FR chemistry and the SVHC declaration before you commit a tool; our REACH and RoHS compliance guide covers what to demand on paper. Glass-filled engineering grades also ship hygroscopic — PBT less than nylon, but both want correct drying before moulding, so confirm desiccant and moisture-barrier packaging on the order.
The practical takeaway: specify the connector resin by all four numbers — glass loading, UL94 class at its tested wall thickness, CTI and GWIT — and lock the grade early, because PBT and nylon are not drop-in substitutes for one another. PBT is the right first guess for precision and dimensional stability; reach for FR glass-filled nylon when reflow survival or impact toughness drives the part. If you are scoping a connector programme and want grade options benchmarked across origins, the OmniaStrata engineering-plastics desk can line up qualified equivalents — or talk to us directly.
Frequently asked
Questions on the desk
Why are electrical connectors usually made from PBT rather than nylon?
PBT absorbs far less moisture than polyamide — typically under 0.5% at saturation versus 2.5–3.5% for unconditioned PA6 — so connector dimensions, contact spacing and dielectric properties stay stable in humid service. PBT also crystallises quickly, giving precise, low-warpage parts straight out of the mould. Nylon swells and softens as it conditions, which can shift tight tolerances and lower insulation resistance.
What does GF30 FR mean on a connector resin spec?
GF30 means 30% glass fibre by weight, added to control shrinkage, warpage and heat-deflection temperature. FR means flame-retardant, almost always to a UL94 V-0 rating at a stated wall thickness. For connectors this combination — GF30 FR V-0 — is the default; the glass content and the flame class are the two numbers that define the grade.
What are CTI and GWIT, and why do connector buyers care?
CTI (Comparative Tracking Index, IEC 60112) measures resistance to surface tracking — the conductive carbon path that can form between live contacts under voltage and contamination; higher CTI (up to 600 V, PLC 0) is safer for high-voltage or dense-pitch connectors. GWIT (Glow-Wire Ignition Temperature, IEC 60695-2-13) measures resistance to ignition from a hot wire, relevant to unattended mains-powered equipment. Both are specified separately from the UL94 rating and are not automatically delivered by a V-0 grade.
Can PBT and nylon be substituted for each other in the same connector?
Not without requalification. They differ in moisture behaviour, melt temperature, shrinkage and chemical resistance, so a tool cut for glass-filled PBT will not deliver the same part in glass-filled PA66, and the electrical safety ratings (CTI, GWIT, V-0 wall thickness) must be re-verified for the new grade. Designers pick one early and stay with it; switching is a re-test, not a drop-in.
When is nylon the better choice over PBT for connectors?
Choose flame-retardant glass-filled PA66 when you need higher continuous-use temperature, better impact toughness, or survival through SMT reflow soldering (PA66 melts around 265 °C versus roughly 225 °C for PBT). High-temperature nylons and PA66 also resist automotive fluids well. The cost is moisture sensitivity, which must be managed by design and by conditioning before dimensional checks.
General market commentary from the OmniaStrata desk, provided for information only. It is not legal, financial, tax, or trading advice, and it is not an offer or a commitment to any terms. Figures such as price ranges, spreads, financing costs, and credit periods are illustrative market context, not OmniaStrata's rates or terms. Actual contract terms — including price, payment instrument, credit, insurance, and Incoterms — are agreed in writing on a per-transaction basis and at OmniaStrata's discretion. Market conditions change; figures reflect the publication date.