PBT vs PA66 for Connectors and Housings: How to Choose

Open a wiring harness and most connector housings are one of two materials: glass-filled PBT or glass-filled PA66. Both mould well, both take flame retardants, and both are available everywhere. They differ in how they handle water, heat and toughness, and those differences decide which one survives your validation plan.

The datasheet comparison

Typical values from two published BASF datasheets, Ultradur B4300 G6 HR (PBT-GF30) and Ultramid A3EG6 (PA66-GF30), for orientation only. “Cond.” means equilibrium at 23 °C and 50% RH; the PBT grade is published with single values because its moisture uptake is small. Values for OSABATE and OSAMIDE grades are on each grade TDS.

Moisture: PBT’s main advantage

Polyamide absorbs water from the air until it reaches equilibrium. In the PA66 reference grade, that takes tensile modulus from 10,000 to 7,200 MPa, a loss of 28%, and it changes the part’s size as it swells. The electrical values move too: volume resistivity falls from 10¹³ to 10¹⁰ ohm·m, and relative permittivity at 100 Hz rises from 4.2 to 9.7 between dry and conditioned.

PBT takes up between a tenth and an eighth as much water, so a PBT connector measured on the day it is moulded is essentially the connector that goes into service. Pin positions, latch engagement and seal compression stay where the drawing put them. For dense, small-pitch connectors and electronic housings with tight tolerances, this is usually the deciding factor.

Moisture is not all bad for PA66. Absorbed water plasticises the nylon, so conditioned notched Charpy almost doubles, from 11 to 20 kJ/m². A PA66 latch that cracks dry-as-moulded may flex happily once conditioned, which is why nylon connectors are sometimes moisture-conditioned before assembly. The effect of moisture on part size is covered in our guide to drying nylon and dimensional stability.

Heat: PA66’s main advantage

PA66 GF30 melts at 260 °C and carries an HDT of 250 °C. PBT GF30 melts at 223 °C with an HDT of 205 °C. For parts close to an engine, an exhaust or a hot busbar, that margin is real. Long-term service depends on the heat stabiliser package as much as on the polymer: the PA66 reference grade quotes a temperature index of 165 °C at 5,000 hours. Compare stabilised grade with stabilised grade, on ageing data at your temperature.

Hydrolysis: PBT’s weak point

PBT is a polyester, made by a condensation reaction that releases water. Given enough heat and humidity, the reaction runs backwards and the chains are cut. BASF’s published work on hydrolytically stable PBT showed a standard PBT GF30 with no hydrolytic degradation after storage at 50 °C and 75% RH or in water at 60 °C. At 85 °C and 85% RH, hydrolysis did occur.

That matters because connector qualification is built around exactly those conditions. Cycle tests in humid air from -40 to 150 °C are common for under-hood connectors, and USCAR-type test plans include high-humidity cycles. For under-hood and powertrain connectors, specify a hydrolysis-resistant PBT and ask for retained tensile properties after the humidity ageing in your specification. PBT also has to be dried well before moulding: the BASF processing data calls for less than 0.04% moisture.

Warpage and appearance

In these datasheets the PBT grade shrinks 0.50% along the flow and 1.30% across it, against 0.53% and 1.02% for PA66. That larger difference can warp flat housings and long, thin connector bodies. A glass-plus-mineral PBT reduces the anisotropy at some cost to strength, which is why our OSABATE 506 GF15 MR15 exists alongside the 30% glass OSABATE 506 GF. PBT also crystallises fast and gives a good surface, which helps short cycles.

Flame retardancy, glow-wire and tracking

Both polymers are routinely made flame retardant for connectors, terminal blocks and relay parts, with brominated or halogen-free packages. The rules that apply are the same for both:

• A UL 94 rating belongs to a thickness and a colour. Ask for the rating at your thinnest wall.
• Appliance parts follow IEC 60335-1. For parts supporting connections above 0.2 A in unattended appliances, the first route is a material GWFI of at least 850 °C, then a GWIT of at least 775 °C, or end-product glow-wire tests.
• CTI sets creepage distances. In the reference datasheets the PA66 grade lists 550 and the PBT grade 400, but flame-retardant packages move CTI, so compare the FR grades you will actually buy.

Our lab runs glow-wire tests during development, and certified UL 94 testing can be arranged at external laboratories. Options per family are on our flame-retardant compounds page; high-voltage connector requirements are covered on EV compounds.

How to choose

Mapping to AAP grades

In our OSABATE PBT range, 503 GF, 504 GF and 506 GF carry 15, 20 and 30% glass, 506 GF15 MR15 combines glass and mineral, and 5000 is an unfilled anti-static grade. Flame-retardant versions are made for connectors and electrical parts. On the polyamide side, OSAMIDE AISG 6 is PA66 with 30% glass and AISG 3 carries 15%.

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