Fixture Materials
G10, FR4 and Phenolic Fixture Details, Machined
The insulating laminate that is stiff, dimensionally stable and does not conduct, for the fixtures that sit next to electronics.
G10 and FR4 are glass-reinforced epoxy laminates used for fixture details that have to insulate and stay stiff: probe and guide plates, standoffs, wave solder pallets and high-voltage barriers. Published data puts dielectric strength at up to 500 volts per mil and flexural strength around 70,000 PSI, with low moisture absorption. FR4 is the flame-retardant version. Send the print and hole data; a person quotes in 24-48 hours, standard lead time 4-6 weeks.
Updated September 2026
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What the laminate is, and why fixtures use it
G10 is a high-pressure laminate: layers of glass fabric impregnated with epoxy resin and cured under heat and pressure into a solid sheet. Wikipedia’s G-10 entry (en.wikipedia.org/wiki/G-10_(material)) covers the construction. FR4 is the same idea with a flame-retardant resin system, which is why it is the substrate most circuit boards are built on and why it is the default when a fixture sits in or near an electrical assembly.
For fixture work the useful combination is: it insulates, it is stiff, it holds a machined size, and it barely absorbs moisture, so a plate cut in a dry month is the same plate in a humid one. Piedmont Plastics’ G10/FR-4 page (piedmontplastics.com/products/g10-fr-4) lists dielectric strength up to 500 volts per mil, flexural strength around 70,000 PSI and low moisture absorption. Nothing else in the plastics bench gives all four at once.
Where it goes in a fixture
The most common use is a test fixture stack: probe plates drilled to a hole table, guide plates dowelled to them as a matched pair, and the board nest that sits between. All three need to hold hole position across a large plate without flexing under probe force, and none of them may conduct. Machined test fixture details covers that job in full.
The other uses follow from the same properties. Insulating standoffs and barriers where a live assembly has to be held clear of a metal base. Wave solder pallets, which mask a board through a solder wave and have to survive the thermal cycle. Mounting plates and spacers in high-voltage test rigs. And ordinary structural details in a fixture that happens to sit inside an electrical enclosure, where an aluminum part would be a hazard rather than a help.
Phenolic, and when it is the better answer
Paper- and cotton-based phenolic laminates sit alongside G10 on the same shelf and cost less. They machine more easily and are quieter and more forgiving as bearing and wear surfaces, which is why phenolic shows up in guides, spacers and low-stress insulating details. What they do not match is G10 on strength, on moisture resistance or on dielectric performance.
So the split is roughly: G10 or FR4 where the detail is structural, drilled to a tight hole position, or genuinely electrically loaded, and phenolic where the job is a modest insulating spacer or a quiet wear face and the cost saving is worth having. Say which properties the detail is bought for and the quote will name the grade rather than defaulting to the most expensive one.
Machining glass laminate: what it means for the print
Glass-reinforced laminate is abrasive. It wears cutting tools fast, it produces a fine dust that has to be controlled, and it will fray or chip at an edge if it is cut carelessly. That shows up in the quote as a real cost difference against an aluminum part of the same shape, and it is worth knowing before a large plate is specified in G10 when phenolic or an anodized aluminum plate would do.
It also shapes the print. Edges that matter should carry a break or a chamfer callout so a frayed edge is not left to judgement. Holes want a stated size and position tolerance rather than a nominal, because that is the callout the plate is bought for. And thickness tolerance on laminate is looser as supplied than on metal plate, so if the stack height matters, say the finished thickness on the print and the faces are machined to it.
Matched plates, dowels and stack height
A probe plate and a guide plate only work if they agree with each other. Ordered separately, each plate is held only to its own print. Quoted and inspected together as a matched set and dowelled together, the pair is held to the stack. State on the print that plates are a matched set, give the dowel scheme with fit classes, and give the stack height and the standoff lengths that set it.
Standoffs are the other half of that. Their length sets probe travel and board clearance, so they are a toleranced detail, not hardware to be picked on the bench. Call the length and its tolerance out and they are machined and inspected with the plates on the same order, which is the only way the stack arrives ready to bolt together.
What to send, what is inspected, and when it ships
Send the print and STEP model naming the grade and thickness, the hole data as a table or drill file with sizes and the datum they reference, the position tolerance you need, edge break callouts, finished thickness if the stack height matters, and the dowel scheme if plates are a matched set. Any file type is accepted, STEP plus PDF plus the hole data is ideal, and the quote lists what is missing.
Every detail is inspected against its print before it ships, and plates ordered as a matched pair are inspected together. One plate or a full stack is a normal order with no minimum. Rush 1-2 weeks applies to machined parts with no secondary operations. Heat treat, grinding, EDM and finishing add time, and the quote gives the real date. Standard lead time is 4-6 weeks, and extended is 6-8 weeks when timing is flexible. Lead time starts at order approval. Put your need date in the part description and the quote answers it.
Questions
Before you send a job.
01 What is the difference between G10 and FR4?
They are the same kind of glass-reinforced epoxy laminate; FR4 uses a flame-retardant resin system and G10 does not. FR4 is the default where the fixture sits in or near an electrical assembly and flammability is a consideration, which is why circuit boards are built on it. Mechanically and dimensionally they behave much the same.
02 When should a detail be phenolic instead of G10?
When it is a modest insulating spacer, a guide or a quiet wear face rather than a structural or electrically loaded part. Phenolic machines more easily and costs less, but does not match G10 on strength, moisture resistance or dielectric performance. Say what the detail is bought for and the quote will name the grade.
03 Why does a G10 plate cost more than an aluminum one?
Because glass-reinforced laminate is abrasive: it wears cutting tools quickly and produces a fine dust that has to be controlled. That is a real cost difference on the same shape. If a large plate does not actually need the dielectric performance, phenolic or an anodized aluminum plate is often the better answer and the quote will say so.
04 How do I get a probe plate and guide plate to agree?
Order them as a matched set. Ordered separately, each plate is held only to its own print. State on the print that they are a matched pair, give the dowel scheme with fit classes, and give the stack height and standoff lengths that set it, so all of it is quoted and inspected together as one stack.
05 Does the print need an edge break callout?
It helps. Laminate can fray or chip at a cut edge, so edges that are handled, that mate, or that are visible should carry a break or chamfer callout rather than being left to judgement. Holes should carry a stated size and position tolerance too, since that is the callout the plate is bought for.
06 Is laminate thickness accurate as supplied?
Looser than metal plate. If the stack height matters, say the finished thickness on the print and the faces are machined to it rather than relying on the sheet as supplied. Standoff lengths should carry a tolerance for the same reason, because they set probe travel and board clearance.
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