How to Choose the Right FR-4 Tg for Your PCB
Glass transition temperature decides how a board behaves during soldering and in service. Here is how to pick standard, mid or high Tg laminate without over-specifying it.
For most commercial boards, standard Tg FR-4 in the 130 to 140 degrees Celsius range is enough. Move to mid Tg at 150 to 160 degrees when the board goes through more than two lead-free reflow cycles, and to high Tg at 170 degrees and above for automotive, industrial and any design whose sustained operating temperature exceeds 105 degrees.
Tg is not a maximum operating temperature. It is the point where the resin changes from a rigid glassy state to a softer rubbery one. Above Tg the laminate expands much faster in the Z axis, which is what stresses plated holes and can cause barrel cracking or corner delamination.
01What Tg actually measures
Manufacturers report Tg by one of three methods: DSC, TMA or DMA. The numbers are not interchangeable. A laminate rated Tg 170 by TMA may read around 150 by DSC. When you compare materials from two suppliers, confirm the test method before concluding that one is better.
The related figure that matters just as much is Td, the decomposition temperature, and the Z-axis CTE below and above Tg. A board can have an acceptable Tg and still fail thermal cycling if its Z-axis expansion above Tg is high.
- Tg: the glass transition point, reported by DSC, TMA or DMA
- Td: decomposition temperature, typically 310 to 400 degrees Celsius
- Z-axis CTE below Tg: usually 50 to 70 ppm per degree
- Z-axis CTE above Tg: often 250 to 300 ppm per degree, the main driver of hole wall stress
- T260 and T288: time to failure at a fixed temperature, used for lead-free qualification
02Choosing by application
The table below is how we normally guide material selection during engineering review. It assumes lead-free SAC305 soldering with a peak reflow temperature around 245 degrees Celsius.
| Standard Tg, 130 - 140 C | Consumer electronics, single and double-sided boards, short assembly cycles |
|---|---|
| Mid Tg, 150 - 160 C | Multilayer boards, lead-free assembly with two or more reflow passes |
| High Tg, 170 C and above | Automotive, industrial drives, telecom infrastructure, boards above 105 C in service |
| Polyimide | Flex and rigid-flex construction, sustained temperature above 200 C |
| Low-loss, mid-loss | High speed digital and RF where Dk and Df matter more than Tg |
03What high Tg costs you
High Tg laminates are more expensive per sheet, and they are less forgiving in processing. They need higher lamination temperature and pressure, and they drill differently: the resin is harder, so tool wear rises and hole wall quality depends more on drilling parameters than on the material alone.
There is also a stack-up consequence. Mixing a high Tg core with a standard Tg prepreg in the same board creates mismatched expansion during lamination, which shows up as warp or bow. If you need high Tg, specify it for the whole stack-up rather than for one or two layers.
04Practical rules we apply
These are the checks that catch most material problems before a board is released to the line.
- Match Tg to the number of reflow cycles, not just to the operating temperature
- Keep the whole stack-up on one Tg class to avoid lamination mismatch
- Confirm the test method when comparing two supplier datasheets
- Check Z-axis CTE above Tg for boards with a high aspect ratio of hole depth to diameter
- For boards above 1.6 mm thick, verify that the press cycle can cure the inner resin fully
Key takeaway
Specify Tg against your assembly profile and your real operating temperature, keep the whole stack-up on one class, and confirm the test method behind any number on a datasheet.
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