Many fabrication drawings carry one line in the material field: "FR-4, High-Tg". The fabricator can meet that line with several laminates. All of them have a high Tg, yet they can behave very differently through lead-free reflow. Tg answers a single question: at what temperature does the resin soften from its glassy state. Whether the board delaminates, or whether the plated barrel cracks, depends on other properties.
The overall order of laminate selection is covered in PCB base material selection: the four filters. This article expands only the thermal filter: what each property measures, how it is tested, and how to write it on the drawing.
Thermal properties at a glance
| Property | Meaning | Common test method | Main effect |
|---|---|---|---|
| Tg | Resin transition from glassy to rubbery state | IPC-TM-650 2.4.25 (DSC) / 2.4.24 (TMA) / 2.4.24.4 (DMA) | Z-axis expansion accelerates above Tg |
| Td | Resin decomposition temperature, usually at 5% weight loss | IPC-TM-650 2.4.24.6 (TGA) | Delamination and blistering after repeated reflow |
| α1 / α2 | Z-axis CTE below / above Tg | IPC-TM-650 2.4.24 (TMA) | Tensile strain on the plated barrel |
| Z-axis expansion 50–260°C | Total dimensional change from near room temperature to reflow peak (%) | IPC-TM-650 2.4.24 (TMA) | Cumulative barrel strain per reflow |
| T260 / T288 | Time to delamination held at 260°C / 288°C | IPC-TM-650 2.4.24.1 (TMA) | Delamination margin |
Each of these has a field in laminate datasheets and in the IPC-4101 slash sheets. Specifying Tg alone constrains only the first row.
Tg depends on the test method
The same laminate gives different Tg values by DSC, TMA and DMA. DSC detects the change in heat capacity, TMA the knee in the expansion curve, and DMA the change in modulus; DMA usually reads higher than the other two. Datasheets state the method. Before comparing Tg across two datasheets, confirm they use the same method, or the numbers are not comparable.
When the drawing sets a Tg minimum, state the method with it, for example "Tg ≥ 170°C (DSC, IPC-TM-650 2.4.25)". SKPCB's registered definition of high-Tg material is Tg >170°C.
A high Tg does not prevent delamination
Tg and Td describe two different events. Tg is a reversible physical transition: the resin returns to its glassy state on cooling. Td is irreversible chemical decomposition. The gas it releases forms voids between layers and around hole walls, which show up as delamination or blistering.
Conventional dicyandiamide (Dicy) cured systems can reach a high Tg without a matching rise in Td. Phenolic-cured and filled systems typically have a higher Td and lower Z-axis expansion, which suits repeated lead-free reflow. Two materials that both exceed 170°C Tg can therefore end up very differently after three lead-free passes. Td and T260/T288 decide that outcome, not Tg.
Z-axis CTE and barrel fatigue
Copper expands far less than epoxy does above Tg in the Z direction. As the board heats, the resin expands through the thickness and stretches the plated barrel. Once the temperature passes Tg, α2 applies and expansion speeds up. Reflow peaks near 260°C, above the Tg of any FR-4, so every reflow pass runs through the α2 region.
The effect on the barrel grows with board thickness. Thicker boards and smaller holes concentrate the strain, which is why high-layer-count thick boards are more sensitive to laminate choice. Aspect ratio and barrel copper distribution are covered in high-layer-count PCB drilling aspect ratio and plating throw; SKPCB's PTH aspect ratio limit is 10:1.
For thick boards and multiple reflows, the 50–260°C total Z-axis expansion is more direct than α1 or α2 alone: it is the dimensional change the barrel has to follow in one reflow pass.
When high Tg is needed
Specify high Tg together with the related thermal properties when:
- The board is thick or has many layers, so cumulative Z-axis strain on the barrel is large.
- Assembly requires several lead-free reflows, for example double-sided placement plus rework.
- The product operates at elevated temperature for long periods, near or inside the Tg range of standard FR-4.
- The stack-up has heavy copper layers or large copper areas, which create internal temperature gradients during lamination and reflow. Resin fill in heavy copper lamination is covered in heavy copper PCB lamination and resin fill control.
When high Tg is not needed
Thin, low-layer-count boards with a single reflow and moderate operating temperature usually work with standard-Tg material. High-Tg laminates are generally harder and more brittle, wear drill bits faster, and cost more. Specifying high Tg without a thermal stress requirement buys cost, not reliability.
If the real concern is delamination, set requirements on Td and T260/T288 directly instead of only raising Tg.
How to write it on the drawing
To turn "High-Tg" into acceptance criteria, use this structure:
- Material class: cite the IPC-4101 slash sheet number, or state "supplier to submit laminate datasheet for designer approval".
- Tg: minimum plus test method, for example "Tg ≥ 170°C (DSC)".
- Td: minimum plus test method (TGA, 5% weight loss).
- Z-axis expansion: maximum 50–260°C total expansion, or maximum α1/α2.
- Time to delamination: minimum T260 or T288, chosen by reflow count and board thickness.
- Substitution rule: any alternate laminate must meet every item above and be confirmed in writing.
UL recognition adds a separate set of limits on material and thickness; see reading UL material recognition as a stack-up design input. Overall multilayer capability and available laminates are on Multilayer PCB and Materials. To have a laminate reviewed against a specific stack-up, submit data through Quote.
Summary
Tg only limits the resin's softening temperature. Delamination under lead-free reflow depends on Td and T260/T288; barrel fatigue depends on Z-axis expansion. When the drawing says "High-Tg", write these properties with their test methods as minimums or maximums, so that any substitute laminate has a basis for acceptance.
FAQ
Q: What is the difference between Tg and Td?
A: Tg is the reversible temperature at which the resin softens. Td is the temperature at which the resin starts to decompose; decomposition is irreversible and causes delamination or blistering.
Q: Why does the same laminate show several Tg values?
A: DSC, TMA and DMA measure different physical changes and give different readings. Compare Tg values only when they use the same method.
Q: Will a 170°C Tg material always survive lead-free reflow?
A: Not necessarily. The reflow peak is above Tg. Delamination resistance depends mainly on Td and T260/T288, and barrel reliability on Z-axis expansion.
Q: Why does Z-axis CTE affect plated through-holes?
A: Resin expands through the board thickness far more than copper, so heating stretches the barrel. The thicker the board, the larger the cumulative strain.
Q: Should every multilayer board use high-Tg material?
A: No. Thin boards with a single reflow and moderate operating temperature usually work with standard Tg. High-Tg laminates are harder and cost more to process.
Q: What does SKPCB define as high-Tg material?
A: The registered definition is Tg >170°C; high-CTI material is CTI >600V. The specific laminate is confirmed during stack-up review.