The coefficient of thermal expansion, or CTE, describes how much a material changes dimension as its temperature changes. On a PCB, that movement matters in more than one direction. Expansion through the board thickness can place stress on plated interconnections; expansion in the plane of the board can affect the mechanical relationship between the board and attached components.
CTE is useful for screening a laminate and discussing reliability, but a datasheet value is not a prediction of when a finished board or via will fail. The stackup, hole construction, copper, temperature history and test conditions all matter.
Read the direction before reading the number
PCB datasheets often distinguish the X and Y directions, which run along the board surface, from the Z direction, which runs through its thickness. Reinforcement and the rest of the material construction can affect these directions differently.
For an in-plane concern, such as the movement of a board relative to a mounted package, start with X/Y data and the relevant assembly geometry. For an interconnection running through the stack, examine Z-axis behavior and the actual plated-hole construction. A single CTE figure without an axis and temperature range leaves out information needed for either assessment.
Where each direction enters a design review
| Direction or interface | What the team is reviewing | Other information needed |
|---|---|---|
| X/Y, along the board | Relative movement between the board and mounted components, connectors or mechanical supports | Component construction, attachment geometry, temperature range and mounting constraints |
| Z, through the thickness | Movement of the stack around plated interconnections during a thermal exposure | Stackup, finished thickness, hole geometry, plating requirements and the exposure profile |
| Between dissimilar layers | How the combined construction responds as materials heat and cool | Actual laminate and prepreg construction, copper distribution, interfaces and fabrication process |
These rows identify questions, not failure predictions. A laminate can look favorable on one property and still need review for the full board construction and use environment.
Why the temperature range matters
Many polymer-based PCB materials change their expansion behavior around their glass transition temperature, Tg. A datasheet may therefore list one Z-axis expansion coefficient below Tg and another above it. It may also report total expansion over a defined temperature interval.
These are different kinds of information. A coefficient in parts per million per degree Celsius describes a rate for the specified conditions. A total expansion percentage describes the overall change across the stated interval. Do not compare the two as if they were the same measurement, or compare coefficients from different temperature intervals without noting the difference.
Tg itself depends on the measurement method. If a calculation uses a transition point from a thermal expansion curve, do not silently substitute a Tg measured by a different method. The Tg guide introduces the transition; the datasheet for the exact material supplies the values and test details needed for a specific comparison.
Read an actual datasheet
Isola's 370HR revision E datasheet lists these typical values. It cites IPC-TM-650 2.4.24C for Z-axis expansion and identifies Tg by DSC.
| Datasheet property | Typical value |
|---|---|
| Tg by DSC | 180°C |
| Z-axis CTE below Tg | 45 ppm/°C |
| Z-axis CTE above Tg | 230 ppm/°C |
| Total Z expansion, 50°C to 260°C | 2.8% |
The coefficients and total expansion answer different questions, and the DSC result should not automatically be used as a breakpoint in a calculation based on another test method. These figures describe one material under the supplier's stated conditions; they are not limits for all FR-4 materials, guaranteed values for a finished board, or a via fatigue-life calculation.
| Datasheet field | Why it matters |
|---|---|
| Material grade and revision | Similar family names do not establish identical properties. |
| Axis and temperature interval | X/Y, Z, below-Tg and above-Tg figures answer different questions. |
| Test method and specimen | Preparation and measurement affect what the reported value represents. |
| Typical or guaranteed | A typical property is not automatically a contractual acceptance limit. |
| Total expansion, if reported | Compare percentages only when the stated temperature ranges and conditions are compatible. |
What does ppm/°C mean in a calculation?
For an unconstrained, uniform sample over a temperature interval where a constant coefficient is a suitable approximation, the change in length can be estimated as:
change in length = initial length × CTE × temperature change
Convert a CTE stated in ppm/°C to a fraction per degree by multiplying it by 10^-6. For a hypothetical sample that is 1.00 mm long, assume a constant coefficient of 45 ppm/°C across a 50°C rise:
1.00 mm × 45 × 10^-6/°C × 50°C = 0.00225 mm, or 2.25 µm.
This is an arithmetic example for a free, homogeneous sample. It does not establish the actual expansion of 370HR over a particular interval, even though the assumed coefficient happens to match one typical figure in its table. It also does not calculate the strain in a copper barrel. A real PCB is a bonded construction in which layers and interconnections constrain each other.
Do not apply one coefficient across a temperature interval that spans a transition without a suitable model and relevant measured inputs. In particular, the DSC Tg in the example datasheet should not automatically become the breakpoint for a calculation based on a different measurement method.
Compare two sheets without creating a false ranking
Place the sheets side by side and start with the temperature interval that matters to the project. If one reports total Z expansion over a stated range while another gives only separate coefficients, leave the fields separate until the measurements and calculation assumptions have been reconciled. A blank specimen-preparation field is unknown; it is not evidence that the two tests used identical specimens.
Next identify the actual material construction proposed in the stackup. A product-family name does not by itself describe every core, prepreg or finished-board configuration. Ask which values apply to the proposed construction, which are typical, and what the fabricator can confirm for the order.
Check the cited test method and specimen preparation before comparing a laminate measurement with expansion measured on a finished, copper-containing board. Confirm the method revision required by your own specification.
Separate assembly heat from the service environment
A board may see soldering and rework excursions during assembly and a different temperature cycle in service. The two histories can involve different temperature ranges, times and numbers of cycles. If an assembly process crosses Tg while the service cycle stays below it, both parts of the material's behavior may be relevant.
Record the planned reflow profile and any anticipated rework. Separately describe the service temperatures, dwell times and expected cycling. Avoid selecting a material on Tg alone or using a laminate's time-to-delamination value as a plated-hole lifetime. The T260 and T288 guide explains what those measurements address.
Build a thermal history the whole team can use
| Stage | Information to collect | Decision it informs |
|---|---|---|
| Assembly | Intended soldering profiles and the number of planned exposures | Material and construction review for the actual assembly route |
| Rework | Expected local or whole-board heating, where known | Whether the proposed repair process should be included in evaluation |
| Service | Expected temperature range, dwell and cycling | A representative use environment for qualification planning |
| Investigation | Observed location and condition of any damage | Which construction, exposure and inspection records to compare |
Where an exposure is not known, mark it as an open input. A guessed rework count or an unspecified service cycle cannot support a defensible qualification plan.
From laminate data to plated-hole evaluation
Imagine a thick multilayer board with plated through-holes that may require component rework. Before choosing between two materials, the design team shares the proposed stackup, finished thickness, hole dimensions and copper requirements with the fabricator. The assembler supplies the intended thermal profile and rework assumptions.
The team can then compare material data over the relevant temperature ranges and decide whether a representative test coupon and a defined thermal exposure are needed. A cross-section or other inspection may help investigate damage, but the method and sample plan need to be selected for the actual design.
An unconstrained expansion calculation can illustrate how a material responds to temperature. A real plated hole is part of a bonded, constrained multilayer structure, however, so that calculation is not the strain in its copper barrel or a prediction of cycles to failure. For coupon planning, see PCB test coupons; for the construction itself, see PCB vias.
Make the coupon representative of the question
Begin by stating the decision the test needs to support. A coupon intended to investigate plated-hole performance should be discussed in relation to the hole and stackup being evaluated, not chosen only because it is available. Document the material and construction, finished thickness, relevant hole geometry and copper requirements, together with the intended assembly preconditioning and service exposure.
Before starting, agree on the inspection locations, measurement method, sample plan and acceptance criteria. Keep the laminate datasheet, coupon results and finished-product qualification in separate records. Each is useful evidence, but it answers a different question. A passing coupon should be interpreted within the construction and test conditions it actually represents.
If results are unexpected, preserve the failed and comparison samples and record their exposure history. Review potential differences in construction, test setup and assembly processing before attributing the result to a single datasheet property.
What to send with the fabrication request
Include the material grade or permitted alternatives, proposed core and prepreg construction, finished thickness, relevant hole and copper requirements, assembly and rework profiles, and service environment. Mark any required test method and acceptance criteria, and identify which material substitutions require approval.
The FR-4 material properties guide provides a broader view of the other datasheet fields. To review a specific stackup and obtain available options, request a quote or contact APTPCB.