Time to Delamination Testing: T260, T288 and Laminate Selection

Time to delamination is the period a laminate survives at a fixed elevated temperature before the resin separates from the glass reinforcement or from the copper foil. It is measured on a small sample in a thermal mechanical analyser, and the result is quoted as T260 or T288 with a number of minutes. The value matters because it describes how the material behaves at the temperatures reached during assembly, particularly during lead-free reflow and during repeated rework.

What the Test Measures

The sample is a small piece of laminated board, clamped in a probe that follows its thickness, and heated to a set temperature. As the resin softens and then degrades, the sample expands and the probe records the movement. Delamination appears as a sudden change in the thickness signal, and the time from the start of the hold to that change is the result.

The test is destructive and quick, which is why it is used both for incoming material qualification and for process control, and why it can be repeated on every delivery without a significant cost. A laminate that passes on receipt but shows a falling time after lamination indicates a problem in the press cycle or in the moisture content of the material.

T260, T288 and T300 Methods

The number in the designation is the test temperature in degrees Celsius, and the higher the temperature the shorter the time for the same material. T260 is the most common screening test, T288 is used for materials intended for lead-free assembly and for thick boards, and T300 is applied to high-performance laminates.

The pass criterion is usually expressed as a minimum time, for example 30 minutes at T260 or 10 minutes at T288, and it is written into the material specification rather than into the board drawing. The test method itself comes from the applicable IPC test method, and the equipment, heating rate and sample geometry are defined by it.

Laminate Tg and Thermal Decomposition

Laminate Tg is the temperature at which the resin changes from a glassy to a rubbery state, and it is related to but distinct from time to delamination. A higher Tg generally gives a longer time at a given temperature, because the resin retains its stiffness further into the range, but the correlation is not exact.

Thermal decomposition, the temperature at which the resin begins to break down chemically, is the second reference point. A laminate can hold its thickness at T260 for a long time and still decompose at a much lower temperature if the exposure is long enough. The two properties together describe the material better than either alone.

Sample Preparation and Conditioning

Moisture is the variable that most easily spoils the comparison between two measurements. A sample that has absorbed water from the air will show a shorter time than a dry one, because the water vaporises and forces the layers apart from inside. Samples are therefore dried before the test, and the drying condition is part of the method.

The sample should also come from the same position on the panel and be the same size each time. Copper content affects the result, so a sample with heavy copper planes behaves differently from one in a signal-only area. For incoming material checks the sample is usually taken from a copper-free area, so that the result describes the resin system.

Interpreting the Result

A result longer than the specification means the material passes, but the number itself carries information. A laminate that gives 40 minutes where the specification asks for 30 has margin; one that gives 31 has almost none, and any change in the press cycle or in the moisture content may push it below.

Trending the result across lots is more useful than comparing each lot with the limit. A slow decline across several deliveries points to a change at the laminate supplier, while a single low value usually points to the sample or to the test rather than to the material.

Failure Modes in the Test

Delamination can appear at several interfaces. Separation between the resin and the glass weave is the classic mode; separation between the copper foil and the resin is a different one and points to the bonding treatment on the foil. A clean break in the middle of the resin indicates that the resin itself has degraded.

The failure mode should be recorded together with the time, because it identifies which part of the system is limiting. A shop that records only the minutes loses the information that would allow the laminate supplier and the press parameters to be separated. Post-test examination of the sample under a microscope is a cheap addition, and the usual microsection preparation methods apply once the sample is cast.

Linking the Result to Assembly Processes

The assembly process has to stay within what the material can survive. A laminate rated for 30 minutes at T260 may see only two or three minutes at peak temperature during a single reflow, which leaves a large margin. Two reflow passes, a wave soldering step and a rework cycle in sequence reduce that margin substantially, and the accumulated exposure is what causes the failure in the field.

Where a board is known to see several thermal excursions, the comparison that matters is between the accumulated time above the relevant temperature and the material rating. Thermal profiling records the exposure, and the standard board level thermal testing methods show how the temperature actually varies across a panel during each pass.

Limits of the Test

Time to delamination measures a small sample under laboratory conditions and says nothing directly about a full board with holes, copper planes and a complex stack. A board can pass the material test and still delaminate in production if the press cycle is wrong, if the board is wet when it enters the oven, or if the drilling leaves smear that creates a weak interface.

The test is therefore a material screening tool rather than a product qualification. For the product, the relevant evidence comes from a combination of thermal stress testing on finished boards and sections taken afterwards to look for separation at the interfaces that the material test cannot reproduce.

Records and Material Qualification

A material qualification record should carry the laminate type, the manufacturer’s lot, the drying condition, the test temperature, the time achieved and the observed failure mode. That set of fields makes it possible to compare deliveries, to investigate a field failure and to support a change of supplier with evidence rather than with opinion.

Where a shop laminates in-house, the qualification record should also carry the press cycle and the resulting Tg, since the press determines whether the material retains the properties it was purchased for. Comparing the incoming test with the test on the finished board closes the loop between the supplier and the process.

FAQ

What is a good time to delamination figure? It depends on the material class and the specification, but 30 minutes at T260 is a common screening requirement and 10 minutes at T288 is typical for materials intended for lead-free assembly.

Why does moisture change the result? Because water trapped in the laminate vaporises during the test and forces the layers apart from inside, shortening the time. Samples are dried before testing so that the comparison is between materials rather than between moisture contents.

Does a passing test guarantee the finished board? No. The test describes a small sample of bare laminate. Press conditions, drilling quality and the number of thermal excursions in assembly all affect the finished board, and those are covered by stress testing on the product.