What damage does the assembly process do to a pcb? (part 2)
In part 1 of this blog post I commented upon the affect the assembly process has upon a printed circuit board. The assembly temperatures applied do in fact burn away the epoxy of the FR-4 composite material. The higher the temperature the faster the rate of burn. I touched upon the relation ship between the glass transition temperature (Tg), decomposition temperature (Td) and the Maximum continuous Operating Temperature (MOT). There is another gauge that can be used to help a designer or contract assembler understand this point and that is the Time to delamination test. These are referred to as either the T260 or T288 tests.
What is the T260/T288 Time to Delamination? This is a test defined by the IPC under IPC-TM-650 Test Method 188.8.131.52. A test sample is incrementally raised in temperature 10°C/min to 260°C (or 288°C) and then dwells at 260°C (or 288°C). When an event occurs such as delamination, cracking, moisture release, stress relaxation, decomposition or a sudden movement a sensor detects the change. The time in minutes from the start of the dwell time to the event at 260°C (or 288°C) is the time to delamination.
How does this value relate to anything in the assembly process? It is a gauge of how durable the FR-4 shall be when exposed to heat. There are some important facts to point out with these tested values.
- These are common values found on the FR-4 data sheet from a laminate manufacturer. Keep in mind FR-4 is but one type of laminate printed circuit boards are made from.
- These values shall vary from laminate manufacturer to laminate manufacturer. No two manufactured grades of FR-4 from different laminate manufacturers are alike. They all meet the minimum requirements to be labeled as FR-4. Some laminate manufacturers are better than others.
- The tested values that appear on a data sheet are from a pristine sample. This is a value that reflects the fully cured laminate straight from the laminate manufacturer.
- The tested value on the laminate manufacturer’s data sheet is as good as it gets. As we say here in the states, “Its all down hill from here.” As soon as you start building a printed circuit board you introduce variables. Variables are seldom a good thing. For example, when you form plated through holes you have to drill holes. When you drill a hole there shall be some fracturing of the laminate at the hole wall. The fracturing is an imperfection but its a result of the printed circuit board manufacturing process. Fracturing is a potential weakness and ultimately a source from where delamination may propagate from.
- The tested value on the laminate manufacturer’s data sheet does not reflect the printed circuit board manufacturer’s over-all process. It is not feasible to require a T260/T288 tested value on every printed circuit board design from every manufacturer. It an indication of the laminates potential, not the manufactured printed circuit board.
The higher the value the better the laminate shall perform when exposed to heat. For conventional eutectic soldering consisting of tin-lead the T260/T288 results are not a critical piece of information to have. For lead free soldering temperatures its an important piece of information to consider. For example, compare two common laminate systems from the same laminate manufacturer…
Epoxy System: Dicey cured Tg 170°C (Traditional FR-4)
- T260 = 10 minutes
- T288 = 2 minutes
Epoxy System: Phenolic cured Tg 180°C (Lead-Free Assembly Compliant FR-4)
- T260 > 60 minutes
- T288 = 30 minutes
As you can see the difference between the two results is dramatic. The T260 test is essentially close to lead-free assembly temperatures. Using a Dicey cured epoxy system for lead-free soldering applications is not advisable. The closer you are to lead-free soldering temperatures the faster the epoxy burns away. The Phenolic cured epoxy system offers much more in regards to thermal resistance and durability. The Phenolic cured epoxy still burns away but at a much slower rate. You’ll notice that the T260 result for the Phenolic cured laminate is greater than 60 minutes. This only means that the laminate exceeded 60 minutes and the laminate manufacturer stopped the test since the laminate didn’t fail.