Card Edge Finger Tarnish (1 reply)
Hi Kevin,
The information on the website is the Glass Transition Temperature of the laminate: (•High Glass Transition Temperature (Tg) (150Tg or 170Tg)
The glass transition is the temperature range in which the amorphous molecules of the resin transition from rigid and brittle to flexible
and pliable. We are not aware of a test method you are describeing (150C for 72 hours). This is not the temperature at which the material is expected to operate and if you could provide the IPC Test Method that you are trying to perform we would be glad to review the test parameters.
The IPC test for Glass Transition is IPC-TM-650 2.4.25. I will attach a copy of it.
Also attaching an article from Isola, a well-known laminate manufacturer.
Understanding Glass Transition
Temperature Measurements of
Printed Circuit Boards by DSC
Written by Peg Conn, Senior Director of the Analytical Services Laboratory at Isola
and Cassie Volden, Analytical Services Laboratory Manager
DSC, Differential Scanning Calorimetry, measures the flow of heat into or out of a sample compared to
a reference. A stepwise change in the heat flow indicates the glass transition. The glass transition is the
temperature range in which the amorphous molecules of the resin transition from rigid and brittle to flexible
and pliable. This range is represented by a single temperature which is identified by an agreed upon method.
Glass and copper do not exhibit this transition; however, their presence can affect the flow of heat into and out
of the sample. IPC-TM-650 defines the use of DSC in the measurement of the glass transition temperature
and the degree of cure in laminates. The sample is heated through the transition region on the first scan, held
isothermally above the glass transition, then cooled and heated through the transition on the second scan. The
glass transition temperature is reported as the temperature at the half height between the two steps. The two
glass transitions are then compared.
Measurements of a laminate core at the time of manufacturing produces a DSC scan which clearly shows the
transition region (Figure 1). Printed wiring boards are comprised of various combinations of laminate cores,
prepreg and copper. The particular combination of prepreg, core and copper can affect the heat flow in samples
measured by DSC causing the different layers of the sample to heat at different rates. This thermal lag causes
the transitions of the different layers to appear to occur at different temperatures, is often exhibited in the DSC
scans by multiple glass transitions (Figure 2), and elongated transition regions (Figure 3). Samples with greater
thermal lag result in transitions, which are distinct enough to be identified. Lower levels of thermal lag appear
as elongated transition regions because the transitions cannot be distinguished.
I have recently got a batch of PCBs back from manufacturing. I performed a thermal soak of the 6 boards at 150C with no issues. I then soaked them at 150C for 72 hours. After that test I saw that all of the card-edge fingers were severely tarnished. I am suspecting there may be a board washing issue where residue on the board reacted with the lead-free solder on the board, but once again, only on the fingers.
In reviewing the FR-4 specs, nothing should have really happened.
https://devtest12345.wpengine.com/fr4-material-details/
Any ideas as to what has caused this tarnish?
Thanks