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Material Witness by Chet Guiles
Anyone who's been in the lamination business for a long time, whether it is laminating full sheets of copper clad material, or fabricating multilayer PWBs, has encountered the "moisture gremlin" at some stage of the process. Over the years we have frequently warned people about the tendency for polyimide, for example, to absorb water when left standing too long before processing. Prepregs need to be desiccated and inner layer details baked dry before lamination, or else excessive frothy flow, blistering or delamination can occur. This logically carries over into assembly or any other post-processing that involves high temperatures, and as we will see, epoxy, while less water sensitive than polyimide, still absorbs enough water when left standing to do harm.I was recent asked if I had any written documentation that discussed drying requirements for polyimide PWBs prior to assembly processing. I had to say "no," but that common sense would suggest that materials well known to have to be dried before they are laminated would logically need to be baked again before HASL, IR reflow or any other heat-shock encountered during subsequent processing.A number of years ago Dr. Subhotash Khan, of DuPont, did some work with levels of moisture in FR-4 multilayer PWBs to determine how much moisture would do positive harm in post processing of PWBs. His results suggested that more than 0.22% retained moisture in an epoxy PWB would result in blistering when floated on a 500°F (260°C) solder pot for 10 seconds. In addition, his work indicated the rate at which a PWB absorbed moisture at different temperature and humidity conditions. Even at relatively low RH, epoxy boards will pick up an unacceptable level of moisture in 24 to 48 hours, as can be seen in the chart of moisture versus time. Polyimide boards will fail by the same mechanism as epoxy, but we know it absorbs moisture more quickly than epoxies, and so may reach the 0.22% danger point in less time, perhaps in as little as 16 to 24 hours.
That means that if PWBs have been exposed for a significant time to atmospheric moisture there is a reasonable probability that they have absorbed moisture, and should be subject to a baking cycle before being post-processed in assembly, field rework or any other point of possible thermal shock. Water expands when it is vaporized by a factor of approximately 1000:1 and, depending on the temperature, can produce steam at relatively high pressure. Saturated steam at 500°F can generate pressure of about 700 psi-obviously, if there's enough of it, delamination and blistering are almost a foregone conclusion. Drying of PWBs prior to application of high-temperature process steps can reduce the risk. The length of time to dry a MLB to a level that is less than 0.22% depends on a number of factors, including how much water is in the board, how thick the board is, how many power and ground layers there are (that normally prevent moisture from penetrating perpendicular to the plane of the board, though not from edges) and any other factors, such as protective coatings or solder masks that might affect the rate of diffusion of water out of the board when held at temperature. Most boards will be relatively dry after 4 to 6 hours at 125°C (or approximately 250°F). A caveat here: Try it out before you commit an entire batch of expensive boards to the process line! Don't get bitten by the moisture gremlins.
Chet Guiles is a graduate of the University of New Haven with a BA in Chemistry (1971) and of the Syracuse University Eastern European languages Institute in Russian Language (1963, while on active duty with the USAF). Subsequent to his military service, Chet worked in the areas of corrosion metallurgy (Olin Metals Research), and PVC and silicone rubber technology (Automation Industries). Prior to joining Arlon in 1979, Chet worked as a Senior Research Chemist at Pennwalt Corporation performing research on PVC stabilization.At Arlon, Chet has served in a number of roles including Technical Manager for the Silicone Rubber Group in DE, Quality Assurance Manager for the Delaware Microwave Materials operation, Division Technical Director and, most recently, as Director of New Business Development for the Electronic Substrates Division. Retired in 2004, Chet now acts as a part-time consultant to Arlon.Chet has several U.S. Patents and has written and published widely over the past 30 years in the area of materials for PCBs.