Welcome to Board Talk. This is Phil Zarrow and Jim Hall of ITM Consulting, but here in Board Talk to answer your SMT process questions. Jim, what is today's question?
Well, it concerns selective soldering, a new topic.
The question comes from K.K. Infrequently we see blisters (delamination) on a board around certain components after selective soldering. The location of the blisters is not always at the same locations. We've tried reducing solder temperature to a minimum, but the problem persists. What could be the root cause of our problem?
Let's assume for the moment that you have your selective machine parameters set up correctly. And they're consistent.
Moisture absorption into the board. Think about it, selective soldering is typically one of the last assembly processes, so the board may have been out of its moisture control bag for a re-flow cycle, maybe some touch-up, maybe some inspection. We don't know what your processes are. But there are concerns.
Your board is always going to absorb moisture, depending on temperature and humidity in the room, and this may be why it's a random problem.
So what you thought might have been related to the selective process itself, it comes back to your storage procedures and practices, particularly with regard to boards. And there is a recommendation for you in the form of a new IPC specification.
IPC specification 1601. People have talked about moisture absorption, baking boards, for as long as I've been in the industry, and that's over 30 years. IPC felt that, particularly with lead free and higher temperatures, it's gotten critical enough that they needed for formalize this. So they produced this document that was just released last September and it talks about measuring moisture, and storage conditions, and bagging boards and sealing them, and using desiccants and so forth.
So I strongly
recommend that you take a look at that specification, and look at what you're
doing in your process in terms of the time that your boards are exposed to ambient humidity up until the time that you're
doing your selective soldering.
just had one other thought. Obviously,
we're assuming that it isn't just a random defect in the fabrication of PCB
itself. Because if you're having inconsistencies with lamination or materials at the vendor,
that could certainly show up.
So cross your T's and dot your I's. Make sure your selective machine parameters are okay, and that you are following best practices with regard to storing and handling your PC boards.
And regardless of whether or not your boards are lying around on a shelf, in a moisture barrier bag, or a dry cabinet, when you go to solder them ...
Don't solder like my brother.
And don't solder like my brother.