Hi, Jerry
For some Level 6 MSD components, the manufacturer does not want to fool around with attempting to keep the parts dry throughout the receiving, component prep (forming and/or tinning, etc.) because the parts can absorb moisture so quickly that they recommend just leaving the parts in their original drypack bag until they are ready for processing. They then recommend some time period from the time the bag is opened until the parts are through reflow, typically 8 to 12 hours.
Likewise, there are certain flex junctions that are easily damaged by direct exposure to molten solder or heat from a reflow process. The vendors are familiar with which types of flex (especially rigid-flex) circuits are susceptible, and will sometimes recommend a bakeout just prior to placement and reflow.
While not something you run into every day, I see this quite often. In fact, I am dealing with this at two different companies as I write this; one is a relay that is not fully hermetically sealed, the other is a flex circuit that likes to wick moisture at the rigid-flex junction, then it expands during reflow. 

-----Original Message-----
From: TechNet <[log in to unmask]> On Behalf Of Jerry Dengler
Sent: Friday, January 10, 2020 6:53 AM
To: [log in to unmask]
Subject: [TN] Flexible PCB's

TechNetters,

 

We reviewed documentation from a flexible PCB manufacturer and saw that they recommend baking of the flex boards prior to assembly even if they are brand new boards in a Moisture Barrier Bag.

 

Why wouldn't these be handled like any other moisture sensitive component?

 

How many of you that work with flexible PCBs are baking them even if they come out of a MBB with A HIC card that shows no real moisture?

 

Thank You,

 

Jerry Dengler

Production Manager

Pergamon Corporation

380 Crooked Lane, Unit# 3

King of Prussia, PA  19406-2567

U.S.A.

(610) 239-0721 Phone