Mick Thwaite <[log in to unmask]> wrote,

We are just starting to use flex PCB's in our production area.

Is it true that the PCBs have to be dried prior to using as they absorb
moisture quickly?

Is it also true that hot air is better than IR reflow?


The profile for flex differs little from any other PCB. You always start
by following the solder supplier's preheat/reflow recommendations. The
two things that can be different is the much lower thermal  mass (though
that depends on fixturing) and the differences in IR coupling. Both of
these issues are so part/process dependent that I can't give you a
general rule to follow. The only way to know is to measure it. Take care
to get an actual reading of the pad surface and watch for hot spots
(particularly with IR as Jowan Iven observed) as evidenced by locally
discolored adhesive.

The real trick with flex is the prebake to eliminate the absorbed
moisture and thereby prevent delamination. Getting that right is far
more important than just about anything you might logically do with the
profile. The prebake has to be immediately prior to assembly/soldering.
Polyimide flex can pick up a full load of moisture in as little as 30
min so prebaking has to be repeated prior to every solder operation.
Also, flex circuit adhesives have varying degrees of solder temperature
resistance so take the time to find out what you're really working with.

Water absorption is thought of in two ways. Total absorption and rate.

The total absorption is in wt% and measures the change in weight after
immersion in DI water for 24 hours. The sample is conditioned at
105-110C for multiples of 1 hour until the "dry" weight stabilizes prior
to immersion.

% absorption =  (dry weight - weight after immersion) / dry weight X
100%

The absorption rate is a measure of the change in % absorbed over time.
While this could be measured by a series of timed weightings (as above)
it's more typically measured as permeability through a thin layer of
material.

Permeability = grams / square meter / mil thickness /day

Be mindful of the subtle distinction. Absorption rate follows an
exponential decay as the total absorption limit for a material is
approached while the permeability is a measure of the materials inherent
flow through rate at steady state.

The permeability of polyimide film (Kapton) is 54 g/sq m/mil/day and the
total absorption is 2.8 wt%. (not 50% as stated by Chuck Brummer, but
still enough to cause problems) The absorption rate changes rapidly over
time with a large percentage occurring in the first 5 min. and nearly
reaching completion in 30 min to an hour depending on the film type and
thickness. Another polyimide film (Upilex) has only 1.7 g/sq m/mil/day
permeability and 1.2 wt% absorption so you must know what specific
materials you're considering.

Because of the many possible variations in dielectric film, adhesive,
copper patterns and layer construction the prebake profiles should be
engineered to match each part. Some might do fine with just 121C (250F)
for 30 min. while others need up to 24hrs at 135C (275F) with vacuum
assist to be dry enough.


Andy Magee - Flex Guru
Senior Consultant, Bourton Group
[log in to unmask]
(937) 435-3629

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