Amol-
Component weights would probably work, although you'd probably need a
bunch of them if they're small. The sample size you weigh must be large
enough to be an accurate representative of the population being sampled.
"Accurate" means that you'd want something like a 98% probability that
the weight of the part as determined by the mean sample weight would be
within the scale accuracy. A very tight lab balance can get to the 100
microgram level (these are fairly expensive). So the sample size
required depends on the variance in the weight of the parts in the
population. This needs to be determined by careful statistical analysis
of the popultation.
PCB weight post reflow would probably require a whole bunch of PCB's
because of all of the variability in solder weights and component
weights from assembly to assembly (the same rules apply).
If you simply take a single part (or a fixed group of parts) and weigh
them before and after baking, then you don't need to bother with the
statistical variability problem.
Wayne Thayer
>>> [log in to unmask] 10/16/2006 8:12 am >>>
Dear Technetters,
How feasible is:
1. measuring the component weight as a quick indicator of its floor
life for MSL considerations, if the "dry" weight of the component is
known?
2. measuring the PCB weight after multiple reflow cycles to determine
whether the laminate is approaching the end of its useful life?
is anybody out there actually using these metrics to draw valid
conclusions? If so, what scales are being used for the measurements?
(least count wise)
Thanks in advance,
Amol
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Paul Reid
Sent: Monday, October 16, 2006 7:52 AM
To: [log in to unmask]
Subject: Re: [TN] T-260 laminate testing
I can't answer your question directly from a specification point of
view. I do not know what the standards may be. From a practicle point
of view I would question material with less than a 10-minute time to
delamination and even then I would be skeptical.
The problem with T260 is that it has only one thermal cycle. Some poor
performing materials can handle one thermal excursion well but are
significantly, adversely affected after 3 thermal excursions, which is
what most PWBs are exposed to during assembly.
Our company has adopted a "cyclic" T260 (cT260) protocol where a
material is thermal cycled to 260 6 times, to simulate assembly (3
cycles) and rework (3 cycles), and then held at 260C until delamination.
The cycle we use is a six-step ramp to 260C followed with a 100C/min
cool cycle. The total cycle, ambient to ambient is about 6 minutes.
Samples from known low reliability boards, where, based on microscopic
examination, there are no obvious process problems like low copper etc.,
fail in less then 10 minutes.
What we have found is that some "high performance" materials degrade
with thermal cycling to 260C. A single thermal excursion to 260C is not
a very accurate tool for measuring materials reliability.
Even though this is a delamination test, it is important to know that
PWBs fabricated with material that have poor cT260 results usually do
not fail for delamination. The common failure is barrel cracks in the
central zone of the PTH.
Paul Reid
613 596 4244 ext. 229
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Wolfgang Erat
Sent: Friday, October 06, 2006 5:22 PM
To: [log in to unmask]
Subject: [TN] T-260 laminate testing
Good day
What are acceptability minimum results for T-260 testing.
The laminate in question is rather basic FR-4 with a Tg of 140
Doing some searching I come up with numbers anywhere between 5 minutes
and 30 minutes. Any insight is appreciated.
Wolfgang
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