-----Original Message-----
From: Smith, Russell (US LA)
[mailto:[log in to unmask]]
Sent: Monday, March 03, 2003 1:56
PM
To: [log in to unmask]
Subject: Re: [TN] First Pass
Yields
You are right, a 100% visual inspection is at best 80% effective, every time it
is done, The big issue with visual inspection is the
effectiveness is extremely variable depending on the class of
defects that you are looking for , and other items such as the visual
acuity, training level,fatigue and moral level of your inspectors.
What you might be looking
for is From Juran-Melsheimer:
percent defects correctly identified
= d-k / d-k+b
where d= number of
defects reported by inspector
k=
number of good units rejected by inspector
d-k = true defects found by inspector
b=
defects missed by inspector
d-k+b =true defects originally in product.
A simple test to show the
effectiveness is the following :
assume that the letter "G" is
the defective product in the following story. Allow someone to read the story
and count all the "G"' s that are used. give them 3 minutes. There
are eighty -five g , determine the percentage they catch and viola you have
your effectiveness and accuracy. It should give you the ammunition you need
WHILE STROLLING THROUGH
A GLEN, A GIDDY ENGLISH
GIRL TRIPPED ON A RATHER
LARGE , ALMOST GIGANTIC
FROG. THE GIRL
STAGGERED BUT REGAINED HER
FOOTING AND WAS ABOUT
TO GO ON WHEN THE FROG
BEGAN TO SPEAK AND
GESTICULATE TO GAIN THE
GIRL'S ATTENTION. "I
HAVE NOT ALWAYS BEEN A
FROG" HE
CROAKED. THE FROG'S GREEN COLORING
SEEMED TO GLOW BRIGHTLY
AS HE CONTINUED, "I
WAS ONCE A GRACIOUS
KNIGHT, A GENTLEMAN
CALLED GALLANT GEORGE
GRENVILLE,BUT WAS
CHANGED INTO THIS
GHASTLY GREEN FROG YOU NOW SEE BY
AN UNGODLY, MAGICAL
GENIE. THE SPELL CAN ONLY
BE BROKEN IF I
GAIN A GIRL'S GOOD GRACES AND
SPEND A NIGHT IN HER
GARDEN." THE AGOG GIRL WAS
SKEPTICAL, OF COURSE. SHE
GAZED AT THE FROG'S
PLEADING EYES AND SOON
HER GIDDY NATURE GAVE
WAY TO HER DOUBTS.
GIGGLING,SHE DECIDED TO
GRANT THE FROG'S WISH AND
TOOK HIM HOME
STRAIGHTWAY, PUTTING HIM
BY HER GARDEN GATE.
THAT NIGHT THE GIRL SLEPT
GRANDLY AND SURE
ENOUGH , WHEN SHE AWOKE
THE FOLLOWING MORN-
ING, THERE ALONGSIDE
HER GARDEN GATE WAS THE
GRACIOUS KNIGHT, GEORGE
GRENVILLE. WELL,
STRANGELY ENOUGH, FOR A
LONG LONG TIME THE
GIRL'S MOTHER DID NOT
BELIEVE THAT STORY.
Inspection accuracy is rarely better than
85%. If you add complexity, like other type fonts, different format , multiple
colors , or distractions such as noise etc. the accuracy can be very low
With the complexity of the product you are
dealing with and the number of defective opportunities per assembly, visual
inspection is not the answer. It should be used as a method to
collect data to be used back in the manufacturing process.It sounds like
at this point in time the only rapid band aid for the problem is to perform
some type of testing. I fought many a customer about paying for inspection ,
and invariably they reply with " I am buying good product, how you assure
I get it is up to you" . That should be accounted for in the original
pricing .
If you follow
modern quality theory then your guiding philosophy should be " If you
control your processes, and all your processes are in control then you don't
need to inspect the product." of course this assumes that you are
controlling the items that effect the outgoing quality of the
product!
While all this seems an
insurmountable challenge, the rewards are vast, and will carry directly to the
bottom line of the ledger sheet assuring that the company achieves it
ultimate goal of amassing great piles of coin of the realm!!!!!
PS: I never was one for the "World
Class" moniker, I always felt that we should strive for "Galactic
Class" ! Yes in my youth I was a Trekky!
Another overused phrase while I am on the
subject is "State of the Art" , Don't you go to a museum
to see art? Wouldn't "State of the Science " be more appropriate?
Thanks all for allowing my ramblings on
this dreary Monday Morning in LA .
I am having a debate with regards to first pass yields and
how testing strategy (ie.. ICT or Functional Test) plays a role.
I am looking for any "factual data" or
"industry studies" that have been done to show the difference in
yields between a batch of assemblies that are subjected to only visual inspection and to those batches that receive visual
inspection and some type of ICT or functional
test. I have my own internal data that
shows the differences but want to compare that against the industry.
Summary - I have a customer that is expecting 99% First pass
yield (including board functionality) to their facility but does not utilize
or want ICT or functional testing done to their products. Now the old rule of thumb that I've gone by
for many years is that visual inspection is approx. 80% accurate and there are
bound to be defects that slide through this visual inspection process. To consistently achieve yields in the high
90%'s additional test strategies should be looked into. Note: The boards in question range anywhere
from 200 components per board to approx. 2000 components per board.
Any feedback or published industry articles documenting the
yield differences between visual inspections only and using ICT/Functional
testing would be very helpful!
P.S. I have a meeting at 1:30pm today (Central Time) so any
feedback before then would definitely be appreciated.
Thanks,
Rich Lasko
Badger Electronics
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