----- Original Message -----
Sent: Monday, March 03, 2003 1:55
PM
Subject: Re: [TN] First Pass Yields
Dear Rich:
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 .
Russell Smith
Original :
Good morning
everyone,
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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