Got 92.9%.. Bragging a bit because my glasses are all
scratched up.
Randy
----- Original Message -----
From: Smith, Russell (US LA)
To: [log in to unmask]
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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