> Joyce,
>
> As I understood the orignal question, the Ionograph measurements were used
> for a sort of product (assembly) qualification per J-STD-001C, i.e. neither
> process development nor process control.
>
> Per-Erik
Techies,
As I am writing this in Finland and unable to get a connection before I
leave, this might seem somewhat tardy but...
Per-Erik and others have made many comments regarding the history,
methodology and future practices for reliable production.
If I might just add my Pennorth:
Jack Brouse of Alfa Metals informed an IPC committee discussing circuit
cleanliness that: "ionic extract cleanliness testing was only EVER intended
as a process MONITOR". Unfortunately, the US Military picked up on the
method, and selected a pass/fail criteria that was modified to several ever
lower levels until 1.5 microgrammes/centimeter/squared of NaCl EQUIVALENCE
was achieved.
This was most misleading - particularly when folks began to produce
fine-line, fine-pitch circuits.
This is NOT a test method and as Per-Erik pointed out so correctly, it is
virtually impossible to determine how clean is clean. Furthermore, it also
states that it is acceptable to leave UP TO 1.5 microgrammes of salt on
every square centimetre of your assembly - OH REALLY?? In modern miniature
circuitry - I don't think so! That is why so many employ their own
(empirical) pass/fail - I prefer "Go / No Go" levels, in my experience
typically less than 0.2 microgrammes.
The method: Involves MEASURING the resistivity of a test solution comprising
alcohol and de-ionised water. This is a good method - except that it NEVER
zero's back to the same start level. The "empirical" bit, is the individual
vendors math's package that converts this MEASUREMENT into an EQUIVALENCY of
sodium chloride, and has to compute the volume of test liquid that is used -
and that certainly varies from machine to machine - PLUS - other factors
involving the filter and resistivity sensor etc..
Speaking of Equivalency Factors - I promised to send a number of you the US
Navy document - and I will, just as soon as I get into the office and find
it! Please stay with me on this....
Now, as to methods to determine end product reliability - as Per-Erik said,
this is a good one, but no specification is yet available. Well, yes it is
now - at least in draft form - because yours truly just finished writing it
and presented it to the IEC committee here in Helsinki yesterday - and it
was accepted.
I have already sent a draft to Jack Crawford to circulate to the various
committee chairmen at IPC, and will be in New Orleans to discuss this in
November.
If anyone wants this latest draft, let me know. Just bear-in-mind that it is
a draft with no formal accreditation at this time.
Finally, just let me say that we are trying to develop a process method
using BOTH our SIR and ionic test machines. Watch this space.....
--
Regards, Graham Naisbitt
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Mobile: +44 (0)79 6858 2121
Concoat Limited - Engineering Electronics Reliability
Web: www.concoat.co.uk AND www.concoatsystems.com
NOTE: NEW ADDRESS :NOTE
Sales:
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