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April 2008

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Subject:
From:
Brian Ellis <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Brian Ellis <[log in to unmask]>
Date:
Mon, 21 Apr 2008 12:11:39 +0300
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Well, yes and no! One contamination tester did indicate a probability of 
whether entrapment occurred. You see, by analysing the shape of the 
conductivity v. time curve, it can be (and was) done. Any given 
non-entrapped contaminant has a given asymptotic curve. Let's say that a 
given hypothetical cleaned flux residue reaches 99% of the way to 
extrapolated asymptote in 2 minutes. If the same flux is entrapped, it 
will not reach 99% of asymptote in 15 minutes, so the curve will look 
different. If you have both cases (the normal) the resultant curve will 
be the sum of both. By analysing the curve, you can separate them into 
the components. The said tester used 9 different curve-fitting 
techniques to do the analysis (hellishly more complex than that in 
Excel!!!) and compared them to the actual curve, choosing the best fit. 
It was able to detect 3 different contamination sources and calculate 
the respective quantitative values of each, extrapolating each of the 
three curves to quasi-asymptote, even if that were many hours longer 
than the test itself. Entrapment tests showed that it worked, with a 
reasonable accuracy. However, what it could not do is to differentiate 
between entrapment and other slow-leaching sources, such as a poorly 
cured solder masks etc. I think the wording on the print-out was 
something like, "Very slow dissolving contaminant averaging n µg/cm² eq. 
NaCl: may be due to flux residues under components, poorly polymerised 
resins or other leaching sources" (I programmed this 19 or 20 years ago, 
so my aged brain may be excused if this is not verbatim, but the gist is 
there). This technique was appreciated by my clients, because they were 
able to judge the proportion of the surface area which was causing 
entrapment and thus guestimating the actual entrapped mass.

Brian

Hernefjord Ingemar wrote:
> Well written, Terry! General areas of a board is seldom a contamination
> problem, but ENTRAPMENTS, yes! And one can not be sure the ionic
> contamination test tells you about them.
> /Inge 
> 
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Terry L. Munson
> Sent: den 18 april 2008 22:35
> To: [log in to unmask]
> Subject: Re: [TN] Ionic Contamination Question (& added info).
> 
> Bill
> As one of the active task group participants during the 90s when we were
> asked to update the equivalence factors table and using low  solids
> cleanable and no clean formulations,  we found it difficult to  assess
> these new types of low solids residues and if you look at the large
> detailed report from the task group (IPC TR 583) we found it difficult
> to  correlate between static and dynamic systems.  The use of higher
> pass fail  criteria were in place for automotive users (one in
> particular used  28 ug/in2 instead of the 14 ug/in2 from
> 1978 to 1991 with no field  performance problems related to flux
> residues with mean levels at 21 ug/in2  while using an 18-24% solids RMA
> flux) that did not see problems.  
>  
> My concern with cleanliness is not the generalized cleanliness of a
> total board but the amount of contamination present between pads, in
> via's or thru-hole devices.  The pocket of contamination below an 0805
> capacitor has  created drained batteries when the entire assembly tests
> clean by ROSE and by  bag extraction Ion Chromatography. It is only when
> we look at the localized  pocket of contamination do we see the direct
> correlation to field and  reliability performance.  Many of the
> localized areas we look at with the  C3 tester are clean and show low
> levels of contamination, but when we see the  combined processing
> effects of selective wave soldering using a pallet to  isolate the area
> we find that the residues that can be trapped between the  pallet and
> circuit board (low solids no clean VOC free) are also protected from
> the heat but are very corrosive due to the water carrier and acidic pH
> 2.35.  Dendrites are growing in these nearby areas, as well as stray
> voltage problems  and intermittent performance issues and No Trouble
> Found (NTF) returns.  
>  
> It is important to understand the cleanliness of an assembly but it is
> more important to understand the cleanliness of the processing steps
> such as 1st reflow, 2nd reflow, wave soldering bottom and top side, bare
> board unsoldered areas, micro via's that have fabrication residues
> trapped inside causing performance problems and the hand solder / touch
> up residues.  This type of cleanliness understanding comes from being
> able to do localized non-destructive residue assessment and ion
> chromatography analysis. 
>  
>  
> Cleanliness testing must predict field performance.  Using localized
> testing is the only way I am aware of understanding how much
> contamination is present in the areas of critical circuitry that tends
> to fail earliest. 
>  
> Terry Munson
> Foresite
> 765-457-8095
> _www.Residues.com_ (http://www.Residues.com) 
>  
>  
>  
>  
> In a message dated 4/18/2008 1:54:54 P.M. Eastern Daylight Time,
> [log in to unmask] writes:
> 
> As  perhaps the only person still around that attended the meetings    
> resulting
> in the equivalence factors and IPC Cleaning &  Contamination  Chair  at
> that time, perhaps a few points would  facilitate the  discussion:
> 
> 1. The Navy set up the ionic testing  development program to  solve a
> serious 
>  
> failure problem in S.E.  Asia. 
> It worked.
> 
> 2.  In the timeframe when the test was  developed and put in place by
> the   
> military, most of the rest  of the electronics industry in the US used
> the mil specs  since they were free.
> 
> 3. As the IPC set up and  adopted Classes  1-3 (basically toys up to
> military/high rel), I asked  the  committee if we used the mil test
> result for Class 3, could we  use  1.5x that limit for Class 2 and 2-3x
> for Class 1? 
> The response  was that with  proper cleaning, the mil limit could
> readily be  
>  
> achieved while serving  to monitor daily production. So the industry
> continued to  use the  (free) mil spec test  standard.
> 
> 4. The ionic contamination test was a  valuable  monitoring tool, since
> the SIR tests were done on coupons,  not  on actual assemblies, and took
> 1-2 weeks to complete. Needless  to  say, a high volume electronics
> producer could turn out 
> 
> a  significant  volume of PWAs during that time, often shipping them
> into the field as  soon as assembly was completed.
> 
> 5. As  noted in my SMT column (offered  yesterday) T. O.  Duyck of
> Northern  
> Telecom was charged with  implementing water soluble flux for   NT 
> electronics
> production. During  that time he observed and reported  the  differences
> in flux residue  release rates, pointing  out that rosin ca 90% of
> rosin flux residues  
> release from  the PWA surface during the 10-15 test time for  ionic test
> 
> equipment, while water soluble flux residues may take up to 2 hrs.  to
> 
> achieve the same
> level of release. Thus the release  rate should be checked  to  ensure
> the flux used, time test  time and the instrument employed  provide
> reliable results and  guidance to the production engineer. 
> (See T. O. Duyck and M. Boulos,  "Water Washes  Reliability into
> Telephone Circuit Packs", IPC-TR-206,  April, 1978)
> Based on this work, I investigated the release rate of   SA flux
> residues, 
> finding it even faster than rosin fluxes. (See W. G.  Kenyon,
> "Synthetic Activated (SA) Flux Technology: Development,
> Commercialization,  Benefits and Future Applications", Internepcon
> Japan, 24 Jan.  1986)
> 
> 6. In the late 1980's, the materials and  acceptance of the  no clean or
> low residue or acceptable dirt  concept became widely  accepted and
> implemented on the  designs  of the time. Outsourcing to  contract
> assemblers (both in the US and
> overseas) became widely practiced,  so much of the former 'in-house'  
> cleaning
> expertise disappeared. 
> 
> 7.  This was seen at IPC as  the number of company sponsored volunteers
> 
> dwindled. Could we  take on projects today to develop an updated ionic
> test  
> for   pr
> ocess monitoring? Find enough participants to conduct   statistically
> sound  
> round 
> robin testing?   
> 
> Bill   Kenyon
> Global Centre Consulting
> 3336 Birmingham  Drive
> Fort Collins,  CO  80526
> Tel: 970.207.9586    Cell:   970.980.6373
> 
> 
> 
> 
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