I would qualify your statement, mentioning the integration time for the
ionic contamination tests. I agree that poorly washed ENIG will give a
rapid response time, whereas HASL will give a very high contamination
level, with full extraction taking ½ hour or more. The reason for this
is that ENIG salts will hardly bond to the substrate, whereas the
thermal excursion of HASL, above the Tg, will open the epoxy matrix and
allow penetration of the flux activators (usually hydrobromides or
hydrochlorides) and the surfactants, which form a hydrogen bond to the
substrate, as deep as a couple of micrometres or so. The extraction
process has difficulty in breaking the bonds and pulling out the
hydrohalides. Those of you who remember the CM-5 Contaminometer will
remember that, under these circumstances, it did a 15 minute test and
then extrapolated the poorly-extractable contaminants to asymptote with
very complex mathematical modelling. With other equipment, it needed
very long tests to ascertain the danger.
I agree that dry film masks were like sponges and would adsorb HASL flux
products practically throughout their thickness. The quality of
photopolymerisable liquid masks depended on the state of cure and, in
many cases, I found post-development curing was inadequate to ensure
good results with HASL. I would say unequivocally that well-cured 2-4
component screened epoxy masks with adequate thermal polymerisation give
the best results. What is often forgotten is that the chain reaction
during polymerisation requires stoichiometric proportions of the
prepolymer and the cross-linker. The MW of the individual molecules will
increase to optimum in the mass, but there is a limit at the surface, so
there may be some very low MW reactive molecules at the air interface.
If these are not removed, prior to contact with other substances, some
other undescribable compounds may be formed, potentially causing
problems at a later date. For this reason, I recommend a hot water wash
before HASL, ENIG, soldering or any other chemical operation to reduce
the risk of bonding of active molecules to the substrate.
Brian
Jeffrey Bush wrote:
> ENIG will in general terms return higher Ionic Contamination levels than
> properly cleaned HASL products. Years ago early in the ENIG development
> process, the contamination results - among a laundry list of disasters -
> were poor with ENIG. The Reflow and early HASL processes had better
> results, but this was a factor of much work done with post-process
> cleaning to obtain levels required by Bellcore. With capabilities to 6
> ugm NACL eq/inch2 on solder boards, the first ENIG parts processed at
> 15-20 were shocking. This issue was also related to the soldermask and
> here also many improvements were made in mask chemistry as well.
> Dryfilm mask for example had very poor results in testing, while some
> flat liquids also had heightened levels.
>
> With many improvements in processing and ENIG chemistries, we have been
> able to maintain all products to a standard level, which for us still
> remains the fairly low some where in the 2 ugm area for all parts ready
> to ship.
>
> Is it possible you requirement is to low?
>
> Jeffrey Bush
> Director, Quality Assurance and Technical Support
> VERMONT CIRCUITS INCORPORATED
> 76 Technology Drive - POB 1890
> Brattleboro, Vermont 05302
> Voice - 802.257.4571 ext 21
> Fax - 802.257.0011
> <http://www.vtcircuits.com/>
>
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Chris Ball
> Sent: Thursday, September 14, 2006 8:16 AM
> To: [log in to unmask]
> Subject: Re: [TN] bareboard cleanliness related to surface finish
>
> Hello Inge-
>
> I asked because we are changing over to a new bareboard supplier for
> several different products. Actually, several different divisions within
> the company are moving a single product each to the new supplier. These
> are
> all mature, already in production parts. The previous shops either had
> no
> problem meeting the spec., or didn't meet it but we didn't know. The new
> shop is asking for exceptions to the requirement, and all the requests
> funnel back here.
>
> I see that they request different levels for different products.
> Sometimes
> different levels for the same product on different days, but that's
> another
> email....
>
> When I showed them an earlier request for a lower level they told me it
> was
> due to the different surface finish. That was a new thing to me, so I
> checked with the experts.
>
> So, not trying to split a whisker, and not even a psuedo-academic. Some
> of
> the other questions, concerns, and exceptions from the shop have made me
> generally suspicious. Maybe these are delay tactics. Just checking. And
> trying to get a little smarter at the same time. Reading this forum is
> pretty educational, IMHO. I appreciate the responses and apologize for
> not
> taking the time to research the archives first, but as a
> not-even-psuedo-academic, I usually follow the easiest path to
> enlightenment. ;-/
>
> Best Regards,
> -Chris
>
>
>
>
>
> "Ingemar Hernefjord
> \(KC/EMW\)" To: "TechNet
> E-Mail Forum" <[log in to unmask]>, Chris
> <ingemar.hernefjord@ericss
> BALL/AuburnHills/VSDS/VALEO@VALEO
> on.com> cc:
> Subject: RE:
> [TN] bareboard cleanliness related to
> 09/14/2006 02:52 AM surface
> finish
>
>
>
>
>
>
>
> Chris,
>
> Why do youi put the question? I've never heard about practical
> in-field-use problems based on ionic level diff between the two
> treatments. Never. Academics can always find hair cleaving topics, but
> we working with design production and marketing are more interested in
> the question "do they work?"
>
> Inge
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Chris Ball
> Sent: den 12 september 2006 20:19
> To: [log in to unmask]
> Subject: [TN] bareboard cleanliness related to surface finish
>
> Hello Gurus-
>
> Is it fair to expect a higher contamination on a HASL finished bareboard
> than, say ENIG? I would have expected the same level of cleanliness when
> receiving the bareboards, but new supplier says they have different
> levels for different finishes.
>
> We specify tighter than std. IPC levels and never varied the spec due to
> surface finish. Requests for exceptions have been rare and not related
> to surface finish.
>
> Any comments appreciated,
> -Chris
>
>
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