Laura,
Is is a long time since we had any conversation together or even crossed
swords! But it's good to hear from you.
I used the term 'chemi-physio-adsorption', which I coined for the
occasion, to describe what I believe is the combination of why glycols
tend to stick to epoxies. If the surface were glass smooth, I believe
the only mechanism would be hydrogen bond adsorption. The broken surface
of etched epoxy probably allows for considerable absorption, which of
course is purely physical. The crunch lies in the fact that some of the
adsorption occurs in the hollows, where removal is very much more
difficult. Of course, acetonitrile is a very effective general-purpose
solvent which can remove both hydrophilic and hydrophobic organics.
Provided that the bond strength between the acetonitrile and a
contaminant is stronger than the bond strength between the contaminant
on the substrate, then the contaminant will be dissolved in the solvent.
In the case of glycols, both OH and H bonds can form simultaneously with
the solvent, so that it is not surprising that it can remove them, at
least partially.
Of course, being retired, I am totally out of touch with the latest
developments. However, I'm a little surprised at your statement that
bromide ions diffused into FR-4 as an affinity to the flame retardant
bromine compounds. These are in no way ionic and, not only are the
bromine atoms covalently bonded to the carbon, they are part of the
cross-linking in the polymerisation of the resin. I would seriously
suggest that the mechanism of migration of bromide ions is more likely
to be due to an ion exchange mechanism with the residual sodium chloride
molecules in the epoxy resin. This would also explain why chloride ions
do not have any effect.
I know you are the guru on the electrochemical migration and its effects
on surface insulation resistance, but this is only one mechanism. In my
opinion (not humble, of course), if you refer to the various
publications on the subject I have made since about 1986, you will see
that I make a very distinct differentiation between surface insulation
resistance and electrochemical migration resistance; although many
confuse the two, they are horses of different colours. The test that I
was proposing in my earlier message was true surface insulation
resistance and, for this reason, I stated that the test should be
conducted without any bias voltage and with the measurements made at 5 V
or less for as short a time as possible. This is to prevent any
electrochemical migration from altering the results or, any dissociation
of the sodium chloride molecules in the epoxy resin from doing the same.
If you apply a bias voltage, there is migration of the sodium and
chloride ions within the epoxy structure and this can seriously change
the apparent surface insulation resistance independently from that due
to the presence of a contaminant such as any form of surfactant.
As you are probably aware, I pioneered the notion of such low voltage,
unbiased, SIR tests when I developed the Insulohmeter IRMA. Much of the
research into the effects due to contamination were done by myself,
while the effects due to the structure of the epoxy was studied by a
graduate student at the Swiss Federal Institute of Technology whose
Master's degree dissertation was on this subject. Unfortunately, I
cannot remember his name or details but I do remember that he worked
under Prof Kausch who had the chair of polymers, at that time in the
1980s. As the guy had become so knowledgeable about the electrical
characteristics of epoxy resins, I suggested to him that there must
surely be many openings in the industry for a person with this
experience; unfortunately, he chose to be attracted by American "big
oil", rather than specialise in a very narrow field. I've never heard of
him since! Incidentally, Kausch told me, after the adjudication of his
dissertation (I was on the panel) that it was the best master's thesis
he had ever had the pleasure of reading! He bought me a dinner in
recompense of having lent the Department the Insulohmeter for three
trimesters!
Of course that brings me to a remembrance of the dinner we had in
Washington DC, together with Barbara K. I recall that we had some very
interesting discussions in that Italian restaurant!
Life goes on in sunny Cyprus, unfortunately with the physically
degenerative effects of old age, about two weeks short of my 81st
birthday! How is it with you in your colder climes?
Best regards
Brian
On 21.06.2013 00:32, Laura J Turbini wrote:
> Hi Brian,
> You always add a little spice to the conversation including references which only us "old timers" know. You are correct that Zado's worked focused on PEG and polypropylene glycol. Today, there are block copolymers such as polyethylene propylene glycol (PEPG), and others that are used in HASL fluids. Jack Brous showed in 1981-82 that the PEG absorbed into the epoxy (it was not a chemi-physio-adsorption) and he was able to extract it from the boards using acetonitrile. When he evaporated the acetonitrile solution and took an FTIR spectrum of the residues he found PEG.
>
> More recently, my former student, Dr. Antonio Caputo published a paper which included extraction of PEG and PEPG from water soluble fluxed FR-4 test coupons. Ref. A. Caputo, L.J. Turbini, D.D. Perovic, (2009), “Conductive Anodic Filament (CAF) Formation Part I: The Influence of Water Soluble Flux on its Formation”, Journal of Electronic Materials, Vol. 39, 85-91 (2010).
>
> In another paper he also showed that if the HASL fluid contained a high bromide content (~15%), the bromide ions also diffused into the FR-4 (because for brominated epoxy - like dissolves like). Chloride from the flux did not diffuse into the epoxy. ref A. Caputo, L.J. Turbini and D.D. Perovic, “Characterization and Electrochemical Mechanism of Bromide-Containing Conductive Anodic Filament (CAF) Failure,” Journal of Electronic Materials, Vol. 40, No. 9, 2011.
>
> You commented below - The only valid way of determining the presence of hydrophilic surface phenomena is by non-biased, low voltage 50/90 or 85/85 SIR qualification tests. What do you mean by non-biased, low voltage? Aren't the two terms contradictory.
>
> The rate of electrochemical migration (dendrite or CAF growth) is affected by the contamination present, but also by voltage, temperature and humidity. Using low voltage testing would require a longer time for the dendrite to form. There is a rule of thumb that says that a chemical reaction doubles for each 10oC rise in temperature. Thus, the use of a lower temperature would also require a longer time for dendrites to form.
> Regarding humidity, FR-4 boards will easily have enough layers of water molecules at 70% RH or higher, to allow the electrochemical migration to occur. So whether it is 85% or 90%, the difference in the rate of dendrite formation will be small.
>
> Regards,
> Laura
>
>
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Brian Ellis
> Sent: June-20-13 11:59 AM
> To: [log in to unmask]
> Subject: [TN]
>
> I'm afraid that some of what you say may be misleading. Frank Zado's paper, at the Anaheim and Philadelphia Nepcon conferences in 1979, explored mainly Carbowax (polyethylene glycol. PEG) of specific ranges of MW. Although he did some tests with polypropylene and higher glycols, these proved to be of much reduced effect. This was also specific to wave soldering. Also the effect was not due to an epoxy-OH bond; it was a hydrogen bond, exacerbated by the structural surface of the epoxy, left by the copper treatment. It could be described as a chemi-physico-adsorption. However, PEG fell largely into disuse in the 1980s, except for some tin-lead reflow and HASL processes in the FAB side. Of course, it was your famous OH group that potentially created any hydrophilic characteristics at the other end of the molecule!
> More particularly, as I have propounded many times since 1969 (Inter-Nepcon), in my book and other publications, in lectures and in my swansong paper in Circuit World, the water-break test is absolutely meaningless, with easily produced false negatives and false positives.
> IMO, anyone who uses it as determinant of any specific reliability conditions needs his head examining. The only valid way of determining the presence of hydrophilic surface phenomena is by non-biased, low voltage 50/90 or 85/85 SIR qualification tests. The oracle hath spoken! :)
>
> Brian
>
> On 20.06.2013 17:21, greg wrote:
>> It is true that WS flux should be removed in the cleaning process.
>>
>> However, many glycols actually bond to FR-4 epoxy through their -OH groups. Hence the surface after soldering and cleaning is hydrophillic. (Frank Zado showed this back in the early 80s.)
>>
>> An easy test is take a board that is clean but not WS soldered and drop DI water on it. It should bead up.
>>
>> If after WS soldering and cleaning a drop of DI spreads you have glycols bonded to the epoxy.
>>
>> Adding a no-clean (with dibasic acids) to a hydrophobic mix may be an iffy proposition.
>>
>> Better to use a Bellcore compliant flux for your final soldering.
>>
>>
>>> -------Original Message-------
>>> From: Steven Kelly <[log in to unmask]>
>>> To: [log in to unmask]
>>> Subject: [TN]
>>> Sent: 20 Jun '13 09:02
>>>
>>> Thanks Dave - customer has not done any testing and for years we have only used no-clean for both operations so now I have some parts to be done one way and some another for Class 3 medical. Not good in my 2 cent opinion. Regards Steve Kelly
>>>
>>> From: [log in to unmask] [mailto:[log in to unmask]]
>>> Sent: June-20-13 9:15 AM
>>> To: TechNet E-Mail Forum; Steven Kelly
>>> Cc: TechNet
>>> Subject: Re: [TN] Mixing solders,
>>>
>>> Hi Steve - I'll have to pay Doug and use his tag line - "It depends"! Anytime you mix two different flux systems, especially a water soluble and a low residue (aka no clean), there may be an issue of incompatibility that could result in a really hard lacquer (best case) or a really cool corrosion cell (worst case). My recommendation would be to advise the customer that the mixing of the two flux systems would not be advised unless some testing can be conducted to ensure no detrimental reactions would occur. A second option would be to check with the flux supplier to see if they have any compatibility data. If the fluxes come from two different suppliers, don't waste your time asking that question as they won't have the answer. Good Luck.
>>>
>>> Dave Hillman
>>> Rockwell Collins
>>> [log in to unmask]<mailto:[log in to unmask]>
>>>
>>>
>>>
>>> From: Steven Kelly <[log in to unmask]<mailto:[log in to unmask]>>
>>> To: <[log in to unmask]<mailto:[log in to unmask]>>
>>> Date: 06/20/2013 08:02 AM
>>> Subject: [TN] Mixing solders,
>>> Sent by: TechNet <[log in to unmask]<mailto:[log in to unmask]>>
>>> ________________________________
>>>
>>>
>>>
>>> Hi All,
>>> I have been looking in the archives but can't seem to find what I want an answer to . I have a customer who wants us to use water soluble RoHS for the SMT process but wants no-clean RoHS for the touch-up. Is this recommended? Regards Steve Kelly
>>>
>>>
>>> If the recipient to whom this e-mail is sent has an NDA with PFC Flexible Circuits Limited this e-mail is considered confidential and is subject to any NDA agreements between the respective parties.
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