Brian, I guess then that you have hot dogs daily.
Inge
On 26 June 2013 08:18, Brian Ellis <[log in to unmask]> wrote:
> Laura,
>
> Yes, I agree with you that once you take the temperature of FR-4 or any
> other resin above the Tg, the opening of the structure becomes a great
> issue in terms of absorbing/adsorbing contaminants.
>
> I don't want to appear more arrogant than I usually am but my experience
> is that there is much misunderstanding both in terms of what the resins are
> and how they behave. I believe there is a tendency to forget that they are
> not electrically perfect and that they are hygroscopic, with an electrical
> "memory", especially under voltage stress of more than a few volts per
> millimetre. In these days of tiny spacing, this becomes especially
> important.
>
> I also agree that there is much misunderstanding, even today, regarding
> surface insulation resistance and the various forms of electrochemical
> migration resistance. Unfortunately, this has become enshrined because of
> the historical errors.
>
> Summer? Today we had a peak heat index of 41° C!
>
> Best regards
>
> Brian
>
>
> On 24.06.2013 00:03, Laura J Turbini wrote:
>
>> Hi Brian,
>>
>> I personally believe that the glycols, and the bromideions diffuse into
>> the epoxy during the soldering process. When the epoxy goes above its Tg
>> it opens up its polymeric structure and allows the contaminants to enter.
>> There is a good description of the epoxy backbone in a web site called
>> Macrogallaria. http://www.pslc.ws/macrog/**level2.htm<http://www.pslc.ws/macrog/level2.htm> You will note that there are a lot of places in the epoxy resin for
>> hydrogen bonding to occur. In the late 90's we were analyzing a field
>> failure due to CAF. The growth was around the 5th layer of a 10 layer
>> board as I recall. When my student polished down to that layer, he was
>> able to extract the ionic residues and found bromide, but no chloride.
>> This board was processed with the high bromide HASL fluid. We have also
>> published some work using different polyglycols in the flux, and we noted
>> that the Cu and Cl ions in the matrix had a different morphology depending
>> on the polyglycol used.
>>
>> I agree with you that the traditional test method described as Surface
>> Insulation Resistance is in fact an electrochemical migration test.
>> According to the IPC, Electrochemical migration (ECM) is defined as the
>> growth of conductive metal filaments across a printed circuit board (PCB)
>> in the presence of an electrolytic solution and a DC voltage bias. The low
>> voltage test could also be described as ECM but it does more accurately
>> show the insulation resistance at the given temperature and humidity
>> conditions because dendritic growth would be rather slow under those
>> conditions.
>>
>> I hope you are enjoying the summer.
>> Regards,
>> Laura
>>
>>
>>
>> -----Original Message-----
>> From: TechNet [mailto:[log in to unmask]] On Behalf Of Brian Ellis
>> Sent: June-21-13 7:28 AM
>> To: [log in to unmask]
>> Subject: [TN]
>>
>> 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 pane
>>
> l) 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:ddhillma@**
>>>>> rockwellcollins.com <[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:ddhillma@**
>>>>> rockwellcollins.com <[log in to unmask]>>
>>>>>
>>>>>
>>>>>
>>>>> From: Steven Kelly <[log in to unmask]<mailto:SKe**
>>>>> [log in to unmask] <[log in to unmask]>>>
>>>>> To: <[log in to unmask]<mailto:TechNe**[log in to unmask]<[log in to unmask]>
>>>>> >>
>>>>> Date: 06/20/2013 08:02 AM
>>>>> Subject: [TN] Mixing solders,
>>>>> Sent by: TechNet <[log in to unmask]<mailto:TechNe**[log in to unmask]<[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.
>>>>>
>>>>> See PFC on "How It's Made`` coming soon on the Discovery Channel!
>>>>>
>>>>>
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