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.
>>
>> See PFC on "How It's Made`` coming soon on the Discovery Channel!
>>
>>
>> ______________________________________________________________________
>> This email has been scanned by the Symantec Email Security.cloud service.
>> For more information please contact helpdesk at x2960 or [log in to unmask]<mailto:[log in to unmask]>
>> ______________________________________________________________________
>>
>>
>> ______________________________________________________________________
>> This email has been scanned by the Symantec Email Security.cloud service.
>> For more information please contact helpdesk at x2960 or [log in to unmask]
>> ______________________________________________________________________
>>
>
> ______________________________________________________________________
> This email has been scanned by the Symantec Email Security.cloud service.
> For more information please contact helpdesk at x2960 or [log in to unmask]
> ______________________________________________________________________
>
______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please contact helpdesk at x2960 or [log in to unmask]
______________________________________________________________________
______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please contact helpdesk at x2960 or [log in to unmask]
______________________________________________________________________
|
