Dave,
The aluminum oxide will stop galvanic corrosion provided there are no vibration issues.
"Fretting" may also create long term thermal issues.
David A. Douthit
Manager
LoCan LLC
Dave Hillman wrote:
> Hi Ian! I am keeping my fingers crossed. I also talked with my buddies at
> the Rockwell Science Center (our old think tank) and we have another
> phenomena working in our favor. The aluminum oxide should impact the
> galvanic couple "connectivity" by impeding the current flow. Dr. Tench felt
> that the Al2O3 and the anode-to-cathode ratio may be enough to inhibit a
> galvanic reaction for many use environments. I am going to have Dr. Tench
> also create the finish polarization curves which should help demonstrate
> that we are in a passive and not active region of the electrochemical
> reactions. Lots of fun - I haven't studied this much galvanic science
> since my junior year in college. It is also a nice data point to see that
> my test results are in agreement with what you experience in real life.
> I'll let ya know who the salf fog testing progresses.
>
> Dave
>
> "FOX, Ian (York
> Rd)" To: [log in to unmask]
> <Ian.FOX@GOODRICH cc:
> .COM> Subject: Re: [TN] Heat transfer from ENIG board to aluminium casing
> Sent by: TechNet
> <[log in to unmask]>
>
> 03/18/2003 08:44
> AM
> Please respond to
> "TechNet E-Mail
> Forum."; Please
> respond to "FOX,
> Ian (York Rd)"
>
> Dave, glad to see your testing so far mirrors what I found, i.e. no
> corrosion with ENIG and conversion coated Al. Good point about the
> anode/cathode relationship though. To contribute to answering Daan's
> question, I agree with both yourself and Brian, contact area is the key to
> thermal performance. Provided that the Al surface finish is reasonable
> there
> should be no practical difference between either of the two finishes. If
> you
> have a rough cast finish for instance you might argue that the HASL finish
> would couple better with the Al due to its ability to deform, but I suspect
> any difference would be marginal.
>
> Ian Fox
> Goodrich Engine Control Systems
>
> -----Original Message-----
> From: Dave Hillman [mailto:[log in to unmask]]
> Sent: 18 March 2003 13:47
> To: [log in to unmask]
> Subject: Re: [TN] Heat transfer from ENIG board to aluminium casing
>
> Hi folks! Let's look at some numbers! The thermal conductivity (in W/ m K
> in
> the 0-100C range) is 237 for Al, 318 for Au and 66.8 for Sn. You should
> have
> better heat transfer by going way from Sn - but as Brian described the
> thermal transfer game is ruled by surface contact. Brian also mentioned the
> issue/question of galvanic compatibility, the same question I queried the
> TechNet community on several weeks back. I received a number of good
> information leads but nothing as definitive as I was looking to obtain.
> Here
> is an update to that question - I began a series of tests: the
> silver/aluminum and gold/aluminum galvanic couples test vehicles (circuit
> boards with the appropriate finishes coupled to Al casings by Al standoffs
> and having a 5 volt bias) have completed 250 hours of 85C/85%RH with no
> issues of galvanic compatibility! The salt fog tests begin next week and
> I'll keep everyone updated as those tests progress. It appears that the
> anode-to-cathode relationship is a key parameter in equation.
>
> Dave Hillman
> Rockwell Collins
> [log in to unmask]
>
> Brian Ellis
> <b_ellis@PROTONIQ To: [log in to unmask]
> UE.COM> cc:
> Sent by: TechNet Subject: Re: [TN] Heat
> transfer from ENIG board to aluminium casing
> <[log in to unmask]>
>
> 03/18/2003 06:54
> AM
> Please respond to
> "TechNet E-Mail
> Forum."; Please
> respond to Brian
> Ellis
>
> Daan
>
> I would have thought exactly the opposite! An uneven HASL surface will have
> fewer contact points to the aluminium than a flat ENIG surface. That having
> been said, I would have thought that both would be bad, electrochemically
> speaking, if subjected to humidity. From very poor memory, the Al-Au couple
> is somewhere about 4 V, roughly double that of Al-Sn/Pb.
>
> Brian
>
> d. terstegge wrote:
> > Hi Technet,
> >
> > We are in the process of converting some of our boarddesigns from HASL
> > to ENIG conductor finishing. Now one of our designers is worried about
> > the effect that this change may have on heat transfer from the plated
> > board-edges to the aluminium casing on which the board is mounted. He
> > feels that the heattransfer from leadtin to aluminium is better that
> > for nickel/gold to aluminium, possibly resulting in heat problems in
> > the end product.
> >
> > Any ideas on this ?
> >
> > Daan Terstegge
> > Thales Communications
> > Unclassified mail
> > Personal Website: http://www.smtinfo.net
> >
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