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March 2003

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Subject:
From:
Dave Hillman <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Tue, 18 Mar 2003 10:04:10 -0600
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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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