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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