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September 2015

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From:
"Stadem, Richard D." <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Stadem, Richard D.
Date:
Thu, 10 Sep 2015 21:54:22 +0000
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There are many good articles out there describing the effects of ENIG on signal loss, and whether or not to mask prior to or after plating the lines. Here is one that you might find helpful, but there are many.
http://www.icd.com.au/articles/Surface_Finishes_PCBD-June2014.pdf


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Thursday, September 10, 2015 4:47 PM
To: [log in to unmask]
Subject: Re: [TN] Board with Nickel/Gold plating

Because this is a high-speed measurement board, please understand that the nickel plating will contribute to a somewhat more lossy circuit due the skin effect. The nickel thickness thus is a critical factor to consider.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Mattix, Dwight
Sent: Thursday, September 10, 2015 4:33 PM
To: [log in to unmask]
Subject: Re: [TN] Board with Nickel/Gold plating

Sounds like an ENIG finish at those thickness for Au and Ni.  That should be specified as SMOBC (soldermask over bare copper). Last thing before legend (aka silkscreen).

Expect 1.8 - 2 mils finished copper on outerlayer. That assumes  starting with 1/2 oz foil and a only single plating cycle (standard Class 2 copper in via and not special extra copper rqmts for VIPPO, pressfit or such).

If those are isolated traces they can plate much thicker due to increased current density. To avoid that be sure to add thieving or allow your supplier to thieve the outer layers to minimize that variation. Just keep make sure they keep back an acceptable distance to avoid creating an unintentional coplanar waveguide (3x or 5x dielectric height).

dw


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ravinder Ajmani
Sent: Thursday, September 10, 2015 2:11 PM
To: [log in to unmask]
Subject: [TN] Board with Nickel/Gold plating

Hi Experts,

I am designing a high-speed measurement board.  The board will have a dozen high-speed signals connected to 2.92 mm connectors.  The signal traces will be routed on the Top layer with through vias at the two ends.  I have been advised to plate the signal traces with 3 - 10 microinches of Gold over 100 microinches of Nickel.  Solder Mask/Conformal Coat will not be applied over the signal traces and via pads.  No components need to be soldered to the signal traces, and 2.92 mm connectors will be screwed to the board.

I am trying to get an understanding of the board fabrication process to determine the finished thickness of the signal traces.  This will help me figure out the trace impedance.  I will appreciate if you can provide answers to the following:


*        The starting Copper thickness for the Top/Bottom layers will be 0.6 mils (1/2 oz).  Will Top/Bottom layers still go through the normal plating process.

*        At what stage will Nickel and Gold be plated on the board.  Also is it plated on the entire board surface or only the exposed locations (signal traces and vias).

*        Is solder mask applied in the final stage (after the ENIG process).

*        What is the expected finished thickness of the signal traces.  I was informed by the Fab personnel that this will be about 2 mils.

Thank you very much for your help.

Regards

Ravinder Ajmani
HGST, a Western Digital company
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