Mail*Link(r) SMTP               FWD>RE>FAB or DES: Z-Axis expansion

Hi Dave,

Several times in the past on technet the subject of non-functional lands has
arisen, you may want to check the IPC's webpage for the technet's archive and
do a search on "non-functional" (I'm not sure if they use the "-" or not).

As has been mentioned, once the lands have "lifted" stress relief has taken
place and experience has shown that failures generally don't occur.

Your statement, "From personal experience, removal of inner layer non
functional pads has resulted in pad lifting rejects as per MIL-P-55110. The
same designs, with
non functional pads replaced, exhibited no pad lifting." is why some of us
require non-functional lands to be located on at least all conductive patterns
layers, and fewer of us require them on all layers.

The physics of non-functional lands, simply stated, is that they will ensure
uniform stress between adjacent-layered lands.  In contrast, when you remove
the non-functional lands the dielectric material expansion stress will be
concentrated
about mid-way between the lands (unless there are "local" thin spots in the
copper plating which serve as stress concentrators), and will eventually lead
to barrel cracking.  That's why some of us for "serious" Class 3 and above
requirements require non-functional lands on all layers, and an "average"
copper barrel plating thickness of 30-38 micrometers (1.2-1.5 millinches), but
this depends on the product's life-cycle thermal environments and reliability
requirements.

Ralph Hersey
[log in to unmask]

--------------------------------------
Date: 9/5/96 9:18 PM
From: D. Rooke
>From personal experience, removal of inner layer non functional pads has
resulted in pad lifting rejects as per MIL-P-55110. The same designs, with
non functional pads replaced, exhibited no pad lifting.

Just out of curiosity, can anyone out there explain why pad lifting is
rejectable? I understand that it's origin was based on solder iron touch up
of leaded components, IE boards that had excessive pad lifting resulted in
pad removal during post assembly rework. Is this still valid? Comments
please...

D. Rooke

From: [log in to unmask] (D. Rooke)

>Dave
>
>Re:  Effect of removing non-functional lands
>
>My recollection is that many years ago we did a set of careful TMA 
>measurements of expansion in the neighborhood of a plated hole with and 
>without lands.  The result was that lands reduce expansion.  As a matter of 
>fact,  I believe that the data fit a rule of mixtures model in which the 
>excess expansion associated with removing a land can be predicted by
replacing
>1.4 mils of copper with 1.4 of resin. I believe the effect was most
pronounced
>below Tg.  At higher temperature, the strength of the barrel resists the
resin
>expansion and the effect is reduced.  Our conclusion was that this was a real

>effect, but that it is small and has a minimal impact of PTH reliability.  As

>a result we approved the removal of non-functional lands.
>
>Bob Holmes
>Lucent Technologies
>[log in to unmask]
>#012#------------- Begin Original Message -------------
>From: kcig1.att.att.com!ipc.ipc.org!TechNet-request
>Date: Thu Sep 5 10:08:56 -0500 1996
>Subject: FAB or DES: Z-Axis expansion
>To: [log in to unmask]
>Content-Type: Text     
>Content-Length: 957
>
>
>Has anyone got any information on z-axis expansion comparisons on boards
>with non-functional pads versus typical boards which do not have
>non-functional pads. We have a customer who is concerned about removing the
>pads from their design. Since the board is a high-layer count the pads are
>causing us much grief at drill by prematurely wearing the drill bits.
>Results of any tests that have been conducted (preferably with data) would
>be appreciated.
>
>David Arivett
>Cuplex Inc.
>
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