TECHNET Archives

July 2010

TechNet@IPC.ORG

Options: Use Monospaced Font
Show Text Part by Default
Show All Mail Headers

Message: [<< First] [< Prev] [Next >] [Last >>]
Topic: [<< First] [< Prev] [Next >] [Last >>]
Author: [<< First] [< Prev] [Next >] [Last >>]

Print Reply
Subject:
From:
Werner Engelmaier <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Werner Engelmaier <[log in to unmask]>
Date:
Mon, 12 Jul 2010 12:19:41 -0400
Content-Type:
text/plain
Parts/Attachments:
text/plain (145 lines)
 Hi Jack,
On this, Paul and I are NOT in agreement.
You are correct, non-functional pads tend to be removed to reduce drill wear.
He is correct in saying "Customers who remove non-functional pads reduce the "anchor" point and stress is transferred to the knee of the hole"— but do I really want earlier failures of the connection at the knee of the hole?
This question is even more important with the Pb-free soldering temperatures than with SnPb-assemblies. I want those  "anchor points" at the layers close to the PCB surface, because they support each other and reduce land rotation. 
 With PTHs/PTVs, there are essentially 2 failure modes: (1) barrel cracks, and (2) innerlayer separation [post-separation].
Barrel cracks typically occur in the barrel wall near the PCB center where the PTH is surrounded by a prepreg layer, because of the huge thermal expansion mismatch between the ED Cu and the surrounding resin above Tg and the CTE(z) is larger for the prepreg layers than the core layers.
Innerlayer separation occurs near, or at the surface [shoulder crack] of the PCB [layers 1, 2, 3 n-2, n-1 and n] due to the combination of radial tensile stresses, as the resin tries to push into the hole, and torque loading due to land rotation.
The radial tensile loads can be reduced by MORE innerlayer ribs supporting the barrel wall [think of a submarine]. One can reduce the land rotation torque loading by NOT removing non-functional lands on layers  1, 2, 3 n-2, n-1 and n—these lands support each other and thus reduce land rotation.  

 All of this is most important during soldering processes, but can also play a role in harsh use environments like automotive and military.
Werner

 

-----Original Message-----
From: Jack Olson <[log in to unmask]>
To: [log in to unmask]
Sent: Mon, Jul 12, 2010 11:52 am
Subject: [TN] non-functional pad removal


I know this topic has been discussed here before, but an article I just read

by Paul Reid put a different twist on it

http://www.pcb007.com/pages/zone.cgi?a=69569&_pf_=1



I wanted to ask about a paragraph where he says:



"We know from many years of reliability testing that a board with

non-functional pads removed tends to be more robust than the same board with

non-functions at every layer. Occasionally, designers will have

non-functional pads at every internal layer. In most applications this

produces a reduction in reliability with an increase in barrel cracks in the

central zone of the PTH. It appears that this produces a number of anchor

points along the PTH and failure occurs in the barrel. Customers who remove

non-functional pads for increased PTH reliability reduce the "anchor" point

and stress is transferred to the knee of the hole."



This is a very important point for me, because I have always heard it

explained a different way. My (unfounded unscientific) understanding was

that fabricators wanted to remove them to save drill bit wear (especially

for high-volume boards in benign environments), but designers often want to

keep them in because the extra ribs provide more support (especially for

harsh environments).

This article suggests that keeping inner layer pads is LESS reliable.

The reason it is important to me is that our boards are expected to survive

20 years in an automotive environment, we have been allowing unused pads to

be removed, but some have suggested we retain them for lead-free processing

temperatures.

We haven't cared about inner-layer pad removal until now, but soon we will

be required to design for RoHS compatibility, and we were about to start

specifying that they be retained. Am I misunderstanding these results?



Jack





______________________________________________________________________

This email has been scanned by the MessageLabs Email Security System.

For more information please contact helpdesk at x2960 or [log in to unmask] 

______________________________________________________________________



---------------------------------------------------

Technet Mail List provided as a service by IPC using LISTSERV 15.0

To unsubscribe, send a message to [log in to unmask] with following text in

the BODY (NOT the subject field): SIGNOFF Technet

To temporarily halt or (re-start) delivery of Technet send e-mail to 

[log in to unmask]: SET Technet NOMAIL or (MAIL)

To receive ONE mailing per day of all the posts: send e-mail to 

[log in to unmask]: SET Technet Digest

Search the archives of previous posts at: http://listserv.ipc.org/archives

Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for 

additional information, or contact Keach Sasamori at [log in to unmask] or 

847-615-7100 ext.2815

-----------------------------------------------------


 
=

______________________________________________________________________
This email has been scanned by the MessageLabs Email Security System.
For more information please contact helpdesk at x2960 or [log in to unmask] 
______________________________________________________________________

---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 15.0
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
-----------------------------------------------------

ATOM RSS1 RSS2