Jim,
 
Yes, I believe the surface area for foil treatments changes dramatically below one ounce foil. Just take a look at the manufacturers bond strength data as a function of foil thickness and you can see the trend. 
 
We should also hear from our foil friends John Andresakis and Rollie Savage for more detail.
 
One way I know to get around this is to use a one ounce OL foil and a process we used to call flash etching. This basically reduces the one ounce foil to 1/2 ounce thickness or less. In this manner you can get the higher bond strength from the treatment used on the one ounce foil and at the same time reduce the amount of background copper to be removed for fine OL spacings. I have also seen this process used in the far east, however it is called "fast etch" over there and used primarily one reducing 1/2 ounce foils to 9 or 11 micron thickness.
 
Gerry
 
> Date: Wed, 31 Dec 2008 09:53:56 -0600> From: [log in to unmask]> Subject: Re: [IPC-600-6012] Pad to trace interface on BGAs.> To: [log in to unmask]> > Gerry,> > Can you elaborate on lower bond strength for thin foils (below 1/2 ounce). Is the tooth structure of 1/4 ounce foil less?> > > Thanks,> > Jim Holme > Sanmina-SCI > Defense & Aerospace Systems > Desk Phone: 256-882-4800 ext.8733 > Cell Phone:  256-759-3895 >   > > -----Original Message-----> From: IPC-600-6012 [mailto:[log in to unmask]] On Behalf Of Gerald Gagnon> Sent: Wednesday, December 31, 2008 9:46 AM> To: [log in to unmask]> Subject: Re: [IPC-600-6012] Pad to trace interface on BGAs.> > Hi Chris,> > Pictures of "cratered pads" are excellent.> > The force to pull a pad off the board surface depends on the pad size and the foil surface area attached to the resin side. Please note that the board side pads are much smaller than the component side pads.> I conjecture that reversing the situation might tend to reverse the failure location to the component side.> > Also, the surface area attached depends on the type of surface treatment the foil received. Many thin outerlayer foils (below 1/2 ounce) do not yield large surface areas and thus have lower bond strength. > > BTW - none of the above are what I would consider fabricator related.> > Gerry> > Date: Tue, 30 Dec 2008 11:53:23 -0500> From: [log in to unmask]> Subject: Re: [IPC-600-6012] Pad to trace interface on BGAs.> To: [log in to unmask]> > Tino,> > Cratering is your culprit, and as pretty common. You shouldn't have much trouble finding help. Werner would top my list of the non-free variety. IPC-7095B is an excellent source of general info.> Without changing the design, "better" laminate material and cool-down will probably fix you up.> > However, I'll bet you a nickel that this is a foil laminated board and that if you tested peel strength (after assembly reflow excursions) it would be 0.5 to 0.7N/mm (much lower than it should be). In other words, the board fabricator might bear some responsibility. We find peel strength to be an good, cheap tool for monitoring adhesion/cohesion degradation at the surface. The military still requires peel testing as periodic conformance (albeit, not after convection reflow, or at elevated temp). 6012 voted against it.> > BTW, does anyone have an action to take "cratering" to T-50?> > Happy New Year to all.> > Chris> > Chris Mahanna> Robisan Laboratory Inc.> 6502 E 21st Street> Indianapolis, Indiana 46219> 317.353.6249> > > > -----Original Message-----> From: IPC-600-6012 [mailto:[log in to unmask]] On Behalf Of Constantino Gonzalez> Sent: Monday, December 22, 2008 10:04 PM> To: [log in to unmask]> Subject: [IPC-600-6012] Pad to trace interface on BGAs.> Importance: High> > Hello, I have a customer with a problem described below, Could you PLEASE HELP ME?> The problem is that several boards are being returned to the plant after failing in the field.> > After Failure Analysis the engineers have found that the reason for the failure is that one of the traces of the PCB connected to a BGA pad is becoming cracked (Micro crack at BGA Pad AJ29 in the corner of the BGA) as showed in picture below:> > image001.gif image002.gif> > They have concluded that Copper qty at PCB layer is not enough to support the normal Mechanical Stress in the unit chassis during the Transportation resulting in the trace and pad crack.> Based on Jedec JESD22B113 standard, they are suggesting that the customer change design on the traces connected to BGA pads as shown below: Does the> IPC7351 suggest anything related to this...> > image003.gif> > I have not heard before about this type of junction from traces to pads, and being honest I did not know the Jedec STD mentioned.> > Have any of you ever heard about this special union from traces to BGA pads to avoid stress or any other recommendation to reinforce trace to pad joints?> Could you please enlight me with your thoughts or opinions about this issue?> > Regards.> > Constantino J. González, ACME, INC. - GALUSAMEX REPAIR Co.> President / Consultant Engineer - Official IPC MASTER INSTRUCTOR - IPC-A-610 Committee Chairman USA Cell Phone: 605-381-5963 - Mexico Cell Phone: 55-2709 - 6392 - <mailto:[log in to unmask]> [log in to unmask] or <mailto:[log in to unmask]> [log in to unmask]> > Does your Company need Rework, Repair or Inspection of Electronic Assemblies Services.> Do you have too much Rework, Scrap or Rejects from your Customer.....Call us, We can help!> GALUSAMEX REPAIR Co.> _________________________________________________________________> Life on your PC is safer, easier, and more enjoyable with Windows Vista®. > http://clk.atdmt.com/MRT/go/127032870/direct/01/> > CONFIDENTIALITY> This e-mail message and any attachments thereto, is intended only for use by the addressee(s) named herein and may contain legally privileged and/or confidential information. If you are not the intended recipient of this e-mail message, you are hereby notified that any dissemination, distribution or copying of this e-mail message, and any attachments thereto, is strictly prohibited. If you have received this e-mail message in error, please immediately notify the sender and permanently delete the original and any copies of this email and any prints thereof.> ABSENT AN EXPRESS STATEMENT TO THE CONTRARY HEREINABOVE, THIS E-MAIL IS NOT INTENDED AS A SUBSTITUTE FOR A WRITING. Notwithstanding the Uniform Electronic Transactions Act or the applicability of any other law of similar substance and effect, absent an express statement to the contrary hereinabove, this e-mail message its contents, and any attachments hereto are not intended to represent an offer or acceptance to enter into a contract and are not otherwise intended to bind the sender, Sanmina-SCI Corporation (or any of its subsidiaries), or any other person or entity.
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