LISTSERV - TECHNET Archives - LISTSERV.IPC.ORG

TECHNET Archives

September 2002

TechNet@IPC.ORG

	LISTSERV Archives
	TECHNET Home
	TECHNET September 2002

	Log In
	Register

	Subscribe or Unsubscribe

	Search Archives

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 >>]

Subject:	Re: Close pitch PCB failure
From:	[log in to unmask]
Reply To:	TechNet E-Mail Forum.
Date:	Wed, 18 Sep 2002 14:37:16 -0400
Content-Type:	text/plain
Parts/Attachments:	text/plain (26 lines)

Hello Rob,

The nominal distance of .007 is tight. In the case of .8mm BGA the PCB planes will have an .0275 clearance and I have drilled .012 diameter holes in the .100 thick board. That renders .00775 PTH wall (drill wall) to edge of copper. It doesn't get much tighter than that. The same configuration; signal layers use a .019 diameter pad with the distance from PTH wall edge (drill) to edge of pad of .0035, with .004 trace and .004 spacing this renders .0075 PTH wall to trace edge. These are tight numbers and are BASIC; all that "slop" is the fabrication allowance. Increase those numbers by decreasing board thickness allowing a smaller drill diameter (aspect ratio <9/1).
FR4 is the best material to achieve success. Polyimide usually brings on need for etch back and there simply is no real estate to do that. Thermount is wonderful material, but is best used for its propensity to have consistent lower Er and its mainstay is low XY CTE. I do not recommend its usage for these tight configurations. Folks used to recommend it because its very nice registration qualities, but that Kevlar may have micro ratcheting -in my opinion- and may induce the CAF condition.
If your stuck in a tuff design I would recommend:
1. lower thickness (mils count)
2. decrease trace widths (half mils count)
3. FR4 170 deg C Tg stuff
4. new drills, one up
5. have a plan "B" ie redesign
6. group crossing of fingers

Good luck,
Boston Brad yer PCB pal

---------------------------------------------------------------------------------
Technet Mail List provided as a free service by IPC using LISTSERV 1.8e
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/html/forum.htm for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315
---------------------------------------------------------------------------------

ATOM RSS1 RSS2

LISTSERV.IPC.ORG