This is a serious issue with flipchip design, not so much for thermal
dissipation but for solder dispersal. Solder mask registration is a real
bear when it comes to flipchip pads, some of which get down to 150u diameter
(6 mils) on 220u pitch (8.7 mils). In this case there is no solder mask
around any of the pads except facing the outer area of a die because a
registration tolerance of 50u (=2mil) and a minimum web requirement of 100u
(=4mils) which makes that a farce. Best solder mask registration I have seen
offered in production is +/- 35u with slight yield loss, or +/- 25u with
step and repeat imaging on a fab panel (super expensive - don't do it!).

If you don't neck a power trace 100u (= 4mils) down to 50u (=2mils) the
wider trace usually steals some of the solder from the die's collapsing ball
and starves the joint of solder, occasionally leading to dry joints in
assembly but more usually to joints that are unreliable in the field. Solder
wicking down a close via hole is a well known phenomenom and the same
problem applies when you get down to really fine lines and spaces on
flipchips.

Strangely, we haven't had much of a problem with solder bridging, even when
an unconnected trace is 50, 75 or 100u away from the pad with no intervening
solder mask. The solder volume on a flipchip ball is so small that surface
tension tends to keep the solder on the pads even as the balls collapse
during reflow. I should add we don't screen paste for flipchips, relying
instead on dipping the pre-tinned and balled die in a flux and placing on
the board, with the balls acting as the only source of solder. Works well.

Andy Kowalewski

[log in to unmask]


-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of John C
Laur
Sent: Thursday, 28 July 2005 06:55
To: [log in to unmask]
Subject: Re: [DC] Trace width ratio to pad

All,

Wide conductors are great, BUT, wide conductors on a fine pitch QFP can
cause bridging.  In affect you solder mask define the pads with the wide
traces creating solderable surfaces that may be ~4 mils closer than
intended.  It happens when power and ground conductors are set up wider
than the signal conductors.

John Laur
Engineering Analyst Sr, HW
Information Platforms Business (IPB)
Rockwell Automation / Allen-Bradley
1201 South Second Street
Milwaukee, WI.  53204-2496  USA

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