I do not have any reliability study but 6 or more years ago I was loading a
Dell work station mother board when I was working at a contract manuacturer
and first experienced oval, "race track shaped" land pads on most all their
passive parts. The smallest passive part in this case was 0603 sized. I
understand that Dell did research on this subject and from that came out
with this design. The logic is there for both lead and lead-free designs to
have the rounded corners. If you look at a solder fillet around a passive
resistor for instance which has a rectangular end and you will see that it
is shaped like a oblong racetrack. You will have 360 degrees of fillet
provided that the spacing between the land pads is such that the part
overlap allows it. Virtually 100% of solder strength here is in the solder
fillet. So, the most economical shape which uses the least amount of solder
(reducing and amount of lead pollution) for virtually 100% of the solder
stength is the rounded corner design. Dell also did the same for it IC's
whose pitch was greater than 20 mils. It makes it easier for stencil design.
I found it excellent for no-clean paste smt processes as it reduced or
eliminated many of the reasons/opportunites for solderballs. You reduce all
the apertures 1-4 mils (depending on the part/pad)or so and misprinting is
not so problematic and solder wetting pulls 100% of the solder into the
fillet as it remelts since there are no corners of solder beyond its reach.
Not very scientific but that is my experience. I do not recommend this
layout for passive parts that will be exposed directly to solderwave but I
do not recommend that any SMD parts go through solderwave. You will have a
hard time getting PCB layout people to do this but it is worth it.
Dorothy Lush
Manufacturing Manager
i2 Telecom International, Inc.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Lee Whiteman
Sent: Wednesday, March 24, 2004 10:48 AM
To: [log in to unmask]
Subject: Re: [TN] FW: Round SMT pads
Has anyone seen any reliability data to show that Lead Free solders
would work better with rounded pads than with squared pads? This is not
to dispute Brian's contention (I actually agree with him). I was just
curious to see if any data supports this claim.
Lee Whiteman
Senior Manufacturing Engineer
American Competitiveness Institute
E-Mail: [log in to unmask]
Ph: (610) 362-1200 x208
Fax: (610) 362-1290
This message is for informational purposes only and does not supersede,
modify, or create any agreements with ACI. Information contained in this
message does not bind ACI or its affiliates to any commitment, either
express or implied, unless ratified in writing by an authorized
representative.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Brian J. Toleno,
Ph.D.
Sent: Wednesday, March 24, 2004 1:29 PM
To: [log in to unmask]
Subject: Re: [TN] FW: Round SMT pads
Another reason for rounded pads is when using Pb-free solders, due to
the
increase in surface tension and wetting angle, people are moving to
rounded pads so that they don't have exposed corners after reflow.
Brian J. Toleno, Ph.D.
Sr. Applications Chemist
Loctite Electronics
15051 East Don Julian Rd
Industry, CA 91746
Ph: 626-968-6511 x304
Cell: 626-215-0879
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