I agree with Roger's recommendations, with the following two additions:
1. The stencil apertures should be 16 mil square as Roger stated, but the LOCATION of the apertures should be closer to the insides of the pads, not in the center.
2. During pick and place, the placement of the 0402 components onto those pads should be at a slight angle, perhaps 20 degrees out of alignment on the pad-to-pad axis. As the solder melts during reflow the part has to be pulled into alignment before it can be pulled up (tombstoned). Placing it at a slight angle makes it more difficult to pull the part up against a corner, and further ensures that both ends of the component are captured in the molten solder at approximately the same time period. This greatly helps reduce any tombstoning.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Roger Mack
Sent: Thursday, March 02, 2017 8:50 AM
To: [log in to unmask]
Subject: Re: [TN] Reduced size PCB pads for 0402 capacitors on back side of BGA
For parts opposite a BGA we usually use 0201's, and the pattern and part sizes were recommended by the BGA datasheets.
If you need to use 0402's i would recommend a smaller pad than you stated below. We always use 20x20 mil pads, 54 mil toe to toe, and print 16mil square. You will never tombstone with this recipe.
We have never used round for chips, but I have read that it is done, and don't homeplate 0402 or 0201, just for larger parts.
Roger Mack P.Eng.
Manufacturing Specialist
Parker Hannifin Canada
Electronic Controls Division
1305 Clarence Avenue
Winnipeg, MB R3T 1T4 Canada
direct 204 453 3339 x7373
[log in to unmask]
www.parker.com/ecd
From: Carl Van Wormer <[log in to unmask]>
To: <[log in to unmask]>
Date: 2017-03-01 03:22 PM
Subject: [TN] Reduced size PCB pads for 0402 capacitors on back
side of BGA
Sent by: TechNet <[log in to unmask]>
We are designing a new PC board with a bunch of 0402 caps on the back side
of a 1mm pitch BGA. The recommended pad size for the caps is rectangular
(0.63x0.64mm). The distance between the corners of these pads and the
pads around the board vias is less than 4 mils (I know I?m mixing metric
and English units). We are discussing the possibility of rounding the
corners to increase the spacing, hoping to reduce the cost of the PC
boards. Another possible choice might be octagonal pads. The extreme
case of rounding the corners is making circular pads. I?ve never seen
octagonal or round pads.
Questions for the group:
1. Have you seen round pads used for 0402 caps?
2. Will properly sized round pads cause problems with 0402 caps?
3. If round pads would work, what is the smallest pad size to
consider?
4. Are there other options that I may be overlooking?
Thanks,
Carl
?????
Carl B. Van Wormer, P.E., AE7GD
Senior Hardware Engineer
Cipher Engineering LLC
21195 NW Evergreen Pkwy Ste 209
Hillsboro, OR 97124-7167
503-617-7447x303
[log in to unmask]<mailto:[log in to unmask]>
http://cipherengineering.com<http://cipherengineering.com/>
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