Hi Dwight,
That is what we thought as well, but for 0.4mm pitch flip chips, we get better solder joint reliability with solder mask defined pads than with Cu-defined pads. Our hypothesis is that the greater standoff height and more consistent solder joint shape associated with SM-defined configuration outweighs the benefit of the increased bond area, but inconsistent solder joint shape with Cu-defined configuration.
Granted, this improvement was noted for temp cycling. I can't speak to drop. I really think Curt is going to need some sort of staking or underfill for that remote control application.
Todd
-----Original Message-----
From: Mattix, Dwight [mailto:[log in to unmask]]
Sent: Wednesday, April 20, 2016 10:17 AM
To: TechNet E-Mail Forum; MacFadden, Todd
Subject: RE: [TN] .031 PCB and WLCSP
S/M define pads on a ball and no underfill or corner staking? Standby for cracked solder joints.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of MacFadden, Todd
Sent: Wednesday, April 20, 2016 5:10 AM
To: [log in to unmask]
Subject: Re: [TN] .031 PCB and WLCSP
Without underfill in this application your greatest challenge for solder joint reliability will likely be from drop rather than cyclic strain and fatigue. It's a small device (9x9mm) so that risk may be relatively low, but here are some things you may consider:
* PCB-side solder joint area should match the device side pad size area (i.e., UBM). The UBM diameter will be smaller than the 0.2mm ball diameter (you may need to ask the device supplier for this info since it's not usually provided on the datasheet).
* Use solder mask defined pads because: (1) a Cu-defined PCB pad <0.2mm is not possible by most PCB fabricators if there are uvias in the pads; (2) solder area of the PCB pads should be of consistent size; this is not possible with Cu-defined pads due to exit traces, which draw solder away and distort the shape of solder joints inconsistently. Solder mask defined also allows for larger Cu pad, for which the PCB fabricator will thank you.
* Is corner staking an option? In the absence of full underfill, your best bet to pass drop test may be to apply epoxy dots or lines on the corners.
Good luck!
Todd
-------------------------------------------------------
Todd MacFadden
Component Reliability Engineering
Bose Corporation
1 New York Ave, MS 415
Framingham, MA 01701
508.766.6259
-------------------------------------------------------
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Curt McNamara
Sent: Tuesday, April 19, 2016 6:56 PM
To: [log in to unmask]
Subject: [TN] .031 PCB and WLCSP
I am reviewing a design with a .4 mm pitch, .2 mm ball WLCSP (9x9) on a 4 layer .031 FR4 pcb.
These will be used like a remote control, so there will be force applied, however there are supports for the PCB.
Due to the presence of switches with cleaning restrictions, underfill is not possible.
Looking for any comments on potential reliability concerns. The design could be changed to .064 if that would help.
Thanks in advance!
Curt
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