Good! The cornerbond option works very well, perhaps better than underfill. It eliminates solder extrusion during thermal cycling. However, you must select an adhesive that has a CTE that is at least close to the CTE of the CSP. For most silicon die, Loctite 3621 is a good one to try.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Curt McNamara
Sent: Thursday, April 21, 2016 9:50 AM
To: [log in to unmask]
Subject: Re: [TN] .031 PCB and WLCSP

Thanks to everyone! We have cleared out the layout in the corners to allow for staking. Seems like a good compromise.

Can anyone suggest a reference / recommendation on best practice for corner staking?

            Curt

On Wed, Apr 20, 2016 at 9:31 AM, David Hillman < [log in to unmask]> wrote:

> I agree with Dwight and Todd - a Wafer Level CSP (WLCSP) is really a 
> flip chip device (silly marketing definitions - just causes 
> confusion!) so without the application of an underfill material, there 
> are going to be solder joint cracks/failures unless the product use 
> environment is really benign.
>
> Dave
>
> On Wed, Apr 20, 2016 at 9:17 AM, Mattix, Dwight 
> <[log in to unmask]>
> wrote:
>
> > 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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