nice post.  i think  you covered everything.... if you do the chip  
stack (multiple chips stacking on top of each other) the size and  
layout would be different... (e.g. memory on top of processor for MCM  
on board).   If you need rework, make sure you got long enough pads  
to put 2 or 3 stitch... some of the gold is so thin, once you pull  
out the old stitch, it expose the nickel,not good for WB...IMHO.
good luck.
jk
On Aug 14, 2017, at 11:33 AM, Wayne Thayer wrote:

> Hi Steve-
>
> For the bond pad on the IC, that's the smashed ball size plus  
> targeting error. Smashed balls "normally" are around 3.5 x wire  
> diameter, but using the best tools and equipment I've seen pretty  
> SEM pix down to 1.5 x wire diameter. The bond pad on the IC is  
> beautifully consistent thin film.
>
> For the bond pad on a gold over nickel over copper over laminate  
> board, in "normal" situations you will be placing a "stitch" bond,  
> and added to overall "smashed stitch" plus targeting error, you  
> have the wire potentially wandering beneath the capillary when  
> you're making that second bond. 4 x wire diameter as a minimum  
> dimension is achievable with producible yields. The best gold is  
> electrolytic (not immersion or electroless). If you are working on  
> a single chip package, then all of the nets can be tied together,  
> and you can electrolytically plate the entire structure post-etch.  
> If you have a very complicated, dense route, that isn't possible,  
> so the common technique is to use the electrolytic gold as an etch  
> resist. This leaves "wings" underneath all of the edges. The depth  
> of the wing is dependent on the etching process and the thickness  
> of the copper being etched during the last step. And some etch  
> chemistries preferentially attack nickel, which makes the "wing  
> problem" even worse. Anyway, the "4x" I cited is at the minimum pad  
> cross section, not what was designed or what you see when you look  
> down on the completed etched circuitry.
>
> With the right tools and equipment, you can reverse bond the IC,  
> which improves the targeting on the circuit board and reduces  
> overall bond height at the expense of increased set up and run  
> time. With this technique it is possible to hit 3 x wire diameter  
> minimum cross section bond pads.  But then there's another limit  
> I've seen: If you have a fairly thick copper build-up, then ultra  
> narrow bond pads sometimes just break off: The thermosonic process  
> ends up to be too much for the adhesive holding on the original  
> copper plating. The thick copper puts the ultrasonic scrub high  
> enough above this adhesive joint to get a little moment arm on it.
>
> If you're flush with space, double the minimal numbers above and  
> you have enough space for a rework cycle!
>
> Wayne Thayer
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Steven Kelly
> Sent: Monday, August 14, 2017 8:03 AM
> To: [log in to unmask]
> Subject: [TN] Wire bond wire size versus pad size
>
> Hi all,
> Is there a general rule of thumb for the wire versus the bond pad  
> size for thermosonic gold wire bonding? Thanks. Steve Kelly