Jack,
Listen to what Joyce and Wayne are saying. Their inputs are all bits of the
puzzle.
It all depends on exactly what you hope to do. Many times, rather than
combining chip & wire and SMT on one board [depending on die size and
pin-out], it is easier to process arrays of die, glob top them and dice them
into individual components that can be placed and reflowed with the SMT
components. This isn't always possible if bypass caps or other components
need to be as close to the die as possible, but it does open up another path
for consideration. I call it the jelly bean approach.
If no one has mentioned ENEPIG as a surface finish. It is both wire
bondable and solderable... It does not bond nearly as easily as the pure
soft gold that Wayne mentions for Au ball bonding. It is also aluminum
bondable. One thing that I've always encountered with ENEPIG is that virgin
incoming boards bond quite a bit differently than those that have been
reflowed. Moral of the story - don't leave your WIP sitting around. The
thermal history of the board does have an impact on bondability
Even automatic ball bonders that can run at speeds of 7-10 wires per second
have targeting accuracies of ±2-3 µm. Isn't that impressive, and that is
slow... :-)?
Steve Creswick
Sr Associate - Balanced Enterprise Solutions
http://www.linkedin.com/in/stevencreswick
616 834 1883
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Wayne Thayer
Sent: Thursday, January 30, 2014 9:39 AM
To: [log in to unmask]
Subject: Re: [TN] Wire Bonding Spec?
Jack-
Wirebonding is micro-welding at comparatively very low temperatures. Only
works reliably if the surfaces are perfect--clean and the right metallurgy.
Getting the right PCB finish can be a nightmare: For wirebonding, most
prefer >25 microinches soft electrolytic ultra-pure gold over >150
microinches nickel (electroless or electrolytic, but virtually all shops use
electroless). And the surface has to be pretty smooth, but this is a lesser
effect. With gold that thick, parts like QFNs can become problems (due to
joint embrittlement), so for ChipOnBoard it isn't uncommon to have two
separate finish requirements. If you can use aluminum instead of gold for
the wiring, it is much easier to weld and can be welded to a wider variety
of surface finishes. ENIG can work--depends on the exact formula and
application process. But aluminum has poorer resolution than the gold.
The big question/challenge for wirebonded COB is whether to wirebond before
or after SMT. SMT is filthy, and as noted above, the wirebonds demand
absolute cleanliness. But if you wirebond first, it messes up the solder
application process. If you have a quarter inch or more of isolation
between wirebonded and SMT, then using thick peelable solder mask can
protect the gold through the SMT process. But we've found that to be
unreliable in tight quarters.
If you are ordering a COB assembled from another company, then creating
specifications to assure that the results will be reliable is relatively
easy, however all reliability is based on visual inspection combined with
statistical process control. For gold ball bonds, you specify acceptable
destruct test statistics for pull tests and ball shear tests. For aluminum
or gold wedge bonds, you just specify pull test statistics. Reading Harman
will assist you in determining the most applicable statistics for your
application. IF your design also has challenges such as extreme
environmental conditions or conducting heat away from the wirebonded part,
then you'll also have to specify statistics for die bond line thickness and
shear strength. The equipment to do all of this destructive testing runs in
the $100K range.
HOWEVER, if you want to slam some wirebonded part down on an ENIG board as a
test case, and the resolution isn't challenging, that can be done quickly
and cheaply with a minimal investment in equipment. It just likely won't be
reliable (although given Murphy's cruelest law, "beginner's luck", you can
easily be led to believe it is). I've even aluminum bonded parts down to
bare clean copper--as I said above, aluminum bonds easily.
Good Luck,
Wayne Thayer
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Joyce Koo
Sent: Thursday, January 30, 2014 9:05 AM
To: [log in to unmask]
Subject: Re: [TN] Wire Bonding Spec?
Jack,
if you are looking for design spec, you are out of luck. Depend upon the
capillary design, and wire bonding machine, you have set of different can
do, can't do, regarding clearance to adjacent components, cleanliness if it
is direct chip on board, height differential (you might need deep access
capillary), etc. etc. The nowadays WB machine are amazing (if you pay arms
and legs). Get a good plasma clean machine prior to bonding, like March,
you got to go. Check UMC website, the operators runs thousands parts a day,
no sweat. Good luck. (if you are running low volume, get an CM-elite
prototype house, set up from ground up for WB is a bit of too big a task.
IMO).
Joyce Koo
Researcher
Materials Interconnect Lab
Office: (519) 888-7465 79945
BlackBerry: (226) 220-4760
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steven Creswick
Sent: Wednesday, January 29, 2014 5:17 PM
To: [log in to unmask]
Subject: Re: [TN] Wire Bonding Spec?
Jack,
All good responses thus far - George's books will provide a great deal of
process and reliability considerations, as well as some plating info. Mil
Std 883 will define visual criteria, pull strengths, and ball shear
strengths. ASTM F458, 459, and 1269 essentially repeat the Mil Std. I'm
sure there is an IPC, JEDEC, or something spec which also repeats the Mil
Std, but I'm an ol' Mil Std boy and I don't know what the other ones are,
off the top of my head.
Can you clarify a bit more specifically what you are looking for?
Are you looking for plating detail, wire bonding process detail,
design/layout details, packaging options etc.?
It is my observation that one doesn't want to just casually get into wire
bonding. Checking things out is a good first step.
Steve Creswick
Sr Associate - Balanced Enterprise Solutions
http://www.linkedin.com/in/stevencreswick
616 834 1883
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Jack Olson
Sent: Wednesday, January 29, 2014 4:17 PM
To: [log in to unmask]
Subject: [TN] Wire Bonding Spec?
newbie question:
Do any IPC publications discuss wire bonding?
What we are trying to do is similar to COB, but this is a new subject for
us...
thanks,
Jack
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