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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