# How thick a gold finish on a pcb for wire bonding?

As a partial reparation for my previous question today.
Please allow this rather esoteric/ but on topic question.
I was sending out a pcb that will have gold wire bonding pads.
It was a option at our board house. I got a call when I was requesting a quote.

"How thick do you want the soft gold bonding layer?"
"I have no idea." I said, which is my normal response.
"What's your "standard" thickness.?", I asked.
I was shifted to engineering. 30-50 micro inches was the answer, after a nice discussion.
(It's always a pleasure to talk with the engineer's.)
So that's what I ordered.
I could order a thicker layer.
Has anyone done this?
I'll be wire bonding soon, so I can let you know if it works.

Edit: Well I got the quote back and my jaw dropped. The price is more than \$2k for a few pcb's. For that cost I could evaporate my own Ni and gold layers.

Edit 2: (Adding more to the question.)
So the reason for the high price is that the board house needed to send it outside for processing. I asked for quotes on their other types of Gold finishes.

The first is called Deep Gold. This is an electrolytic process and puts down 30 u inches of gold. (I don't know if there is any underlying Nickel.) This is also called hard gold and from limited reading I don't think it works for wire bonding.

The second is Immersion Gold. or ENIG and puts down 3-10 u in (75-250 nm) of gold over a thicker nickel layer. Again from limited reading it seems that one can not wire bond to this with gold. But maybe with aluminum. (Though there can problems with the Al/Au interface.)

And my final crazy idea is to make a mask for my pcb and evaporate my own Ni/Au layer.

• I don't know how thick the gold should be, but I know it is sometimes gold over nickel. Also, there is some adjustment of the process that can be done on the bonding machine. When I have worked with this technology, the assemblers always test the bonding wire pull-force on the first sample (or first few samples) to make sure that the process is dialed in. The test consists of pulling the wires until they break or detach from the bonding pad. Then they tweak the bonding time or force if needed. Good luck! – mkeith Nov 12 '14 at 1:03
• @mkeith, thanks, I think I'm getting gold over copper. I've got an expert showing me how to do the wire bonding... hey I should ask him how thick! – George Herold Nov 12 '14 at 1:22
• Gold over copper will corrode. The gold will peel right off the copper after a couple weeks. Guess how I know. – The Photon Nov 12 '14 at 2:33
• @ThePhoton, Interesting. Did you have a board house make it for you? I'm still waiting for a firm quote from advanced circuits, I'll ask them if they do a nickle layer in between. (Certainly they must know how to do it.) – George Herold Nov 12 '14 at 13:19
• @GeorgeHerold There's a place in PA that offers training in wire bonding, next course is in early Dec. I think all gold over copper requires a barrier layer of some kind (usually Ni, but I guess other things would work). Unfortunately Ni is a bit magnetic so that that doesn't always work in sensitive applications. – Spehro Pefhany Nov 12 '14 at 17:01

## 2 Answers

According to documents I have from a PCB vendor, a typical spec for wire-bondable electrolytic soft gold is and 1.97 $\mu$in (min) gold over 188 $\mu$in (min) nickel. These are called out as IPC minimum values, though I don't have the IPC documents in front of me.

ENEPIG (electroless nickel, electroless palladium, immersion gold) plating can also produce a wire-bondable surface. A typical spec for that is 197 $\mu$in nickel, 12 $\mu$in palladium and 1.1 $\mu$in gold. Again these are all minimum values.

Recently I had success with a design that spec'ed the gold thickness as 25-30 $\mu$in, but really you don't want a very thick gold layer because excessive gold does bad things to solder joint reliability.

• Thanks, I've spent the morning reading all about ENEPIG and all sorts of other wire bonding stuff. The soft gold thickness was 30 u in. (1 u in = 25nm) the reason for the large price in the quote was because they needed to send it out for processing. Does the place you used do small proto-type quantities? OK I'm stopping here and just editing my question. – George Herold Nov 12 '14 at 18:12
• Most PCB shops, afaik, will send boards out to a plating shop for gold plating. This might be related to the environmental/safety issues with some of the chemicals involved in plating processes. Certain processes might be easier to get done in-house than others. If you visit chat I could drop names of a couple of shops I've used. – The Photon Nov 12 '14 at 20:15

Wow, I want to answer my own question.
Well, it's not my answer at all. Credit goes to JL, the first response here.
And then while chatting 'The Photon' found the following link. Which looks just about perfect, for prototype wire bonding. (I hope someone will give him another +1 for me.)