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gold wire bonding

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#38699

gold wire bonding | 27 December, 2005

Hello,

does anybody have experience in thermosonic gold wire bonding to electroless nickel/immersion gold??

regards, Sergey

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#38704

gold wire bonding | 27 December, 2005

What's the issue and situation?

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#38709

gold wire bonding | 28 December, 2005

I would like to know if ENIG is widely used for the thermosonic gold wire bonding or this application is an exotic one. And I also wold like to talk to someone who uses printed circuits with ENIG finish for such an application. I do not quite understand why (according to literature and standards) ENIG is only suitable for the ultrasonic aluminium wire bonding. What's the problem with TS gold bonding?

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Chris

#38720

gold wire bonding | 28 December, 2005

I have lots of experience with thermosonic gold ball bonding. You can read the literature and you will probably find some papers that say you can do it. I have never been able to do it. We gold ball bond all day long with little problems at all but this is to soft gold over nickel. The gold is 25 micro inches and the nickel is about 200 micro inches. I can tell you that if I have any problems with stitch lifts or nonsticking stitches, I XRF (xray fluorescence)the PCB and the gold is thin at about 3 to 6 micro inches. Why? My PCB supplier forgot to selectively plate up the wire bond areas. I have never been able to thermosonic gold ball bond to ENIG. Have tried this with Palomar, Hughes, and Panasonic bonders. No luck!

Here are a few suggestions.

For prototypes, you might be able to do make a ball bond to the substrate. This will leave a little gold ball on the stitch pad. Then you can go back and ball bond to the die and stitch bond to the ball stuck to the substrate pad previously. The idea is that the ball bond will stick to ENIG better than a stitch bond and you are building up the substrate gold thickness using a ball bond placed on the stitch pad.

Reverse your bonding. Ball bond to the substrate and stitch bond to the die.

Use MacDermid Sterling Silver plating instead of ENIG. It works for me. You can gold ball bond to it. I can anyway. Not nearly as good as thick gold but it works well with little rework. One caution here. The PCB has to be clean and stored properly so it does not oxidize. The advantage is that you can solder to it. This is not the case with thick gold due to the gold embrittlement issue.

The process that works the best is to use selectively plated PCB. Adds about 15% to the cost of a PCB plated with thick gold. This is what my supplier charges me. The soldered areas are 3 to 6 micro inches of flash gold over 200 micro inches of nickel and the wire bonded areas are 25 micro inches of soft gold over 200 micro inches of nickel.

Good luck,

Chris

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#38721

gold wire bonding | 28 December, 2005

You don't TS bond gold wire to nickel [of ENIG] for the same reason that you don't TS bond gold wire to hard gold. [Nickel or maybe cobalt is used to harden soft gold.] Together nickel and gold produce unreliable bonds. Look here: * http://www.smta.org/files/SMTAI01-Holcomb.pdf

* Wire Bonding In Microelectronics: Materials, Processes, Reliability, and Yield; Harman, GG; McGraw-Hill 2nd ed 1997 0070326193

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#38722

gold wire bonding | 29 December, 2005

Hello Chris, To be honest, it's hard to believe you could not bond to ENIG at all. Yes, the gold layer is thin and hard, but it just narrows the process window and does not make the bonding impossible. I say this, because we have been using TS gold wire bonding to ENIG on a regular basis (on a Delvotec bonder). What the pull test results did you have when you tried to bond to ENIG? What the pull test results do you have now, when you bond to thick gold? We have chosen ENIG as a surface finish for our PCB substrates, because we have to both solder and wire bond chips to the substrates and ENIG seems to be suitable for this application and gives the lowest price of substrates. Are we alone in the universe?

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Chris

#38730

gold wire bonding | 29 December, 2005

The paper Dave posted the link for is great. I have to go back and read it in depth.

I can build prototypes by wirebonding to ENIG. I can do it on a manual wirebonder but I don't have the patience to do it on a magazine to magazine automatic machine. Just way to many errors and pull strengths well below 4 grams with 0.0033 inch diameter wire. I also do not plasma clean at all. Plasma cleaning may give you that extra advantage that I do not have. With 0.0033 inch diameter wire, I get at least 12 gram pull strengths and average about 14 gram pull strengths. My stitches are security bonded.

I use a Panasonic KME 27U-H gold ball bonder. My process runs better than 100 ppm but it is 200 micron (0.004") pitch.

I could never allow my design group to introduce a product where I was required to wirebond to ENIG. It just does not work for me but it may work for others. I have over 200 products on my wirebonder and they all run perfect with selectively plated gold, 25 micro inches in COB areas and 6 micro inches in the SMT areas.

Great paper. Thanks Dave!

Chris

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#39225

gold wire bonding | 20 January, 2006

Hi, i'm david, new to this forum. Came across your posting about using a delvotec bonder to TS bond to a ENIG pad. Given that the immersion gold layer is thin, does the capillary scrub through the gold layer to expose the underlying nickel? Does your capillary pick up material from the pad (capillary build up) really quick? thanks!

david.

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#39240

gold wire bonding | 20 January, 2006

Hello David, No, the capillary does not scrub through the gold layer. At least, we have never seen it. We replace the capillary after approx. 200k bonds. Regards, Sergey

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#39260

gold wire bonding | 22 January, 2006

Hello Sergey, thanks for your input. The capillary i'm using seems to pick up material from the bond surface fairly quickly. Can't think of a solution at the moment.

regards, david.

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