Ask the ASC Expert Ask the ASC Expert: What is a micro rectifier?

Written by: on May 22, 2020

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John Bushie, ASC's Director of Technology, explains what a micro rectifier is and how it functions.


SW: Steve Williams here I'm at American Standard circuits today I'm with John Bushie, director of technology here hey John how you doing?

JB: Very well, Steve.

SW: So recently, you and I were at a function where you were doing a technology presentation to an audience and you were talking about some new equipment that American Standard has just put in place.One of the things I found really fascinating was was some kind of a micro rectifier. Can you fill us in on what that is and how it how it helps you?

JB: Absolutely, Steve. We're getting asked nowadays to do a lot of what we'd call selective gold plating. In those cases were trying to meet two different requirements on the same panel. One, we're trying to provide a solder ball gold surface that won't damage or embrittle the solder joint for the components, and the second is basically a thicker gold ball wire bondable surface (obviously for gold ball wire bonding. The differences in thicknesses are generally we like to be less than 10 micro inches of gold for solder ability purposes; IPC I believe agrees in that respect and for gold ball wire bonding and may range anywhere from 50 to over a hundred and fifty micro inches.

So you'll see the significant difference in the amount of area that we're plating to do those two things and it's for some very specific reasons. Now to give you a little bit of background generally when we're talking about gold ball wire bondable gold, we're talking about a soft pure gold, Generally that's a type 3 with 99.9 0 purity and Knoop hardness of less than 90. We control that through monthly analysis by our chemistry supplier and it allows us to do this plating in a reliable fashion to give people consistent gold ball wire bonding results.

Now, when we do that for a full sized panel and a full panel plating configuration to be able to provide the solderable surfaces and the product protection to those surfaces. This is an example of this would be an 18 by 24 inch panel but we've decreased the the scale a little bit for demonstration purposes. But in this case it would be a 432 square inches for a full 18 by 24. This actual image is about 303 square inches, so in essence if we're talking about soft gold plating which generally in our case uses 3 amps per square foot which is the desired current density, we're talking at plating roughly 6 amps. In this instance, we're using a plating rectifier that goes up to 10 amps. The problem comes in is when people want those selective areas which tend to be very small in size, as demonstrated by what is the elective artwork for this particular job. You can see how sparse the plated area is you can see that the borders are even reduced and we're not using the thieving area that we were using on that particular design. Well obviously there's a very good reason for that. Gold at 150 micro inches thick is extremely expensive and you have to control costs for your customer because obviously somebody's got to pay the bill. In this particular instance, we go from roughly 70% of plated area down to about 7.2 in those cases.

If we were using a relatively large rectifier at 10 amps, we would have extreme difficulty getting down to those low current settings with the resolution of a rectifier designed to be able to plate that high. The other issue is that when you start to plate at the very bottom end of a rectifier's capabilities start to get into it's noise, and as you get into it's noise it starts to create plating anomalies and other issues.

In this case when we go to select a plate we utilize a specific we'll call it a micro rectifier and basically what it is is a very low current rectifier designed for very fine resolution control and in this instance about 0.1 milliamps resolution and it's generally used when we're plating very small areas in some cases when we talk about the board area specifically we're plating less than 10 square inches.

In this example we're in about 31 when we calculated all of the panel border area, and if you imagine this is an 18 by 24 inch panel you kind of get a sense for it.

This is just another way that we're able to provide a surface finish for our customer and do it reliably. We felt that was absolutely critical which is one of the reasons we teamed with Dynatronics on this, and came up with an excellent solution

SW: Awesome John. Appreciate that so I mean this is significant this is a 10x reduction in surface area and normally you'd be frying all these nicely.

JB: That's exactly what happens in that case the bath becomes very inefficient. It starts to generate hydrogen that creates localized alkalinity or high alkalinity which breaks down the resisting gives you imaging problems -basically your circuitry or your pads are no longer the same size as what was intended and you have subsequent etching problems. So this is really a the right way for us to go about this.


SW: And you guys have added a bunch of new equipment here recently to help your capabilities and increase your technology, so hopefully we'll be talking about some of those in future sessions. Totally appreciate this this is fantastic and look forward to seeing it in action.

JB: Excellent Steve.

SW: Thanks again absolutely once again Steve Williams John Bushie American Standard circuits and thanks for watching.

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