Electroforming on to electroforms

A. It's strange that I have been in the plating industry a long time and haven't heard this question because it doesn't sound like it would be an unheard of requirement. I can't give you the benefit of actual experience, but I believe that it should not be a problem if you can maintain good current distribution. Remember that for precision electroforming (stampers, molds, etc.) that it is the first deposited surface (the side closest to the mandrel), not the last deposited surface, that offers the best replicability.

A. Dear Rupert,

What you are describing is possible. As you can imagine the problem is one of registering the second image to the first and also maintaining uniform thickness for the electroformed components across the mandrel.

As an alternative and if the features of your electroform allow, it is possible to electroform the parts so they are thicker than the resist and then lift off the parts. The resist is left on the substrate and a second set of parts can be made from the same patterned area. Lifting off the parts slightly damages the resist edges, so the mandrel does have a finite life.

Good luck with your process.

A. Multilayer electroforming is possible, but I do not understand what you mean by "stop the process before removing unexposed resist". If you do not develop the image (i.e., dissolve the unexposed resist) you do not have a pattern to electroform. Hewlett Packard have a couple of patents granted in the mid to late 1980's on multilayer electroforming to make ink-jet nozzles. They used positive and negative resists in sequence to get their desired shapes. The problem with using the same resist is that a developer is very similar to a resist stripper, so as you image the second (or later) layers, you start to strip off the initial layers. However, this is easy to overcome, by using different types of resist! Secondly, as you build up the electroform it will become a 3-dimensional shape and that will give problems is getting a good image resolution. Nevertheless, this can also be easily overcome. Thirdly, Pat is right about registering the image; it may be difficult to accurately do so, but there are systems available that can achieve micron scale accuracy if you are prepared to pay for it! Multiple runs with electroforming is not unheard of and relatively high numbers can be achieved by using the correct systems. There is a lot of exciting work being done with LIGA and micro technology at present, so it may be worth looking at these areas. Better still, look at nano-engineering techniques.

A. Hi Amit.

I don't think we have enough information for specific suggestions, but shields are the general way to control relative thickness in electroforming. Remember that Faraday's Law tells us that the plating will go where the current goes, and that Ohm's Law tells us that the current will go where the resistance steers it. Good luck.

Regards,

A. Hello Ozum. The thickness of the plating is directly proportional to the current at that point, so you control the thickness by controlling the relative current density as desired. While auxiliary anodes are one way to do that, and thieves (dummy cathodes to steal the current) are another, there are two easier and usually better ways for electroforming --

The easiest way to reduce build-up at edges and corners, when possible, is simply to make the anode smaller than the cathode. That way, for the current to reach an edge or corner, it needs to travel further than it has to travel to reach the middle; this means more resistance, so less current, so less plating.

Where that isn't practical, shields are usually the best answer. These are non-conductive plastic plates that are placed between the anode and the cathode so that the current cannot take a direct path through them, but must go around the shields, or only through limited holes in them. In your case, conceptually you might have a shield with a rectangular hole in it that is smaller than the cathode, so that the current must travel a long way to get to the edges and corners. (In my sketch the distance from the shield to the cathode is exaggerated so things aren't drawn on top of each other and thus invisible; in reality the shield would probably be only a couple of inches from the cathode and even closer if the cathode is very small).

The design of shields can be an art or a science. Experienced electroformers will have a good feel for how to make a geometrically appropriate shield, and how to tweak it to improve it, and software is also available for the purpose. Good luck.

Regards,

A. Hi again. Shields have to work, Ozum. Plating will not deposit where there is no current flow, will deposit lightly where is is little current flow, and will deposit heavily where there is high current flow. Picture a shield very close to the object you are plating, and with only a pinhole in the center. All of the plating would of necessity be right by the pinhole; there would be no plating at all at the corners. I suspect your shields are too far from the cathode and/or the holes are too big to be effectual.

Pulse plating can be of some value by chasing ions back through the boundary layer during the reverse current portion of the cycle, but it is yet another difficult variable to tame in a process that you are already having some trouble with.

Nickel plating is quite efficient, about 95%+, across the plating range -- so your plating thickness map is already a quite accurate current density map; you don't need to develop and alternate way to check it. Good luck.

Regards,

Hi. We are leaving my area of experience. Hopefully another reader will be able to comment on your situation. Good luck.

Regards,

A. Hello Ozum,
Let's get down to basics. What current density are you applying? As you may know from your research, a higher current density will favor the high current density areas. When you lower the current density, you will achieve a more uniform thickness across the part to be plated. What Ted had recommended with shielding and anode to cathode spacing makes perfect sense. What is the anode to cathode spacing? What is the anode to cathode ratio? Is there cathode bar movement? How about solution agitation? If you will answer these questions we can better get an idea of how to help you. Please advise.