How are leveling and Throwing Power Related?

A. Hi Anna,
It's hard to know exactly what you did when we don't know what grade you are in, or anything beyond the fact that you called the two things you did 'electroless plating' and 'electroplating' ...
... but yes, electroless plating will give complete coverage more 'easily' because, with no external electricity required, all points on the surface are essentially equally exposed to the available nickel ions and the reducer which converts them to nickel atoms -- so quite even deposition is expected. When electroplating there will invariably be points on the surface which are exposed to higher and lower current densities and will thus be expected to get more or less plating.
Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Hi Tony. As you no doubt realize, a plating experiment for high school chemistry is easy, but good industrial electroplating is very hard. One of the differences is that industrial plating must be bright, must fully cover, and must be of consistent thickness. To get those properties, you don't merely need to move electrons from anode to cathode and let the ions follow willy-nilly, but you must carefully engineer the transport :-)

Micro and macro throwing power are not actually closely related, as we can see by contrasting bright nickel plating (which is exceptionally good at leveling /micro throwing -- so good that roughly polished steel will brightly shine after nickel plating -- but very poor at macro throwing power) to cyanide-based plating solutions (which have very good macro throwing power but are not as good at leveling and brightness).

Remember that what drives the metal to deposit is that electrons on the surface of the cathode reduce a positively charged ion in solution to an atom of metal, but that electricity tries to follow the path of least resistance. As a result of this a couple of undesirable results happen in plating that you are battling against. First, the plating "wants' to deposit like the pins in a pin cushion because the atoms deposited first are asperities that attract more electricity. Micro throwing power is about using organic additives that are attracted to these asperities and shield them so that new adatoms form on the "cushion" rather than on the "pins" (I don't know of plating solutions with good leveling power but without organic additives).

At the same time, from a larger scale view, the ions want to flow to edges and protruding corners, and not travel the extra distance to internal corners because the solution resistance to those areas is greater. Macro throwing power is about shielding the higher current areas with a brightener, and/or otherwise making it difficult for excess plating to occur in those high current density areas. Picture why cyanide solutions have good macro throwing power: the metal ions are so tightly complexed that there are few free metal ions available at any one time and one place ... so once one plates out you don't have another to immediately available to take its place, and it can be "faster and easier" for a metal ion in a low current density area to deposit than for another free ion to migrate through the boundary layer in the high current density area.

In more recent times a third type of brightener was developed to help with through-hole plating of circuit boards. The brightener molecules are so big that they don't fit into the holes; so they shield everything else but the holes.

These explanations tend to be pictorial and illustrative rather than detaied science, so forgive me that they are simplifications of complex stuff. You might contact ASM International about buying a copy of their Course 22, "Electroplating", edited by Charles Faust. It has several pages of in-depth discussion about throwing power, including the math and the test equipment for quantifying it. Good luck.

A. Hi Luke. We appended your inquiry to a thread where I explained my understanding of micro vs. macro throwing power. It seems to me that micro throwing power and PEG would be important for trench plating though, and the macro throwing not very relevant. The large-molecule brighteners used in through-hole plating are probably applicable to trench plating, although I would expect that areas outside of the trench, which you don't want plated, would probably be masked.

Regards,