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June, 2019

Challenges in Printed Circuit Board Acid Copper Plating

By Akber Roy, CEO, Rush PCB, Inc.

of this method of plating PCBs is the poor throwing power of the electrolyte. This results in excessive plating times for achieving mini- mum copper thickness in areas with low cur- rent density, while high current density areas get too much copper deposit. In a PCB with through holes, the high


current density areas are at traces and at the edges around the holes and the low cur- rent density areas are at midway to the length of the hole. This not only leads to wasted copper, but also adds to the cost. On the surface of the PCB, there can be

over-plating on isolated traces when com- pared to larger ground planes, since the for- mer are areas with high current density, while the latter are areas of low current density. Within the hole, the throwing power of the electrolyte sets the challenge in achieving the minimum copper thickness re- quired at the center. The challenge increases lin- early with an increase of the aspect ratio of the hole, decreasing as the throwing power of the elec- trolyte increases In acid copper plating, the challenge is to

sing direct current during acid copper plating has been a staple method for PCB manufac- turers. However, one of the basic drawbacks

fecting the throughput of PCBs drastically. Rush PCB adopts a two-pronged approach to reduce the overall plating time, and to achieve a uniform dis-

specific duration cyclically, creating current flow in a square pulse waveform. Although the current flow is not a constant, the pulse plating technique may cause the plating current density in the on-pulse to increase to more than ten times what is achieved during regular DC current plat- ing. This high current density reduces the excessive potential of the metal ions, mak- ing the deposit a layer of fine grains. This leads to a more uniform deposition of cop- per as a detailed crystalline coating. The ratio between the on-time to the

Rush PCB uses pulse plating and reverse pulse plating to apply uniform copper coatings and thicknesses.

tribution of copper deposit. These two steps are as follows: using pulse plat-

ing and reverse pulse plating techniques rather than traditional direct current for acid plating; and using organic additives during acid copper plating.

achieve proper thickness distribution and surface uniformity without unduly compromising metal- lurgical properties, such as percent elongation and tensile strength of the deposit. Sometimes it is dif- ficult to achieve surface uniformity, because of the formation of nodules. Reducing the current density can equalize the

copper thickness to some extent, but leads to an in- ordinate increase in the overall plating time, af-

Pulse Plating Technique Rush PCB uses the technique of pulse plating

to control the electrodeposition of copper rather than a constant supply of current. Pulse plating relies on the time function to start and stop the current sup- ply, thus improving the physical and chemical prop- erties of the plating film. By modulating the current supply, the technique switches the current off for a

cycle time of the pulse train is its duty cycle. Therefore, a duty cycle of 100 percent essen- tially produces a continuous current as in the traditional DC current plating. The pulse plating technique uses a duty cycle al- ways below 100 percent, the actual value de- pending upon the characteristics of the PCB undergoing the plating. While for most PCB plating applica- tions the pulse plating technique is ade-

quate, more complex PCBs require an enhanced version of pulse plating. Rush PCB uses a reverse pulse plating technique in such cases. This im- proved technique uses an additional pulse in the cycle, with an opposing polarity. Therefore, each current cycle now consists of a forward pulse, a re- verse pulse, and an off period. The time period and amplitudes of the two pulses are individually ad- justable and an electronic circuit controls them. In most cases, a computer generates the necessary waveform. For proper plating thickness and uniformity, Rush uses an amplitude of the reverse pulse that

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