The Reverse Pulse Rectifier
Since many years rectifiers are used to generate DC current for electrochemical deposition of copper on printed circuit boards.
With increasing complexity, thickness, track density and decreasing hole diameters and track width, the conventional DC plating process turned out to be more and more inefficient or resulted in bad quality.
Forced to lower current density for equal copper thickness over the surface and especially in the holes and micro vias, process duration appeared to be prolonged to unacceptable long plating times. With the arrival of the reverse pulse rectifier and a chemistry adapted to this process much shorter plating times can be achieved.
Talking about the rectifier’s output we always use the unit of current.
For plating purposes, the voltage is not so important, as long as this voltage is high enough to support the current we want. This is because the current multiplied by the time is directly proportional to the amount of deposited copper.
Many years ago this was found out for us by Faraday.
In general a rectifier is an electronic device to convert AC into DC, with or without galvanic isolation from the input, with or without a controller to stabilise the output voltage and/or output current. This isolation between input and output can be combined with a voltage level change and that will be accomplished by a low frequency (50/60Hz) power transformer. Control can be realised by using bulky linear dissipative regulation or much better thyristor regulation. The third possibility is using a secondary switched pulse width modulated switch mode power supply. Mentioning these possibilities it is not my intention to list them all. There are mixed versions and magnetic control methods, but the above mentioned ones are (were) the most widely used types. State of the art today however is to use a pulse width modulated direct off line (no 50/60Hz transformer) switch mode power supply, which works with a high conversion frequency (e.g. 100kHz). This means a relatively small and light weight unit, because a 100kHz transformer of the same power rating as a 50Hz type, is orders of magnitude smaller and have much less weight.
The Pulse Rectifier
A pulse rectifier converts AC into a DC pulse train with a rectangular shape. The ratio between the DC presence and the DC presence plus pause, is called the duty cycle [%]. This duty cycle may be constant or may vary depending on the process where the rectifier is used for. See figure 1. At a duty cycle of 100% you will have a continues DC output current.This on/off switching of the DC output can be done by a separate electronic switch and can in that case easily be combined with all above mentioned topologies. In case of the direct off line switch mode power supply, this on/off switching can be simply integrated in the control circuitry by periodically inhibiting the pulse generator of the pulse width regulator, and no extra power switch is necessary. Important is also that efficiency will not be negatively affected by this on/off switching and even more important is that no expensive extra power components, which require heat sinking, are necessary.
The Reverse Pulse Rectifier
The reverse pulse rectifier is a relatively new product. The difference compared to the pulse rectifier is, that beside the normal forward pulse, also a reversed polarity pulse is generated.
This can be achieved by reversing the normal forward pulse with an electronic H bridge circuit. The disadvantage of this topology is that the amplitudes of forward and reverse pulse are equal and cannot be adjusted individually. See figure 2.
For some plating applications this will be sufficient. In the more advanced reverse pulse rectifier like the products of DRPP, forward pulse amplitude, as well as the reverse pulse amplitude, will be individually controlled and adjusted. The latter is more or less claimed by the today’s chemical process, developed for pulse plating application in the printed circuit board industry, where the reverse pulse boasts an amplitude of maximum the threefold of the forward pulse amplitude. Fortunately, for power requirement’s sake, the reverse pulse duration is 2,5 to 10% maximum of the forward pulse duration. A very commonly used value is 5%.
See figure 3.
Plating complex boards with a wide variety of hole diameters and other special properties, it would be nice to have the possibility to change the amplitudes as well as the time adjustments during the plating process. This is necessary to be able to take full advantage of the best performance of the process, as these parameters will change with certain board properties. In that case you can work with a batch program, where during the process one or more times amplitudes and time adjustments will be changed. This feature is also available in the reverse pulse plating rectifiers of DRPP.