How we galvanise boards ?
Now on Eurocircuits.tv
In the production flow, galvanisation comes after the imaging with dry-resist of the copper image.
During the imaging process we put a negative image of the copper design to the panels. This way we can electroplate copper into the holes and onto the tracks. The aim of this process is to deposit at least 20 um of copper on the tracks and in the holes.
After the copper plating we plate a thin layer of tin over it. The tin serves as etch resist layer.
Before starting the galvanisation process we run a simulation to specify the required current density and cycle time to achieve a homogenous copper layer. The software we use for this simulation is called Smartplate and it is developed by Elsyca in Belgium.
Already in the PCB configuration stage, before placing an order, our customers can run a similar analysis of the plating on their individual boards.
This analysis is part of our PCB Visualizer set of tools and the results are available in the DFM section of PCB Checker. For every board we calculate a plating index, and show a graphical image that highlights areas on the board that risk to be underplated or overplated during the galvanisation process. The aim is to achieve a plating index as close as possible to 1, the score for a perfectly balanced copper distribution.
To start the galvanisation, we first fix the production panels to fly-bars . A set of maximum 4 panels run together on a fly-bar through the galvanic line. This fly-bar creates an electrical connection to the power supply. The production panels act as cathodes in the process. The production panels travel through different tanks, each with a different function, such as cleaner, micro-etch, copper and tin galvanisation, rinsing and drying.
Copper is electroplated onto the surface of the exposed copper of the production panel. We can deposit copper in the holes as well, thanks to the conductive carbon layer we built into the holes before the imaging process.
For copper galvanisation we can use two different power supplies:
- Direct current, called DC
- Pulse plating, called PP
These two methods have a different influence on the distribution of the copper over the panel. Let us look at it more in detail now.
Galvanising using DC rectifiers
We can influence the process with three different parameters:
- The copper-surface on the boards, both top and bottom surface
- The galvanisation time
- The current density in A/dm2.
The galvanisation time and the current density are variable. We have plating programs with different cycle times . The longer the galvanisation time and the lower the current density, the more equal the copper deposition on the boards will be.
On the other side, changing the cycle time between two different runs has a huge negative influence on the capacity.
With the DC- method a continuous current is switched ON between the anodes and the panels. So we have a constant current with ions moving always from the anodes towards the panels.
When the design of the boards on the panel has an unbalanced or uneven copper distribution, we will have higher copper deposition on the areas with less copper in the image and lower copper deposition on areas with more copper in the image.
To get a uniform copper deposition during the galvanisation we need panels with a plating index as close as possible to 1. The plating index is an indication of the copper distribution in the design, both on top and bottom side of the PCB.
Pulse Plating process
For pulse plating we can define 6 different steps within one galvanisation cycle and we can set reverse current. An example will make it clear.
- The total galvanization time, this is the total time the panels are in the copper bath is 100%.
- This 100% can be divided into 6 different timezones with a different length.
- So the first thing we have to define is the time of the different zones.
- Then inside the zones we can set the amplitude and time for the forward current and the amplitude and time for the reverse current.
- The accuracy is in milliseconds.
What happens exactly?
The panels (which are the cathodes) are in the copper bath and when the positive current is switched on we have copper deposition on the copper surface.
Because of the unbalanced copper distribution there are places where we have higher deposition, typically these are the places with less copper in the image. To compensate, inside the cycle periodically a negative current is switched on.
So for a very short time our panels are the anodes and the accumulated ions move away again from the panel.
With this technique we achieve a more equal copper distribution on the final product. On the other hand, the cost for such a pulse plating rectifier is 10 times more than for conventional DC rectifiers.
Our panels are now ready plated with DC or PP technology.
When the panels come out of the galvanic line we check the thickness of deposited copper of every technological panel. To get an idea of the average thickness of the plating in the holes, we use an eddy current sensor. This is a non-destructive measurement device.
For process control and to get more accurate and detailed information we make cross-sections. Cross-sections are a destructive inspection method that take more time ,but give deeper insight into the processes. In a next episode of this technology series on Eurocircuits TV, we will explain in detail how cross-sections are made and what information we can get from it.
Now we will compare the plating results between DC plating and PP plating on the same circuit.
We can see on the cross-section from the DC plated board that the deposition of the copper is not even. The thickness of the electroplated copper is quite high in the holes and on the surface of the tracks where the drawing is less dense and it is much lower on the places where we have full copper areas in the layout.
In the cross-section we made from a board galvanised with PP it is visible that the distribution of the galvanised copper is more homogenous. We need to get more experience with this PP technology to finetune the plating parameters, but we are confident that in the future we can guarantee better plating results for boards with lower plating indexes. The goal must however remain to design boards with an even copper distribution.