An Overview Of PCB Outer Layer Processing

In a previous two-part article, I described the PCB fabrication operations relative to inner layer processing, lamination, drilling and plating. The last step in the process is outer layer processing which is described below. For a detailed description of outer layer finishes; how different vias are formed and the steps involved in the multilayer build-up process, please refer to my previous blog. 

The Start of Outer Layer Processing

Once the desired plated copper thickness of a PCB has been achieved, it’s necessary to etch away the copper between the features in order to define the outer layer pattern. A concern arises as to how to remove the unwanted copper without destroying the integrity of the wanted copper. The answer is shown in Figure 1 which depicts the processing steps involved in finishing the outer layers of a PCB. 

Here, a different metal is plated on top of the copper that is to remain on the PCB after processing, in this instance, tin-lead, solder or a ROHS (reduction of hazardous substantives) compliant material. The tin-lead protects the copper pattern while allowing the unwanted copper to be etched away. Figure 2 is a typical line that is used for this purpose. The steps in this process include:

Stripping off the plating resist to expose the copper that is to be etched away.
Etching away the unwanted copper. Etching away the solder plate (Referred to as SES—strip/etch/strip).

At this point in the process, the PCB appears as shown in step 12 of Figure 1. A solder stop mask and legend or silkscreen are still required.

Figure 3 is a typical liquid solder mask application machine. The PCB panel is suspended vertically in the machine and a curtain of liquid solder resist is applied to each side. 

After the solder stop mask is applied, it is dried to the point that it is tack dried (dry to the touch). The soldermask is photosensitive. As soon as it is dried, it is placed in a photo printing machine, such as that shown in Figure 4.

Developing the solder mask is the next step in the process and that involves washing away the mask that is over the areas that are to be soldered to, or holes into which components such as connectors are to be installed. Because the traces and pads are copper, this process is referred to as SMOBC (solder stop mask over copper). 

After the solder mask is applied, the legend or silkscreen is applied using the same screening methods that are used to place a pattern on a T-shirt. Because this is a screening process, the size, location and accuracy of features that can be placed on a PCB with this process are limited. Care must be taken to ensure that letter sizes and line widths are within the limits of this process. In some instances, legends are created using a photosensitive material similar to liquid photoimagable solder resist.

Following the foregoing, the PCB is essentially finished except for the application of a protective coating for the exposed copper. This coating is necessary to prevent the corrosion of the component mounting pads and the exposed copper that will occur in ambient air in a very short time. The application of the outer layer finish will preserve the solderability of the board. Those finishes, and the pluses and minuses of each, which are described in detail in the earlier referenced blog noted above include:

• Hot air solder leveling
• Organic coatings or OSP such as Entec 106
• Plated tin
• Electroplated solder
• Electroplated gold over electroplated nickel
• Electroless nickel under immersion gold (ENIG)
• Electroless nickel, palladium, immersion gold (ENPIG)
• Electroless palladium, immersion goad (EPIG)
• Electroless silver
• Electroless tin

Following the application of the selected outer layer surface finish, the remaining step in the PCB fabrication process is depanelization. This operation is done with a machine that resembles a drill.
However, instead of a drill bit, the unit has a router bit much like that used to route wood parts. This bit travels around the perimeter of the PCB and cuts it from the panel. If there are to be any breakaway tabs, these will be formed at this stage in the process. At this point, the PCB is finished and all that remains is bare board testing.

Summary

The process of applying outer layer surface finishes is the last step in the PCB fabrication process. This process involves stripping off the etch resist; etching away unwanted copper, etching away the solder plate, applying a silkscreen or legend, and applying a protective coating to the component mounting pads and the exposed copper. Following the steps, outlined during the entire PCB fabrication process, will ensure that a manufactured board will perform as designed for the life of the product.

Have more questions? Call an expert at Altium or continue reading about planning your multilayer PCB stackup in Altium Designer®.

References:

1. Ritchey, Lee W. and Zasio, John J., “Right The First Time, A Practical Handbook on High-Speed PCB and System Design Volume 2.”