Rigid PCB

Design
Before the manufacturing process begins, the rigid PCB needs to be designed by a CAD engineer based on a working circuit schematic. Once the design process is complete, a set of documents is provided to the rigid PCB manufacturer. The documentation includes Gerber files, which consist of layer-by-layer configurations, drill-down files, pick-and-place data, and text comments. What's more, the documents should also provide processing instructions critical to manufacturing about the PCB specifications, dimensions, and tolerances.

Making preparations before the fabrication
Once the PCB manufacturer receives the files from the designer, they can begin creating the manufacturing process plan and the product packaging. The plan will be defined by listing such things as material type, surface finish, plating, work panel array, process route, etc. In addition, a set of physical artwork can be created with the film plotter. The artwork will include all layers of the rigid PCB as well as artwork for the solder mask and terminology markers.

Preparing materials
The PCB specifications stated by the designer determine the type of material, core thickness, and copper weight required in the material preparation process. Single-sided and double-sided rigid PCBs do not require any inner layer handling and go directly to the drilling process. If the rigid PCB is multi-layered, a similar material preparation will be performed, but it will be carried out in the form of an inner layer which is usually much thinner and can be stacked to a predetermined final thickness.

Drilling
The drilling process is performed by a CNC-controlled multi-station drilling machine. The drilling process creates a clean, smooth hole wall that does not damage the inner layer, but the drilling can provide a pathway for inner layer connectivity after plating and the non-through hole eventually becomes the site of the through-hole assembly. Non-plated holes are typically drilled as an auxiliary operation.

Copper Plating
Electroplating is widely used in rigid PCB production where through-hole plating is required. The objective is to deposit a layer of copper on a conductive substrate through a series of chemical treatments, and then increase the thickness of the copper layer to a specific design thickness, typically 1 mil or greater, through subsequent plating methods.

Outer Layer Treatment
Both sides of the top and bottom layers are coated with photoresist. The two sides are aligned using the outer layer artwork and tooling holes, and then each side is exposed to UV light to detail the optical negative pattern of traces and features. The UV light falling on the photoresist bonds the chemicals to the copper surface, and the remaining unexposed chemicals are removed in a development bath. The next step is to remove the bare copper through an etching process. This leaves copper traces hidden beneath the photoresist layer. The resist is then peeled off, leaving traces and features on the outer layer. Automatic optical inspection can be used to find defects in the outer layer before the solder mask.

Solder mask
The application of the solder mask is similar to the outer layer processes. The main difference is that a photo-imageable mask is used over the entire surface of the production panel instead of a photoresist. The artwork is then used to take images on the top and bottom layers. After exposure, the mask is peeled off in the imaging area. The goal is to expose only the area where the components will be placed and soldered. The mask also limits the surface finish of the rigid PCB to the exposed areas.

Surface Finish
There are several options for the final surface finish, such as sink gold, sink silver, OSP, lead-free solder, leaded solder, etc. All of these methods are effective, but making a choice is based on the design requirements. Gold and silver are applied by plating, while lead-free and leaded solder is applied horizontally by hot air solder.

Marking
Most rigid PCBs have markings on their surface. These markings can prompt reference marks and polarity marks during the assembly. Other markings can be as simple as part number identification or manufacturing date codes.

Depanelization
Rigid PCBs are produced in complete production panels and need to be moved out of their manufacturing profile. Most rigid PCBs are set up in arrays to improve assembly efficiency.

Testing
Rigid PCB manufacturers typically use a flying probe or bed-of-nails testing process. The test method is determined by the number of products or available equipment.