The schematic, material selection, and etching process comprise the beginning of board realization.
Lamination, drilling, and plating make up the final stages of bare board manufacturing.
Pick-and-place and soldering machines populate and bond components to the board.
How are PCBs made? For those designing for manufacturing, this is a constantly influential question on design rule parameters and general best practices. After all, design practices do not spring out of the blue - there is feedback between engineers, designers, testers, floor operators, suppliers, and more that live in the PCB ecosystem. To better serve customers and understand the hows and whys of circuit board design, an overview of the process of bringing a board from conception to finished product is provided below.
From Design to Etching: Initial Stages of PCB Design
Before manufacturing can begin, design documents and drawings must be rendered in accordance with the engineer and any manufacturer guidelines. For an entirely new board, design will begin from the schematic. It is here where all the interconnections are stored as well as the associations between symbols and land patterns in the parts library. When the schematic contains all the parts information, the design can be netlisted, which carries the information of the schematic to the board file, including populating the board with the aforementioned land patterns. Following the completion of the layout, the board, pending any changes on the manufacturing end or revisions by the engineer, is ready to proceed to initial production.
Material selection will chronologically occur prior to layout. Vector field calculations will determine trace widths and spacings for single and double-ended traces; these values will then be directly implemented in the board design rules. Material properties can also be used to tailor a design towards a particular emphasis, as in high-speed design. Different substrates will contain different average dielectric constants and loss tangent values, and even the weave itself can be manipulated to provide a more consistent dielectric constant across the trace’s transmission. Compared to a common 106 or even 1080 weave tightness, tighter weaves drastically reduce the gap between orthogonal threads, providing more uniform material properties.
In addition to the substrate, the thickness of the copper (or other conductors) foil will be under consideration. Most boards will use the standard 1 oz. plating, as it is easier to process and allows for a smaller gap between copper features. Thicker copper-clad laminates require more time in the acid baths, but the etch resist (which covers the areas where the designer wants to express copper features) only covers the top layer of the copper. As the acid etches its way down, it also begins to move in a plane normal to the top side. Assuming the layers are within registration tolerance and pass optical inspection, the board is ready to continue onto the next step of processing.
How Are PCBs Made?
At this point, the individual layers are now ready to be joined together through lamination. During lay-up, pins are used to fix the layers in position and ensure the stack remains in alignment prior to pressing. Pre-preg is applied between the layers of substrate and copper foil as an adhesive before applying heat and pressure to fuse the layers of the board together. After removing the press plates and alignment pins, the board proceeds to the drill machine. Using location data generated from the board file, varying hole sizes are drilled into the board by computer-operated bits.
Although it has been decided well in advance of this point, the aspect ratio, or the ratio of the circumference of a drilled hole to its depth, will play an integral role during plating. At the conclusion of the drilling step, the board is taken to an electroplating tank to create the conductive barrels for the plated through holes, including vias. Importantly, plating establishes the vertical conductivity path that links the traces and other copper features together between layers. With the plating process complete and the board fully formed, the board’s outer layers have etch resist applied and are acid-etched in the same manner as described above.
Finish Solder and Silk Layers
The major work on the bare board is done; what remains is the final steps that will finish the solder and silk layers on the top and bottom of the board before the final finish. The board undergoes a quality assurance and cleaning intermediary step following outer layer acid etching. Once verified, the board will have a solder mask and silkscreen layer applied sequentially. The solder mask image will be negative - that is, the indicated sections in the GERBER files will show where the solder mask is not to be present (and vice versa), and operators will strip the extraneous solder mask from the board before a baking oven cures the layer.
The silkscreen layer in comparison is far simpler: an inkjet will place any labels, graphics, reference numbers, or other pertinent information (company logo, assembly part number, etc.). The major role of the layout designer at this stage is to ensure the font and size are easily readable at a distance and emphasize important assembly information, such as pin 1 dots and polarity indicators. With the board fully detailed, it submits to the final step before assembly: finishing. Finishing indicates a protective layer over exposed copper that prevents oxidation and is primarily achieved with coinage metals or other similarly unreactive alloys.
Inspection Through Assembly: The Home Stretch
Visual Inspection and Testing
Finally, the bare board is prepped and ready for visual inspection and testing. Technicians will verify the physical connections of the schematic and board production files using continuity and isolation testing. Test point verification will be performed using a bed of nails machine or flying probe test, depending on the size of the production lot. As all work to this point has occurred on boards as part of a panel (a note for new designers: you’re paying for services by the panel, not by the board), routing needs to be performed to remove the boards from the panel prior to assembly. A router will remove the majority of the material along the perimeter of the board, but leave behind breakage points spaced along the edge of the design. This format ensures the boards remain affixed to the panel, yet can be removed without causing damage to the connections.
Although assembly occupies its own sphere of design in PCB manufacturing, the overall process is dwarfed in time and complexity by board fabrication. For surface mount components, a stencil is used to place solder paste on the pads in a precise amount and even layer to prevent manufacturing issues such as tombstoning. Next, a pick-and-place machine rapidly attaches components to the board based on the orientation and positional data generated during assembly file production. With the components in place, the board enters a reflow oven, which over the course of multiple heating and cooling cycles, liquifies and resolidifies the solder paste to form the solder joint, which provides electrical continuity from the component pin to the copper pad as well as fixed mechanical support.
Through-hole components, which may or may not be mutually exclusive to surface mount components on a board, require an alternative soldering method. While it may be possible to use short-pin through-hole components in a standard pick-and-place machine, manual insertion may be necessary. Thankfully, the relative size disparity in favor of through-hole to surface mount components means the task should be much less difficult and time-consuming.
After placement, the board either undergoes manual soldering or an automatic process known as wave soldering. Wave soldering, like the name suggests, drives a standing wave of molten solder across a side of the board, simultaneously soldering all through-hole pins in one pass. However, because of the indiscriminate nature of the solder wave, layouts with SMT components on the side of the through-hole pins are discouraged. The layout will have a huge impact on the viability of the design in terms of solderability and the total amount of processing during the assembly step. If at all possible, the layout designer can reduce costs by moving all components to a single side of the board, especially in the case of high-volume lots.
Learning how PCBs are made requires an understanding of the standard manufacturing process of an entire industry and its dizzying amount of exceptions and variations. If you are working on creating PCBs, Cadence offers a suite of PCB design and analysis software to assist users across all aspects of board development.
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