If you're interested in pursuing in-house PCB prototyping and testing, there are several routes you can take to create functional prototypes for new products. The best one depends on a number of factors, with product complexity being the primary consideration. For highly complex, low-volume products, using an additive manufacturing system is often the best choice for rapid prototyping thanks to its high throughput and fixed cost structure.
In-House PCB Prototyping Over Time
Although PCBs have been around since the early 20th century, their manufacturing and in-house prototyping methods were not always standardized. In 1936, the first PCB was used to support an electronic system, and the prototyping process was not significantly different from the manufacturing process in those early days.
As fabrication and electronic component technology has progressed over time, so have the available options for in-house PCB prototyping. The story of in-house PCB prototyping methods can be traced back to the 1960s when electronics were making the transition from vacuum tubes to transistors. At this point, a large metal chassis was needed to build electronic circuits and the planar geometry of integrated circuits made a planar geometry for electronic circuits the natural choice.
As more electronic devices were being built as integrated circuits with transistors, boards were built on plywood workbenches using breadboarding. The top layer of the plywood sheet was replaced with a material called Bakelite, which has a standard thickness of 1/16’' or 1.57 mm. This labor-intense process was done manually and all components were wired using standoffs. Eventually, engineers started gluing sheets of copper foil on top of the sheet of Bakelite, allowing wires to be etched between components.
Advancements in plating and etching processes allowed full-scale production of PCBs with through-hole and/or SMD components by the 1980s and similar two-layer prototyping boards (protoboards) were available to accommodate these components. Sometimes called perfboards, these boards included pre-drilled holes in a regular grid with plated copper for soldering. Similar prototyping boards included copper pads on the surface layer, allowing prototyping with SMD components.
Today, designers have access to evaluation boards for specific components and microcontroller development boards. These prepackaged options can be used to create basic prototypes and experiment with different features but finished products are unlikely to resemble a prototype created from these boards. The adventurous designer can even etch their own boards, effectively mimicking traditional PCB manufacturing processes.
Traditional In-House PCB Prototyping Options
Today, there are several traditional options for in-house PCB prototyping, each with benefits and drawbacks to consider.
Protoboards. Protoboards are still used for some less-complex circuits that run at lower speeds and frequencies. These simple, two-layer boards are normally used to mount through-hole components in plated holes on each surface of the board. The components can then be soldered to these plated holes. These boards can also be cut down to size with a hand tool and placed in an enclosure. While protoboards are fine for DC devices and low-speed (such as sub-MHz) devices, they suffer serious signal integrity problems at higher speeds. First, the lack of ground planes and printed traces causes circuits to have large loop inductance, leading to strong crosstalk and susceptibility to EMI at high frequencies. These prototypes will have little resemblance to a finished product and are best used to build a proof-of-concept.
Development Boards. Today, a micro-controller board like an Arduino is an excellent choice for in-house PCB prototyping for many applications, ranging from industrial or environmental embedded systems to IoT devices. Prototypes for applications that require more powerful computing capabilities might best be built on top of a Raspberry Pi or BeagleBone board, as these boards include the connectivity required to interface with other devices or a PCB. Development boards carry a low price tag and are reusable. They also allow a designer to focus on functionality rather than becoming mired in the finer points of PCB design; however, they don't allow a designer to integrate more components on top of the existing development board unless you purchase an add-on board or you build your own breakout board. Without an add-on board, you are limited to the functionality provided by the components you see on a development board.
Evaluation Boards. Many component manufacturers will release evaluation boards that are designed to mount to a specific component for a specific application. Some example components are high-speed FPGAs and high-frequency transceivers. Evaluation boards are very useful for interfacing with a small number of other components as part of the in-house PCB prototyping process. The useful aspect of an evaluation board is that the board is optimized to ensure signal integrity, allowing a designer to focus on designing functionality. Evaluation boards suffer the same drawbacks as development boards. Your functionality is limited to what you see on the board and you are unable to integrate additional features and functions without incorporating an additional board. With more advanced components, you risk introducing signal integrity problems when integrating additional boards, as evaluation boards are normally designed for measurement, rather than prototyping.
Manual Etching. Some designers are known to mimic etching in the traditional PCB manufacturing process using some common household chemicals. This starts with a CEM laminate that is covered with a copper foil. Regions of the board to be etched can then be traced within a handmade mask. Once the etchant is washed with a solvent (usually isopropyl alcohol), what remains is a functional two-layer board with copper traces and planes. Instead of a chemical etchant, a CNC mill can be used to cut away the copper foil, leaving behind traces and conductive planes on a two-layer board.
Manual etching can produce rough traces and planes that can create signal integrity problems and increase losses at high frequencies. The resolution of traces and between traces is also limited by the size of the drill bit. As such, these boards tend to be larger than the same system on a multilayer PCB and designers will not be able to experiment with a unique interconnect architecture or non-planar PCB substrates. With manual etching or CNC milling, you will only be able to create two-layer boards with limited trace size, you won't have plated vias, and the resulting structures may create high-speed/high-frequency signal integrity problems that are difficult to solve. Furthermore, these boards will bear less resemblance to a mass-manufactured PCB in an advanced electronics system.
Outsourced Rapid Prototyping. Outsourcing has always been an option in manufacturing and PCB prototyping is no different. If you are not in the business of pursuing in-house PCB prototyping, you can outsource your board to a traditional manufacturer for rapid prototyping. As long as you design your board within the constraints of traditional PCB manufacturing processes, you can rest assured that the boards you receive will be functional.
The downside to outsourcing is that not all rapid prototyping houses will provide a single board. Typically, there is a minimum panel order that must be satisfied. The ability to quickly create a single prototype is critical for advanced electronics. Some examples are boards that include embedded components, unique printed antenna arrays, non-orthogonal via and interconnect architecture, and non-planar boards. You can also expect longer lead times with outsourced electronic prototypes. Some fabrication houses may accommodate as fast as a 48-hour turnaround time, not including shipping. You could be looking at a week to a few-week turnaround time from a larger manufacturer, depending on the complexity of the board. In addition, you could be risking confidentiality and intellectual property security by sending your design to a third party.
The Move to More Advanced In-House PCB Prototyping Methods
While the above methods may seem more familiar or convenient, the fact remains that they work for simpler prototypes and more advanced prototypes on planar boards. If your goal is to create a finished product that takes advantage of prepackaged components, only has two planar layers, or runs at very low speeds/frequencies, then prototyping in these ways is perfectly reasonable, as your finished product will more closely resemble your prototype.
For more complex products that require greater customization, run at high speeds, and incorporate unique functionality, using these methods quickly becomes insufficient from a functionality and signal integrity perspective. Working with traditional manufacturing processes carries high lead times and costs, especially with more complex PCBs. This is where better options are needed to create fully functional prototypes of highly complex devices.
3D Printing Methods for In-House PCB Prototyping
Newer electronics devices have become progressively more complex in terms of architecture and functionality and the level of complexity is only expected to increase as PCB form factors and functionality demands continue to mount. This includes the shape of the board itself, spawning rigid-flex and multilayer PCBs with complex interconnect architecture. The aforementioned in-house PCB prototyping methods are appropriate for simple devices but they don't reflect the form factor of more complex electronics, and they are prone to signal integrity problems.
Even as these prototyping methods remained popular, 3D printing systems were being developed for mechanical prototypes and finished products. The original process is still known as stereolithography (SLA), which is a photochemical process involving light-induced cross-linking reactions in monomers. This stereolithography process is still used to form plastic products.
Other deposition processes are based on extruding a heated filament of plastic or soft metal through an aperture in a print head, and the print head was moved in a two-dimensional plane, known as fused-filament deposition (FFD) method and the related fused deposition modeling (FDM) method. The movement of the print head follows the structure of the part being fabricated, yielding a completed mechanical component as the filament solidifies. These components require some level of post-processing, such as polishing and sanding, to create a usable finished product.
Related methods, like selective laser sintering (SLS) and powder-bed fusion (PBF), are useful for 3D printing of metal products and these processes are widely used to fabricate complex parts for aircraft engines. This ability to directly fabricate a finished component in a layer-by-layer printing process reduces the use of fasteners to join multiple parts, eliminates assembly steps, and almost completely eliminates material waste. These simplified parts tend to have higher mechanical strength as stress does not concentrate at fastener holes and welds in mechanical products.
The technology used in 3D printing has evolved significantly in recent years. As more materials have become available and new systems have been perfected and adapted for 3D printing conductors on planar substrates, these additive manufacturing processes can now be used for in-house PCB prototyping.
When it comes to PCBs, inkjet printing and aerosol jetting are two prominent methods for 3D printing a substrate and conductors simultaneously. While some of the methods mentioned earlier could be adapted for in-house PCB prototyping, they are limited in that they cannot be used for co-deposition of a substrate and conductors. Advanced systems have been adapted to use one or more of the aforementioned processes to 3D print a functional PCB directly on planar or non-planar substrates.
Different systems are adapted to a specific set of materials and processes, meaning not all PCB designs are usable with every 3D printer. There are some important design guidelines that should be considered when designing your prototype for 3D printing. Although this requires some additional up-front design work, the benefits are worth the effort, especially for smaller companies that cannot keep a replica of the traditional PCB fabrication process in-house. As long as you conform to the design guidelines for your additive manufacturing system, you can fabricate a more complex product than with traditional planar processes.
Advantages of 3D Printing for In-House PCB Prototyping
Compared to traditional PCB prototyping methods, using an additive manufacturing system provides a number of advantages in terms of cost, time, and innovation.
Reduced lead time and cost structure. The lead time associated with a prototype, and the costs for producing a prototype, only depend on the weight of the materials used in the prototype. Lead time and cost are independent of a product's complexity. This is not the case with traditional manufacturing processes and prototyping, where the costs and development time increase with product complexity.
Greater design freedom. When traditional manufacturing or in-house prototyping processes are used, the design of a product is limited by the processes themselves. Using an additive manufacturing process, especially an inkjet 3D printing process, provides much greater design freedom, allowing designers to experiment with unique architecture, board shapes, and component embedding.
Intellectual property security. Keeping your prototyping capabilities in-house eliminates the opportunity for an external party to steal your design data.
Hastened R&D cycles. Using an additive manufacturing system for in-house PCB prototyping allows an R&D team to produce a single prototype and immediately test it. This eliminates the lead time associated with traditional processes and allows a design team to quickly implement changes to their design.
New applications. Because additive processes can be carried out for nearly any level of complexity, your finished prototype will closely resemble a finished product. Your testing results will closely match the behavior of your device in the field.
Designing for In-House PCB Prototyping
While different processes and systems have different capabilities, any PCB designed for in-house PCB prototyping with additive manufacturing should consider the following design guidelines:
Printing resolution. This will determine the minimum feature size that can be printed in a PCB. This limits the layer thickness, trace thickness, trace spacing, and via size to some minimum value.
Interconnect architecture. Depending on the exact process used for printing, you may still be limited to an orthogonal architecture. As an example, FDM of conductors directly on FR4 limits designers to vertical vias and horizontal traces. Alternatively, using an inkjet process allows direct fabrication of diagonal or curved vias and interconnects in your device.
Material selection. Different materials are more suited to different applications. As an example, FR4 is not the best material for working at GHz frequencies, which motivated the development of more advanced PTFE-substrates. In additive manufacturing for in-house prototyping, some polymer materials are ideal for 3D printing PCBs for use at high frequencies.
Substrate shape. Not all printers for in-house PCB prototyping can accommodate non-planar PCBs. In some cases, you may need to include structural support for your board as you print, just as is done with mechanical parts.
Generation of printing instructions. Printing instructions need to be generated for complex PCBs, just like with other products. You can quickly generate printing instructions for your device when the manufacturer of your additive manufacturing system provides the right software tools.
Following these design guidelines, electronics developers can realize a significantly streamlined prototyping process when using in-house additive manufacturing technology.
Going Beyond In-House PCB Prototyping
With the right additive manufacturing system, you can go beyond in-house PCB prototyping and incorporate additive manufacturing into a lights-out digital manufacturing process for high-complexity, low-volume products. Additive manufacturing systems are inherently digital and can be incorporated into newer digital manufacturing processes, either as a complement to existing processes or as a replacement for obsolete processes.
This article was contributed by Nano Dimension, Santa Clara, CA. For more information, visit here .