What is a PCB?

A PCB or Printed Circuit Board is an electronic assembly used in almost every modern electronic device. It is a multi-layered component that uses copper tracing to create communication pathways. Through these pathways, electricity travels to the various components that are mounted onto the surface of the PCB. 

How is a PCB Manufactured?

Generally, PCB manufacturing varies greatly based on the type of PCB and manufacturing techniques. PCBs typically start out as a copper-clad laminate, which is then fused to an insulating substrate made of fiberglass-reinforced epoxy, commonly known as FR-4. The PCB design and pathways are created through specialized PCB design software, and then the design is generated and used to guide the manufacturing process. It is then dipped in a chemical bath which etches away the copper in the design not covered by the stencil so that when the stencil is removed the copper conductive traces remain.

After many processing and inspection steps,  electronic components like resistors and capacitors are soldered to the board. To create functional circuits, the leads are connected to the conductive traces. Then, a silkscreen layer is printed on top of the board, which contains text and symbols that act as identifiers for the different components.

Finally, the sheet is covered in a solder mask, protecting the board from corrosion and debris. This mask is what gives the PCB its characteristic green color, however, solder masks also come in colors like blue or black. Most importantly, solder masks are what provide the board with electrical insulation.

What is a Flexible PCB?

A category of PCBs is a flex PCB. As the name suggests, this printed circuit board is made with flexible materials, opening up more opportunities for use in constrained areas where flexibility is required.  Like a rigid PCB, the flex PCB contains copper tracing communications pathways. The main difference is that instead of a rigid fiberglass-reinforced epoxy substrate, flex PCBs are made with a non-conductive polymer substrate like polyimide or polyester.

Manufacturing a PCB with these materials makes it bendable and light, which can be extremely beneficial in applications where there are space constraints or weight requirements.

In some cases, rigid and flex PCBs are combined to create a rigid-flex PCB. This is a hybrid circuit board that combines both technologies to create a customizable PCB, which can be very fitting to specific applications where rigidity but also flexibility is required. 

PCB Assembly Process Flow

PCB assembly is a manufacturing process that involves attaching electronic components to a printed circuit board (PCB). It typically begins with a bare PCB and progresses through several stages including component placement, soldering, and inspection to create a functional electronic product.

The process starts with thoroughly inspecting the bare PCB to ensure it meets the design specifications and is free of defects. On the PCB board, a stencil with openings corresponding to SMD (surface mount device component) component pads is placed. Solder paste, a viscous mixture containing solder particles and flux, is applied through the stencil onto the designated pads where SMD components will be placed. After applying the solder paste, the solder paste on the PCB board will then be inspected using the solder paste inspection (SPI) machines.

Automated machines precisely pick up surface mount components (SMDs) and place them onto the PCB pads with the solder paste. The PCB is conveyed through a reflow oven. Controlled heating melts the solder paste, creating strong electrical and mechanical connections between the components and the PCB pads. The assembled PCB is carefully flipped over to access the second side and the process is repeated from the solder paste application using a stencil, component placement and the PCB undergoes the reflow soldering process again to create solder joints on the second side. 

After reflow, the PCB is cleaned to remove any residual flux, a cleaning agent used in the soldering process. For some designs, components with leads are inserted through designated holes in the PCB. For through-hole components, the PCB is passed over a wave of molten solder, creating solder joints on both sides of the board. It's also common to add individual components after the main assembly process using conventional hand soldering. The assembled PCB then undergoes a rigorous visual inspection and automated optical inspection (AOI) to detect any defects including misalignment, missing components, and solder issues. The final stage involves electrical testing to ensure the assembled PCB functions as designed.

The evolution from lead-containing Sn63Pb37 wire to Pb-free micron-sized solder spheres

Semiconductors and printed circuit board technology are relentlessly evolving industries. Their constant development has forced solder technolgy to change and evolve too. Solder technology, as a whole, has moved from lead-based solder wire and separately applied solder flux to solder spheres. The latest industry break-through being a move away from wire bonding to micron size solder spheres for flipchips. To understand how this happened let\'s start by breaking down the product name Sn63Pb37 into its parts:

1. Sn = Tin
1. 63 = 63%
1. Pb = Lead
1. 37 = 37%

Alloys melt at different temperatures based on their composition or ratio of metals. Sn63Pb37 solder liquifies at exactly 183°C. Because the liquidus phase is so perfectly defined, Sn63Pb37 is known as a eutectic solder. This is very useful for two reasons. The first is that there is no "slurry phase" as some of the metals melt and others have not yet melted. The second is that the 183°C temperature is incredibly useful for circuit boards because it was high enough to melt the solder but low enough not to damage plastic components or the circuit board itself.

Solder wire needs flux. Flux removes oxidation.

Without flux the layer of oxidation acts as a barrier between the solder and the two conductive contact points thus preventing a clean and solid solder joint.

Pre-coating parts in flux was, therefore, an essential extra step that cost more time and labor. So the next step in solder development was hollow solder wire filled with an internal flux. Further developments included multi-core wire with multiple hollows for better distribution of flux.

The industrial mass-produced Printed Circuit Board (PCB), as we know it, started out with through-hole technology. Semiconductor devices and chips had pins which were pushed through holes in the printed circuit board. The bottom side of the entire circuit board would then have flux applied in one go and then be dipped into a solder bath. This automated the soldering process and eliminated unnecessary extra steps. Which, in terms of mass production, saved millions of dollars in energy and material.

Evolving ICs – Pushing soldering technology to evolve

After through-hole technology PCBs evolved towards surface mounted technology: where solder paste was used. Solder paste is a suspension of powdered flux and powdered solder together. This allowed solder to be printed on circuit boards by a solder paste printer and then melted in one go in a solder reflow process: thereby allowing for much more complex and densely packed installations of semiconductors on the PCB.

Integrated circuits have been getting smaller and smaller and with larger and larger numbers of tiny transistors. This can be best seen in the evolution of Small Outline Integrated Circuits or SOIC: evolving at first by doubling the number of inputs and outputs or pinouts: from SO8, on to SO16, SO32. Eventually for the sake efficiency, instead of two rows of pinouts, all four sides would be used in the Quad Flat Pack (QFP). This also meant that the legs got thinner and thinner and got packed closer and closer together.

Correlation between solder ball size and BGApitch

Solder ball diameter (mm)	Variation	Pitch size
0.75	0.90 - 0.65	1.50, 1.27
0.60	0.70 -0.50	1.0
0.50	0.55 - 0.40	1.00, 0.80
0.45	0.50 - 0.40	1.00, 0.80, 0.75
0.40	0.45 - 0.35	0.80, 0.75, 0.65
0.30	0.35 - 0.25	0.80, 0.75, 0.65, 0.50
0.25	0.28 - 0.22	0.40
0.20	0.22 - 0.18	0.30
0.15	0.17 - 0.13	0.25

How to Protect a PCB with Conformal Coatings

An easy way to protect a PCB and increase its longevity is to apply a Conformal Coating. A conformal coating is a thin film specifically designed to safeguard electronic assemblies. It can protect from dust, corrosion, and moisture, by creating a coating that envelopes the PCB and all its components. Conformal coatings also insulate the PCB, protecting it from electrical failures.

There are different types of conformal coatings based on requirements, including acrylic (Loctite Stycast PC 62), polyurethane (Loctite Stycast PC 18M), silicone (Loctite SI 5293), uv-based (Loctite Stycast PC 40-UMF) and rubber based. 

How to Protect a PCB with Aculon's Electronic Grade Coatings

Another protection method for electronic components and circuitry are 3M Novec Alternatives, or Electronic Grade Coatings. Aculon's Nanoproof™ 3 Series are specialized coatings designed to replace the now discontinued 3M Novec coating line, which protects electronic components like PCBs from environmental factors such as humidity, heat, and corrosion. They are used in various industries to ensure the longevity and reliability of electronic devices by providing a protective barrier against moisture, dust, and other contaminants that can interfere with their functionality.

Electronic grade coatings are typically applied as a thin layer over electronic components or circuitry to provide a barrier against environmental contaminants, moisture, and corrosive agents. These coatings can be applied through various methods, including spray, dip, or brush application, and can be cured through heat or UV light.

The target thickness of the coatings is based on fluoroacrylate % and varies from as low as <1 μm (Nanoproof 3-001X) up to 6 μm (Nanoproof 3-10). 

 

Nanoproof Alternative	3M Novec Product
Nanoproof 3-001X	Novec 1720
Nanoproof 3-002	Novec 1702
Nanoproof 3-02	Novec 1700, 1902
Nanoproof 3-04	Novec 1904, 2704
Nanoproof 3-08	Novec 1908, 2708
Nanoproof 3-10	Novec 1710
3 Series Extender	Novec 7100

 

How to Waterproof a PCB with Aculon's PCB Waterproofing Treatments

Certain applications may require waterproofing, such as underwater equipment or electronics like smartphones and watches. In these cases, nanocoatings are a great way to achieve various levels of waterproofing. Aculon's PCB Waterproofing Treatments provide an extremely thin coating that adds hydrophobicity without altering the surface of the PCB. The goal of these treatments is to protect the PCB from short circuits and other damage caused by moisture, which can cause corrosion and degrade the performance of the circuit over time. 

Aculon's NanoProof™ Series offers customers a range of PCB waterproofing solutions from protecting against accidental water damage to IPX7, immersion in water at one-meter depth for 30 minutes, to greater barrier properties that can withstand 100 hours of immersion in sweat solutions and some of the most stringent test methods developed for non-hermetic components. Some Waterproofing Treatments Caplinq offers include:

• Nanoproof 1.0 - IPX4, best for a broad range of substrates
• NanoProof 2.1 - best for PCBs and electronic components

Treatments depend on various factors including substrate, hydrophobicity or oleophobicity level, coating thickness, etc.