Using vias in a PCB design allows the designer to shorten the distance that a trace must be routed in order to complete its connection. Vias are metallic lined holes connected to the metal circuitry of a PCB that conduct an electrical signal between the different layers of the board. Although vias can vary in their size, pad shapes, and hole diameters, there is only a handful of different via types or structures:
Thru-hole via: This is the type of via that is used most often in a circuit board. The holes are drilled all the way through the board with a mechanical drill bit and can get down to 6 mils in size.
Buried via: This via only connects internal layers of the board and is useful for PCBs with very dense routing.
Blind via: This via starts on either the top or bottom of the board but doesn’t go all the way through it.
Microvia: For hole sizes smaller than 6 mils, a laser-drilled microvia is used. These vias connect only two adjacent layers of the board and can be on the surface or buried within the board layer stackup. Microvias are extremely versatile and can be stacked together, or on top of a buried via, but have a higher fabrication cost associated with them.
Via-in-pad (VIP): These vias can either be standard thru-hole vias or microvias, but their position in a surface mount pad makes them unique. If a standard mechanical drill is used, the via will require extra fabrication steps to prevent solder on the pad from flowing down through the hole. Microvias on the other hand don’t have this problem, but they can be more difficult to fabricate due to the tighter trace and space tolerances in a high-density design.
Ultimately, which type of via to use depends on the technology of the printed circuit board, the circuitry needs, and the targeted cost of PCB fabrication. For instance, a microvia is very desirable to use because of its smaller size, but that doesn’t necessarily make it the best choice. A microvia has more steps involved in its fabrication and, therefore, is more expensive compared to a mechanically drilled thru-hole via. But if you are designing a high-density interconnect board, the microvia becomes the better choice. Here is a breakdown of the common uses of vias in PCB design:
Signal routing: Most circuit boards will use a thru-hole via for signal routing placed on a grid. Denser boards, however, may also use blind or buried vias, while very dense boards will need microvias.
Escape routing: Larger surface mount (SMT) components can usually have their escape, or fanout, routing done with thru-hole vias. In some cases, blind vias or microvias will be used, and on very dense packages such as high pin-count BGAs, a via-in-pad will be used.
Power routing: Since the vias used for power and ground nets will conduct more current, they are usually restricted to larger thru-hole vias, although blind vias may be used as well.
Stitching vias: These vias are used to provide multiple connections to a plane and are, therefore, thru-hole or blind vias. For example, a sensitive area of circuitry may be surrounded by a strip of metal with vias stitched in it to connect to a ground plane for EMI protection.
Thermal vias: In this case, a via is used to conduct heat from a component out through the internal plane layer that it is connected to. This usually requires a larger thru-hole or blind via and these vias are often in the pads of these devices as well.
Deciding on the best testing method for your printed circuit board can be a daunting task. There are plenty of factors to take into consideration, including costs, coverage, and development lead time. However, there are two popular test strategies you’ll often find yourself choosing between: ICT testing vs flying probe testing.
Which of the two is a better automation test solution to fully inspect whether your specific product will perform well in the market? Would it be wise to use a combination of both to achieve the desired test coverage? How complex is your design, and would it be better to your PCB manufacturer who can offer an array of test options beyond just these two?
All these are questions you should ask yourself. Discuss your options with your PCB manufacturer, too. Each option has its strengths and weaknesses, and your PCB manufacturer can help you settle on the best one by analyzing each test’s:
Up-front and per-unit costs
Scope of coverage
Development lead time
Customization
Ability to adapt to varied designs
CONTRASTING ICT TESTING VS FLYING PROBE TESTING
In-circuit testing (ICT) and flying probe testing (FPT) offer similar coverage in their tests, discovering most manufacturing defects that often occur in printed circuit boards (PCBs). These include:
Opens
Shorts
Resistance
Capacitance
Component orientation
The two, however, differ on their:
Test time periods
Per-unit costs
Custom tooling
Non-recurring engineering charges
Digital logic testing
IN-CIRCUIT TESTING
ICT is a powerful tool for PCB testing. It uses bed of nails in-circuit test gear to access the circuit nodes of a board and check the performance of each component. It can also test some functionality of digital circuits, although the complexity involved can make it economically prohibitive.
ICT is most suitable for testing products that are more developed and high-volume. However, the up-front costs and development lead time with ICT are higher and longer, respectively, than those of flying probe testing (FPT). This is because your manufacturer must explicitly create a custom ICT fixture for each PCB.
The good thing with ICT is that after the tool is developed, costs per unit tend to be lower than with FPT because it only takes about 1 minute for one test cycle. With FPT, it can take up to 15 minutes per board.
STRENGTHS
Quick tests per PCB unit
Lower costs per unit than FPT
Checks for shorts, opens, resistance, capacitance, and component tolerance
Tests components individually
Tests for logic functionality
Capable of performing on board verification FPGAs
Can be set to turn on and test LED components – say by enabling verification of color and brightness
Ability to check the integrity of BTC components’ soldering using a pressure test
WEAKNESSES
Long development lead time
High up-front costs
Programming and custom tooling are required
Doesn’t test connectors or non-electrical components
Doesn’t test components working together
FLYING PROBE TESTING
Unlike an ICT machine, an FPT does not utilize a bed of nails fixture. Instead, it uses a small number of movable and fixed probes to easily make a simultaneous in-circuit test of the top and bottom of your PCB. It’s made up of high-precision needles — some machines use as few as four needles, while others can use as many as 20 per PCB side. They’re programmed to contact component pins and perform electrical and functional tests to determine if the board is fit for the field.
FPT is most suitable for products that are in the early stages of development and are low-volume orders. It requires no custom tooling, and customization for each PCB is carried out through programming using the CAD data you provide to the manufacturer. With FPT, costs-per-unit are higher compared to ICT because of longer test cycle time periods per board (up to 15 minutes).
STRENGTHS
No custom tooling needed
Programming requires less time
Checks for shorts, opens, resistance, capacitance, and component tolerance
Tests components individually
Low up-front costs
Ability to test LEDs
Capable of performing on board verification FPGAs
WEAKNESSES
Higher cycle test period times and cost-per-unit costs
Doesn’t test connectors or non-active components
Doesn’t test components operating together
WHICH SUITS YOUR PROJECT?
All in all, the choice between ICT and FPT will depend on several important factors of your project. To recap, these include:
Expected volumes
Budget
PCB design/complexity
Lead development times
Remember to consult a well-rounded PCB manufacturer for professional guidance. There may be other PCB testing and inspection options that are better suited to your design. Learn more about ICT Testing and Flying Probe Testing, please contact us via [email protected].
The manufacture of flexible PCBs involves several complicated steps, and extreme care must be taken. Some design or manufacturing defects can cause the entire circuit board to fail.
During the PCB manufacturing process, please follow the precautions listed below:
1. Use the right type and quantity of materials to maintain “flexibility”. For example, if an application requires a 6-layer rigid-flex PCB that will bend during assembly and then should be kept in a fixed position, the correct type of copper and adhesive must be used.
2. It is recommended to keep the circuit small. It is better to use a set of smaller circuits rather than a single larger circuit.
3. It is not a good habit to bend flexible circuits around corners. However, some design situations require bending at the corners. In this case, you can make a gentle bend or use a conical radius bend.
4. The copper used in the 6-layer rigid-flex board is easy to fall off the substrate. Therefore, it is essential to provide support for the copper to prevent its separation. Even if you are using surface mount pads and non-plated pads, you should consider using them with some other measures, such as fixing the cover mask to the pads.
5. The recommended tolerances must be adhered to as closely as possible.
6. It is best to avoid laying traces on each copper layer in the same direction. It is recommended to stagger the wiring to distribute the tension between the copper layers evenly.
7. It is recommended to carry a power or ground plane on the flexible circuit to enhance/maintain the flexibility of the circuit board. You can consider using solid copper casting or shaded polygons.
In order to produce reliable rigid-flex based products, you must be careful enough to pay attention to details. Otherwise, the final assembly may not provide the expected functionality and durability. By following the above recommendations, these troublesome situations can be avoided.
Key Takeaways
A PCB’s response to electrical, thermal, and mechanical stimuli depend primarily on the material of the substrate materials.
Today’s designers have a range of options for PCB substrate materials, ranging from typical FR4 substrates to specialized low-loss laminates.
Different material properties are more desirable in different applications, and designers should choose specific material properties that are important for their designs.
The thermal, mechanical, and electrical behavior of every PCB is governed by the material properties of the PCB substrate, conductors, and component materials. Among these different materials, designers have the most control over board behavior by selecting the right PCB substrate material. The PCB material properties, particularly of the resin and laminate materials, will dominate how your board reacts to mechanical, thermal, and electrical stimuli.
When you need to select a PCB substrate material, which PCB material properties are most important for your board? The answer depends on the board’s application and the environment where your PCB will be deployed. When you’re selecting prepreg and laminate materials for your next PCB, here are some important material properties that you should consider for your application.
Important PCB Material Properties
Your substrate selection is no longer limited to FR4, but you should not make the decision of PCB laminate selection lightly. You should first understand how different material properties affect your PCB and choose a laminate that satisfies your operational requirements accordingly.
Some data for PCB material properties can be found online, but it’s best to consult with manufacturers, particularly for specialized laminate materials as no two laminates are exactly the same, nor are two lots exactly the same. More exotic materials such as ceramics and metal-core PCBs give a range of unique material properties.
The important PCB material properties all designers should understand fall into four areas: electrical, structural, mechanical, and thermal properties.
1 Electrical Properties
All the important electrical properties that need to be considered in today’s PCB substrate materials are embodied in the dielectric constant.
1.1 Dielectric constant
This is the primary electrical property to consider when designing a stackup for a high speed/high frequency PCB. The dielectric constant is a complex quantity that is a function of frequency, which gives rise to the following forms of dispersion in PCB substrates:
•Velocity dispersion: Because the dielectric constant is a function of frequencies, different frequencies will experience different levels of loss and travel at different speeds.
•Loss dispersion: The attenuation a signal experience is also a function of frequency. Simple models for dispersion state that loss increases as frequency increases, but this is not strictly correct and some laminates can have a complex loss vs. frequency spectrum.
These two effects contribute to the level of distortion the signal experiences during propagation. For analog signals operating over very narrow bandwidth or at a single frequency, dispersion does not matter. However, it is incredibly important in digital signals and is one of the major challenges in modeling and interconnect design for high speed digital signals.
2 Structural Properties
The structure of a PCB and its substrate will also affect mechanical, thermal, and electrical properties in the board. These properties are largely embodied in two ways: the glass weave style and roughness of copper conductors.
2.1 Glass weave style
The glass weave style leaves gaps in the PCB substrate, and it relates to the resin content in the board. The volume proportion of glass and impregnated resin combine to determine the volume-average dielectric constant of the substrate. Furthermore, gaps in the glass weave style create what are known as fiber weave effects, where the varying substrate dielectric constant along an interconnect creates skew, resonance, and losses. These effects become quite prominent at ~50 GHz and higher, which affects radar signals, multi-gigabit Ethernet, and typical LVDS SerDes channel signals.
2.2 Copper roughness
Although this is really a structural property of printed copper conductors, it contributes to the electrical impedance of an interconnect. The surface roughness of a conductor effectively increases its skin effect resistance at high frequencies, leading to inductive losses from induced eddy currents during signal propagation. Copper etching, copper deposition method, and the surface of the prepreg all influence surface roughness to some degree.
3 Thermal Properties
There are two groups of thermal properties in PCB laminates and substrates that need to be considered together when selecting substrate materials.
3.1 Thermal Conductivity and Specific Heat
The amount of heat needed to raise the board’s temperature by a single degree is quantified in the substrate’s specific heat, while the amount of heat transported through the substrate per unit time is quantified in the thermal conductivity. Together, these PCB material properties will determine the final temperature of your board when it comes into thermal equilibrium with the environment during operation. If your board will be deployed in an environment where heat needs to be quickly dissipated into a large heatsink or enclosure, a substrate with higher thermal conductivity should be used.
3.2 Glass Transition Temperature and Coefficient of Thermal Expansion (CTE)
These two PCB material properties are also related. All materials have some coefficient of thermal expansion (CTE), which happens to be an anisotropic quantity in a PCB substrate (i.e., expansion rate is different along different directions). Once the temperature of the board exceeds the glass transition temperature (Tg), the CTE value will abruptly increase. The CTE value should ideally be as low as possible within the desired temperature range, and the Tg value should be as high as possible. The cheapest FR4 substrate will have Tg ~ 130 °C, but most manufacturers offer a core and laminate selection with Tg ~ 170 °C.
The thermal properties listed above are also related to the mechanical stability of conductors on the PCB substrate. In particular, CTE mismatch creates a known reliability problem in high aspect ratio vias and blind/buried vias, where the vias are prone to fracture due to mechanical stress from volumetric expansion. High-Tg materials and other specialized laminates have been developed for this reason, and designers working on HDI designs might consider using these alternative materials.
If you’re looking to learn more about PCB material properties, talk to us and our team of experts.