What is an Interconnect Board?
Electronic devices are getting increasingly smaller with every passing year. This miniaturization of electronics would not be possible without the use of interconnect technologies.
Interconnect boards are multi layer PCBs that are incredibly complicated with a very high wiring density with smaller micro via PCBs, thinner spaces, and higher connection pad density. Working with interconnect boards is an art form because it requires a careful selection of tiny components such as micro, blind, staggered, and buried vias across multi-layered circuits.
The Challenges of Working with Interconnect Boards
There are several challenges that designers have to overcome when working with interconnect boards. These include switching speed limitations, current capacity, and temperature problems. The board’s real estate is also incredibly limited so designers have to become creative on how to create more space.
This is why it is common for interconnects to feature surface mounted technology instead of through holes. Two of the most important parameters for interconnect boards are thermal conductivity and signal integrity. It is also important to choose the right connector for spark planning. Let’s discuss a few examples below.
i) Backplane Connectors: Also referred to as backplane, these PCBs are used to provide a support structure to other printed circuit boards. Backplane connectors provide stability and mechanical strength to circuits while also connecting with system components known as daughterboards.
Backplane connectors are a popular choice for designers because of their modularity, ease of use and scalability. These components should not be confused with motherboards because they do not have active components such as microprocessors.
ii) Standard Board Interconnects: The most popular type of board to board connectors are male to female and pin/socket headers.
iii) Wire to Board: Wire to board connectors are designed for low-profile mating between PCBs. They may utilize a full lock mechanism or a friction lock mechanism. Some of the most popular options include FPCs (flexible printed cables), FFCs (flexible film cables), and ribbon connectors.
iv) PCB Edge Connectors: Edge connectors are designed for high signal integrity on circuit boards and allow outside cards to be inserted into devices. PCI-e slots are common examples of PCB edge connectors. They are incredibly robust and can withstand extreme forms of wear and tear.
What are Some Design Considerations of Interconnect Boards?
There are few design considerations when working with interconnects in your HDI PCB design. Let’s take a look at some of them below.
EMI and EMC
- There is a slight cost to stacking HDI PCBs together including EMI (electromagnetic interference) and crosstalk impedance.
- Common mode current,
- Accidental antennas.
- Isolating signals from each other. As a rule of thumb, digital and analog signals should be kept separate.
- The vias and interconnects that are connecting the PCBs together will affect the overall signal of the entire device.
Steps must be taken to minimize noise generation and cross talk when working on interconnect boards.
It is recommended to work with design software to identify return paths and split planes between the boards. You can use plane decoupling capacitors to deal with offending signals. You should avoid routing traces in broadside and parallels because they can result in a high degree of PCB crosstalk compared to those that are routed perpendicularly.
For the most part, parallel trace design will result in poor performance. However, broadside trace design may be used by experienced designers by properly calculating it into the HDI PCB design. In any case, it is recommended to work with perpendicular traces where possible.
Use of Via in Pad Technique
Via in pad techniques are becoming increasingly common with HDI boards because they make routing easier. Furthermore, via in pad interconnects can increase circuit density without substantially increasing the assembly cost of the board.
A major advantage of using via in pad technology is that it allows the use of finer pitch packages and reduces inductance. The result is that it can greatly save space in HDI PCBs compared to blind vias and buried vias.
Some of the advantages of using via in pad design include:
- Increased space sayings
- Improved heat dissipation
- Compatibility with fine pitch components
- Lower inductance
The Production Challenges of Interconnect Boards
With all the miniature components used on interconnect boards, the actual production process has to be flawless. The board’s tolerance must be extremely high to support the components. This definitely applies to lamination cycles, routing, annual rings, and drilling to achieve desirable results. The more layers you add to the HDI PCB, the more sophisticated your tools have to be.
It is also important for manufacturers to incorporate test points into their overall design. Test points should allow us to check the board’s functions.
For obvious reasons, the board should be designed with future serviceability in mind because it may require troubleshooting and repair.
Dealing with the Risk of Corrosion
Corrosion is your interconnect board’s worst enemy. It poses a serious threat to connectivity and can affect board functionality. Materials such as copper and lead are extremely vulnerable to corrosion. Materials such as gold, graphite, and silver have more resistance from corrosion.
It is important to understand the conditions that your circuit will be subjected to in order to make informed decisions on material selection. Here are a few things to consider:
Atmosphere: Metals can be oxidized when exposed to water and oxygen. Even small amounts of humidity in the air can corrode copper contacts and affect conductivity.
Fretting: Fretting refers to the removal of surface oxide layer due to frequent wear and tear of moving parts. This can lead to oxidation.
Electrolytic Problems: The presence of an ionic liquid may lead to dendritic growth. This could result in a short circuit - which is detrimental to the circuit’s performance.
In practice, most interconnect boards will be affected by corrosion and fretting. The designer should understand the conditions that their product will be subjected to for making an informed choice in material selection.
Interconnect boards play a pivotal role in making your PCBs more functional, durable, and smaller. We recommend speaking with an interconnect board expert at Hemeixin Hdi PCB who can help you develop the perfect components for your next project.