One concern you may have with respect to your circuit boards concerns interconnect defects, or ICDs. What exactly are interconnect defects, and what can you do about them? Here is the basic information you need to know regarding this troublesome issue.
What Are Interconnect Defects (ICDs)?
An interconnect defect is a problem in your printed circuit board that could lead to circuit failure. There are internal connections in your printed circuit board, typically called vias, in which the manufacturer drills through the inner-layer circuit. When the manufacturer processes the PCB, they put copper into the drilled hole to connect the inner-layer circuits to each other and to the surface of the printed circuit board. This allows you to place connectors or components to the board’s surface, and allows the circuit to connect between layers.
Problems can occur when the manufacturer fails to properly manufacture the PCB design, creating a defect near the plating and inner-layer copper. The result of these interconnect defects may be open circuits, or possibly intermittent defects at higher temperatures. Thus, an interconnect defect may cause the circuit ultimately to fail.
One of the challenges of interconnect defects in printed circuit boards is that you may not be able to detect the anomaly while building the PCB. The board may work fine during testing, but then reveal problems during assembly or usage, when it can actually cause serious damage to your system.
Interconnect defects are becoming a problem of increasingly greater concern among PCB manufacturers and suppliers because they can be hard to detect until it is too late. This issue has been cropping up more and more in recent years.
There are two major types of interconnect defects: debris-based ICDs and copper bond failure ICDS. Each brings with them their own concerns and approaches to troubleshooting the issue.
Debris-based ICDs occur when debris from the hole-drilling process gets into the interconnection hole. One presumes that as part of the manufacturing process, the manufacturer clears out all debris after drilling a hole in the circuit board, but this does not always happen. Sometimes drill debris residue, drill smear, inorganic fillers or fiberglass is overlooked. They then embed themselves to the inner-layer copper surface, which can result in an interconnect defect.
Why do debris-based ICDs happen, given that all PCB manufacturers should be aware of the issues surrounding leaving debris behind when drilling a hole in the circuit board? It may be because more manufacturers are using low DK/low DF materials, which use inorganic filler types. While these materials may be more cost-effective in some ways, they can create more debris when drilling and are often more chemically resistant as opposed to the standard FR-4 epoxy materials. More debris that resists cleaning efforts naturally means it is more likely for debris to be left behind, resulting in a defect.
Copper Bond Failure ICDs
In a copper bond failure ICD, high stress during the assembly process or during PCB use, in conjunction with a weak copper bond, causes the copper connection to physically break off. Naturally, the weaker the copper bond, the less stress is required to break it. You may find copper bond failure ICDs on HDI microvias, as well as standard printed circuit boards.
Why is the rate of copper bond failure interconnect defects increasing? More manufacturers are using higher lead-free soldering temperatures and thicker printed circuit boards in the modern era. Larger hole sizes, thicker printed circuit boards and wave soldering are all identifiable factors that can lead to copper bond failure ICDs.
Copper bond failure ICDs occur when the copper connection breaks. This is caused by high stress during assembly or use, the copper bond being weak or a combination. This failure mode is design related. Increased hole size, PCB thickness and wave soldering all tend to raise the risk of copper bond ICDs. There is a higher rate of this ICD type, which is related to increased board thickness and higher lead-free soldering temperatures over the past 10 years.
Reliability testing has found copper bond failure ICDs to be a significant issue. Debris-based ICDs have not shown significance in studies with respect to reliability, but they can still be a costly issue and one that all users of printed circuit boards should look out for.
What Causes Layer Separation?
ICDs have two common causes: copper bond failure and excess debris. While some experts categorize ICDs based on their cause, for the purposes of this article, we will use the types based on defect location. To learn more about those categories, skip to the next section. For now, we will focus on ICDs’ two main causes:
- Debris-based ICDs: During the drilling process, debris can get into the interconnection hole. Drill smear and particles from glass and inorganic fillers can also build up in these holes. Cleaning these substances out is a standard part of the manufacturing process, but sometimes this step isn’t done correctly or gets overlooked. Debris most often causes ICDs in low-loss materials featuring inorganic fillers.
- Copper bond failure ICDs: In other cases, the ICD doesn’t come from a debris blockage. Instead, the inner copper layer fails to connect or breaks. This can happen when the bond goes through high stress or it doesn’t have a strong enough bond. Thicker PCBs and larger holes increase the risk of copper bond failure ICDs occurring. Manufacturers have seen this type of ICD more frequently in recent years due to changes in technology such as board thickness. As PCB design advances, we will hopefully see this type of failure less often.
Factors like high levels of resin, lower amounts of copper and use of materials with lower temperature resistance increase the risk of ICDs. Under-cured boards are also incredibly vulnerable to layer separation.
Types of Separation
When categorized by location in the PCB, ICDs can fall into one of three categories:
- Type I: Occurs at the inner layer copper and electroless copper interface
- Type II: Found at the electroless copper and electrolytic copper interface
- Type III: Located within the electroless copper layer
Type I and Type III ICDs usually happen because of poor controls during the electroless copper process (copper bond failure), while Type II ICDs occur due to contamination (debris-based). To determine the location of the ICD, manufacturers use microsectioning techniques and surface etching to get an easy-to-see cross-section of the board. Type III ICDs require precise testing to detect, so it is important to pay close attention to ICD testing.
Interconnect Defects in Printed Circuit Boards Troubleshooting Tips
As with most printed circuit board issues, avoiding ICD problems comes down to a solid PCB design. A reliable design and consistent, thoughtful manufacturing processes can go a long way toward creating defect-free circuit boards. Drill bit heating and use of inorganic filler materials are factors leading to debris-based ICDs that you can avoid. Using the right materials and being more aggressive with your desmearing process can help significantly cut down instances of debris-based interconnect defects.
When it comes to potential copper bond failure ICDs, cleaning the inner-layer copper surface in order to allow a strong bond to form is critical. Making sure you have the right thickness and grain structure for the electroless copper deposit, so you have the necessary strength, is also a good way to avoid copper bond failure ICDs.
In addition to the above, controlling hole size and board thickness can help keep interconnect defects down. One effective solution can be to get rid of your soldered through-hole connectors.
Order Quality, Reliable Printed Circuit Boards From MCL Today
To ensure getting quality printed circuit boards consistently, order your PCBs from Millennium Circuits Ltd. MCL has one of the most stringent quality assurance programs around when it comes to PCB and PCB-related products, so you can order your PCBs in bulk with confidence. To find out more about how MCL has become a leader in high-volume, quality printed circuit boards, or for a free quote, contact MCL today.