Understanding Interconnect Defects (ICDs) in PCBs: Causes, Prevention, and Solutions

What you need to know about Interconnect Defects (ICDs)

Inner-layer separation, or interconnect defects (ICDs), can lead to circuit failure in PCBs. To create a fully functional PCB, a manufacturer must look out for ICDs and their causes. Here is a look at ICDs and how manufacturers prevent and correct them.

What Are Interconnect Defects (ICDs)?

During PCB production, the manufacturer drills through the inner-layer circuit and plated the hole with electroless copper to connect the inner-layer circuits together. This copper plated hole, usually called a via, brings the circuits to the top layer of the board. The via enables the PCBs’ different layers to connect, giving it functionality.

However, sometimes a defect occurs in or near this drilled hole. This flaw is called an interconnect defect (ICD) or inner-layer separation. As the term implies, ICDs involve a separation between the copper filler and the circuits. These components must connect properly for the PCB to function correctly. Inner-layer copper reliability is key to creating a functional PCB that doesn’t damage your system.

An ICD most often happens in response to thermal shocks like soldering. You usually find an ICD on the first inner layer on both sides of the board and much less frequently on deeper layers.

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.

How to Prevent Separation

Preventing inner layer separation in your PCB seems obvious — simply test for it before moving onto the next stage of production. However, ICDs don’t always appear during testing because they happen in response to stress. Instead, they tend to occur during assembly or usage, when a lack of defects is more important than ever. It takes a proactive approach to truly prevent ICDs from forming in your PCBs.

To reduce the risk of ICDs, a manufacturer must pay careful attention to every part of the production process. More specifically, they can take the following steps to tackle the root causes of ICDs:

• Thoroughly desmear the drill hole walls, especially when using methods that result in hotter drill bits
• Use materials with high temperature resistance
• Ensure that the electroless copper has the proper grain structure and thickness to withstand high stress

Improving inner layer copper reliability depends on the quality of the manufacturing process. The lowest risk of ICDs comes from using the right materials, drilling parameters and chemical controls.

To learn more about the PCB production process, contact us online or call 717-558-5975.