Dec 3, 2020|General
Printed circuit boards require protection from delamination, a potentially damaging condition during production caused by humidity, which erodes the adhesive and allows the base layers to separate. Here’s a closer look at what PCB delamination is, why it occurs, and how to help prevent it.
What is PCB Delamination?
Table of Contents
- 1 What is PCB Delamination?
- 2 Difference Between Delamination and Measling
- 3 Causes of PCB Delamination
- 4 Types of Tests to Measure Delamination
- 5 How to Prevent Delamination
Delamination is a type of damage that can occur to a printed circuit board. It results in the layers of the base material separating from one another. The process is easy to identify because the board will have small, visible gaps or bubbles, typically referred to as blisters.
They occur if the laminate on the base material is exposed to humidity. Heating during the production process turns the humidity into a gas, causing blistering and layer movement.
PCBs consist of multiple layers. The base materials typically consist of FR-4, a laminate material made from woven fiberglass. A flame-resistant, epoxy resin binds these FR-4 layers. During delamination, heat and humidity either loosen the resin from the fiberglass or loosen the resin from the foil or laminate.
Delamination occurs in two distinct steps. Adhesive failure occurs first. As the resin loosens and disintegrates, the blisters appear. Next, the boards move apart, as they’re no longer bound by adhesive. It has a cumulative effect: As the boards shift position, they put pressure on the already-weak adhesive, resulting in additional separation.
Once delamination begins, a few remedies are possible, but they’re not guaranteed. Instead, most anti-delamination efforts focus on prevention. Delamination is almost always the result of issues on the production side. The most effective solutions against delamination are batch testing, high-quality materials, and controlled production methods.
Difference Between Delamination and Measling
Aside from delamination, measling is another potential problem. Telling the two conditions apart isn’t always easy, as they share many similar symptoms. However, they have different causes and represent different production problems.
Like delamination, measling also involves problems with the lamination process. If an insufficient amount of resin is applied to the board during the lamination process, white spots on the PCB weave interior will appear. These white spots are the “measles.”
If you see raised blisters on the PCB base materials, that’s delamination, and it’s caused by humidity buildup. However, if you see white spots, that’s measling. A lack of resin during production, or excess stress at any point in the PCB life cycle, are the two most common causes.
When delamination occurs, the board’s functioning is almost certainly impaired. Fortunately, that’s not necessarily the case with measling. Small amounts of measling are common. As long as the measling doesn’t bridge conductors or soldering eyes, your PCB can likely continue normal operation.
Some measling will likely occur during the board’s life and isn’t normally a source of concern. However, like delamination, measling can also occur during production. Manufacturers must implement correct resin-application techniques to ensure the PCB is produced without measling, as it’ll almost certainly occur later on.
Causes of PCB Delamination
What is delamination’s most likely cause? It can happen due to one factor or a combination of several:
Moisture is the most common culprit. Even a small amount trapped within the board can result in damage. High temperatures turn the moisture into steam, and the resulting expansion can cause both internal and external cracking.
It’s essential to understand why moisture can enter a PCB. Typically, it’s the result of poor bonding and a lack of proper adhesive when attaching layers.
Additionally, PCB delamination is possible due to thermal stress when production or repair. Thermal stress is any change to the size, fit, or placement of a component, typically a mechanical component, caused by excess heat.
Thermal-stress delamination occurs in a similar way to the damage from moisture. The excess heat creates a gas from inorganic materials, causing layers of the PCB to separate.
Delamination doesn’t always have a single cause. Instead, a combination of poor manufacturing processes, low-quality materials, and more can all contribute to moisture buildup. Even something as simple as using FR-4 material with the incorrect type of Tg can result in delamination.
Types of Tests to Measure Delamination
How do I know if my PCB is bad? When delamination first occurs, it’s often difficult to see because it’s happening internally. By the time you notice blisters on the board, it’s functionality is almost certainly compromised.
Most delamination testing and prevention strategies occur at the production level. A few different tests are available to help measure PCB delamination and identify any potential issues with the base materials.
Manufacturers use a time test to determine how long the resin takes to separate from either the copper or reinforcement. During the test, the board is brought to a specific temperature, and the time until delamination is measured.
This test commonly uses temperatures of 260° and 288° Celsius. The 260° test is typically considered the most accurate of the two. The Tg (glass transition) rating of FR-4 also plays a role here. FR-4 with high-Tg levels delaminate quicker than those with low levels.
How to Prevent Delamination
Learning what causes delamination is only half of the equation. Manufacturing PCBs in a way to prevent delamination is equally important. Prevention involves several strategies employed during the manufacturing process.
Dry Production Environment
First, the manufacturer must maintain a moisture-free environment during the entire production process. The oxide layer on the board must remain dry before thermal processing. Otherwise, any moisture will turn into a gas due to the heat, resulting in delamination.
Next, boards require baking before thermal processing. The temperature of the oven should exceed the water’s boiling point (100 C). The placement within the oven is also important. PCBs shouldn’t stack. Instead, they require space for circulation around all sides.
Use High-Quality Components
Proper manufacturing procedures don’t provide guaranteed protection against delamination. A manufacturer can do everything right, and the boards can still suffer from layer separation. Aside from the procedure followed, high-quality materials are also needed.
The inner layers of the board require an oxide coating. It’s a vital protective barrier against both thermal stress and environmental hazards (such as inorganics). The oxide coating must be applied evenly across the PCB for maximum protection.