Printed circuit boards are made from layers of substrates, prepreg (adhesive), and electrical components. PCBs are stable for use in a range of temperatures, are resistant to ion migration, feature low dielectric constants, and have great processing performance. They are used in a range of products, from computers to medical devices.

While PCBs undergo the manufacturing process, there is one factor technicians must watch out for: warpage. Warpage is essentially any deformation of the PCB, including bending, bowing, and twisting. And while modern PCB fabrication technology ensures that the possibility of warpage is minimal across all stages of the assembly process, warpage still happens.

Why PCBs Must Remain Flat During Fabrication

During the PCB construction process, it is imperative that the PCB remain perfectly flat. Flush circuit boards are critical for the proper mounting of electrical components. If the PCB is warped (bent, bowed, or twisted), the automatic insert machine is unable to mount the components accurately. In serious cases, this machine can even break, costing the manufacturer a significant amount of money and lost time. Additionally, warpage may cause these components to slide out of place in the reflow oven. This causes undesirable effects such as solder bridging, parts falling off, open circuits, and other soldering problems. Further, inaccuracies such as these demand reworks, which cost the manufacturer and client time and money.

Fortunately, manufacturers can come to understand the main causes of warpage in PCB assembly and how to prevent it. With this knowledge in mind, companies can reduce unnecessary costs, consistently create quality products, and maintain a positive reputation.

The Main Causes of Warpage in PCBs

Before we dive into how to prevent it, here are the main causes of warpage in PCB assemblies:

Exposure To Heat and Chemicals

Heat is the number one cause of warpage on a printed circuit board. Exposure to harsh chemicals also causes warping. During the manufacturing process, PCBs are exposed to numerous heat treatments. Technicians must be careful to heat both sides of the substrate equally, evenly, and quickly. Warpage will occur when the substrates are not heated evenly, or if the substrates are exposed to high temperatures for too long.

In terms of chemical exposure, PCBs are silk screened and dried with heat or UV light. When the PCB encounters hot air, the substrate may experience thermal shock. Processes such as these also contribute to warping.

Weight

Heavier circuit boards may cause the PCB to sag, contributing to warpage.

Uneven Layers

Occasionally, technicians may erroneously distribute the circuit board’s copper layers unevenly. In effect, these uneven layers cause the PCB to warp. During the fabrication process, technicians should be careful to ensure that all layers are added evenly, whether they’re added manually or via machine.

V-Scoring

Depending on the design, PCBs may need to be split. This process is referred to as V-scoring, or V-cutting. During this process, machinery cuts a V-shaped groove on the top and bottom of the circuit board. This cut is just deep enough to leave a small amount of material to hold the board together. If the V-score is placed incorrectly, the PCB may experience warpage.

Expansion and Contraction

Printed circuit boards feature copper foil and one or more substrates. When the copper foil and substrate(s) expand and contract in significantly different ways, the stress that results causes the PCB to warp.

Improper Storage Conditions

A printed circuit board may make it all the way through the assembly process without warping, only to be stored incorrectly or exposed to the elements. Storing or transporting PCBs in hot, humid, wet, or dirty conditions will cause these boards to warp, discolor, rust, and sustain otherwise significant damage. To store and transport PCBs properly, the PCB must be placed on a flat surface in an area that is free from water, heat, dirt and debris, and corrosion. An additional precaution to prevent moisture from damaging PCBs is sealing the PCB in moisture-proof packaging.

How To Prevent PCBs From Warping

Fortunately, manufacturers can prevent circuit boards from warping. Some of the ways in which technicians can avoid warped PCBs include:

• Distributing all copper layers evenly while the PCB is undergoing construction.
• Balancing each layer’s copper pattern with the PCB’s circuit area.
• Balancing the layout of the electrical components with the assembly and thermal distribution.
• Ensuring that prepreg layers (fiberglass strengthened by an adhesive layer of resin, which acts as an adhesive in the PCB) are uniform in thickness and in number.
• Using the same supplier for all prepreg layers and core sheets.
• Understanding warp and weft directions before cutting and laminating the PCB.
• Releasing stress from the PCB after it has been hot and cold-pressed, sheared, and milled. This is done by baking the PCB in a 302-degree Fahrenheit oven for approximately four hours.
• Meshing the copper foil on large copper areas to reduce stress.
• Avoiding heavy pressure while fabricating the PCB.

What Happens if a PCB Is Warped?

Warped PCBs cause a significant amount of trouble, not only for the technicians but for the machinery, the manufacturer, and the device it powers. When the electrical components are placed on a warped board, the placements will be incorrect. Additionally, the components will not match up with their intended holes on the circuit board. As a result, the warped PCB may even damage the pick-and-place machinery and never make it to the soldering process.

Individuals working on an assembly line will also find working with a warped PCB incredibly difficult, if not impossible. Bent circuit boards are extremely challenging to install in their intended device and may slow down the production line. They may also come misaligned in a device after installation and cause the device to malfunction or not work at all.

Device manufacturers who wish to outfit their products with quality PCBs need look no further than Advanced Assembly. Expert technicians utilize experience and state-of-the-art technology to build a range of warpage-free printed circuit boards, including quick-turn PCB prototypes. Inquirers can contact our team to discuss PCB assembly options.