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PCB Issues Guide

Guide to PCB Issues

Printed circuit boards (PCB) are essential components of many electrical devices today, connecting different components to one another through a complex array of circuits. These boards consist of several layers of copper traces to make the connections, as well as plastics and other materials to protect those connections from the environment. The design and quality these PCBs directly affect how well the boards work in the final product, or whether they will work at all.

The complexity of the PCB designing and manufacturing processes means there are numerous opportunities for PCB failure issues to arise. Some of these failures are a result of design oversights, such as insufficient clearances or incorrect measurements, which can negatively affect the functionality of the finished product. Others may result from problems in the manufacturing process, such as drilling errors or over-etching, which can be equally catastrophic.

Fortunately, most of these errors can be avoided with knowledge and consideration for the manufacturing process, as well as awareness of the more common PCB manufacturing issues. To help you and your company better understand and avoid the potential errors in your printed circuit board designs, we’ve compiled a list of the most common issues experienced in PCB manufacturing, why they occur and how they can be prevented.

Plating Voids

Plated thru-holes are copper-coated holes in a printed circuit board. These holes allow electricity to be carried from one side of the circuit board to the other. To create these holes, the PCB fabricator drills holes through the circuit board, puncturing the material all the way through. A layer of copper is then added to the surface of the material and along the walls of these holes through an electroplating process. This process deposits a thin layer of electroless copper onto the circuit board in a process called deposition. After this step, extra layers of copper are added and etched to create the circuit image.

While effective, the deposition process is not perfect, and under certain circumstances can result in voids in the plating. Plating voids are effectively gaps or holes in the plating of the circuit board, and are usually the result of problems during the deposition process. These plating voids are particularly problematic because imperfections in the plating of a thru-hole prevent an electrical current from passing through the hole, resulting in a defective product.

These plating voids happen because, for one reason or another, the material does not coat evenly during the deposition process. The reasons for this include contamination of the material, air bubbles caught in the material, insufficient cleaning of the holes, insufficient catalyzation of the copper in the deposition process or rough hole drilling. Any of these problems can result in plating voids along the walls of the circuit holes.

Defects as a result of contamination, air bubbles or insufficient cleaning can be avoided by cleaning the material properly after drilling. Additionally, defects from faulty drilling can be avoided by closely following the manufacturer’s directions during use, such as recommended number of drill hits, drill in-feeds and drill speeds. Both problems can be avoided by hiring a well-qualified and experienced PCB manufacturing company.

 

Insufficient Copper-to-Edge Clearance

Copper is an incredibly conductive metal, which is used as an active component of PCBs. However, copper is also relatively soft and vulnerable to corrosion. To prevent corrosion and protect the copper from interacting with its environment, this copper is covered with other materials. However, when a PCB is trimmed, if the copper is too close to the edge, part of this coating can be trimmed as well, exposing the copper layer underneath. This can cause numerous problems in the functionality of the board.

copper-pcb

For one, it is possible for the exposed copper planes to make contact with one another by simultaneously touching a conductive material, causing a short. This exposure also leaves the copper open to the environment, making it vulnerable to corrosion. This exposure also increases the chance of someone contacting the PCB and receiving an electrical shock.

This problem can easily be avoided by making sure the space between the edge of the copper and the edge of the board, also known as the copper-to-edge or plate-to-edge clearance, follows acceptable standards for the type of board being manufactured. A good DFM check by your manufacturer will usually catch any potential problems.

 

Slivers

Slivers are narrow wedges of copper or solder mask produced during the PCB manufacturing process, and can cause serious problems during the fabrication of circuit boards. These slivers are often produced during the etching process, and can occur in one of two ways.

First, slivers can be produced when an extremely long, thin feature of the copper or solder mask is etched away. In some cases, this sliver detaches before it fully dissolves. These detached slivers can float around in the chemical bath, and can potentially land on another board, adding an unintended connection.

Another way to produce slivers is to cut a section of the PCB design too narrowly or too deeply. Even if they are intended to stay attached to the board, if an etched section is narrow enough or the etching is deep enough, a sliver of material can completely or partially detach, either producing a floating sliver or a peeled-back sliver. Both of these options can have serious negative consequences for the circuit board’s function.

These slivers can either connect to other pieces of copper or expose copper plating that would normally be covered by the solder mask. The former problem can cause a short, therefore producing a defective circuit board, while the latter option can result in corrosion of the copper over time. Both of these problems reduce the lifespan of the circuit board.

Slivers can be avoided by designing sections with minimum widths, reducing the chances of producing slivers. A manufacturer will usually spot potential slivers with a DFM check.

 

Missing Solder Mask Between Pads

Diagram of solder mask

The solder mask is the layer on top of the circuit board’s copper layer. This solder mask is laid on top of the copper layer to insulate the copper traces from accidental contact with other metal, solder or conductive bits. It also acts as a barrier between the copper and the environment, preventing corrosion and protecting the circuit board’s handlers from electrocution. Pads are the portions of metal left exposed by the solder board, which components are soldered to.

In some circuit boards, the solder mask may be partially or completely absent between pads. This exposes more copper than is necessary, and can result in solder bridges forming accidentally between pins during assembly. This can result in a short, as well as reduced corrosion protection, both of which can negatively affect the functionality and longevity of the circuit board.

This defect is commonly due to a design oversight, where the solder mask is undefined or the settings for a larger board are carried over to a smaller board, resulting in pad holes that are simply too large for the smaller PCB. This can be avoided by double-checking the design before sending it to the manufacturer.

However, oversights do happen, so make sure you have a good manufacturer with a proper DFM check protocol, since these manufacturers can catch such issues before they become a real problem.

 

Acid Traps

“Acid trap” is the common term for acute angles in a circuit. They are referred to this way because these acute angles trap acid during the PCB etching process, allowing the acid to build up in the nook of the angle. The angle functionally keeps the acid in the corner for a longer period than the design calls for, causing the acid to eat away more than intended. As a result, the acid can compromise a connection, making the circuit defective and causing more serious problems later on.

Most designers are aware of the problems caused by acute angles in a circuit board, and are therefore trained to avoid them. However, mistakes do happen. Often, acute angles are the result of simple human error, although some design software programs may also set circuits to acute angles if the settings are not properly adjusted.

Most designers will catch acute angles as they double-check their work, though oversights are possible. If this is the case, a good manufacturer will catch these mistakes with a DFM check.

 

Starved Thermals

Thermals are small traces surrounding pads, and are used to connect pads to the plane. These thermals allow the pads to more effectively disperse heat, and are important components during the soldering process. Sometimes, however, voids between the thermal and the rest of the plane, or the thermal and the pad, can result in an incomplete connection, reducing the effectiveness of the heat transfer system these thermals create. This can result in several functional problems.starved-thermals

Starved thermals take much longer to transfer heat from pads to the rest of the plane, which can be problematic during soldering or if the circuit is under heat stress. A thermal pad with improper heat transfer may solder oddly, and will take an abnormally long time to reflow, slowing down the assembly process. After manufacturing, circuit boards with starved thermals may suffer from insufficient heat transfer and may be more prone to overheating and heat damage.

The source of this problem is most often in the manufacturing. These thermal connections are usually tied correctly to a plane layer in a CAD system, but are somehow manufactured with a reduced connection to the rest of the plane. Manufacturing problems during thermal molding or machining, such as over-machining or improper molding, can all result in this issue, and can often be remedied by replacing the thermal.

An experienced and highly rated manufacturing company can identify such faulty thermals easily and replace them before they have a chance to cause problems in the circuit board.

 

Electromagnetic Issues

Electromagnetic compatibility and electromagnetic interference are two problematic areas for circuit board designs. Electromagnetic compatibility (EMC) involves the generation, propagation and reception of electromagnetic energy, while electromagnetic interference (EMI) refers to the unwanted and damaging effects of this EMC. Too much EMI can result in a defective product. Normally, these issues are a result of one of many possible design flaws.

EMI can be reduced by increasing the circuit board’s ground area and compartmentalizing the board specifically for EMI. Additionally, 90° angles should be avoided for most components, because they tend to increases EMI interference. When in doubt, use shielded cables and metallic packaging to absorb EMC and reduce EMI.

A PCB manufacturing expert can help identify these potential design flaws and work with you on a redesign if EMC and EMI are noted as problems during the prototyping stage.

 

Not Using a DFM

As exemplified by the issues listed above, a number of things can go wrong when designing a printed circuit board. These issues can decrease manufacturing yields and increase costs for both the manufacturer and the designer. Worst of all, it can severely extend the time it takes for the product to go from the drawing board to the consumer.

Often, these issues are the result of flaws in the design or manufacturing of the product, both of which are preventable. This is why it is so important to hire an experienced PCB manufacturer that runs DFM checks on your design before they get to working on it.DFM-PCB

DFM is short for “Design for Manufacturability.” In short, DFM checking is the process of inspecting a circuit board layout to minimize any problems that could happen during the fabrication and assembly process. There are several key factors in DFM, but in short it ensures that the design will result in a consistent product that performs correctly and as expected. Unlike other standards that only look at how the circuit board will potentially work, DFM looks at how the circuit board will potentially break.

DFM issues are typically problems in the topology of the circuit board — those that may or may not result in problems during the manufacturing process. Some of these issues include those listed previously, such as slivers and missing solder masks. Often, the great majority of circuit boards produced with these design issues function perfectly normally, with only a small percentage of circuit boards showing any signs of defects. However, in the PCB industry, every scrapped board adds to the cost of production.

Minimizing these problems requires a specific kind of software and expertise. Few of the aforementioned design flaws are flagged by CAD software, because it’s not designed to look for these potential problems. A good DFM check, however, is designed to identify these potential issues, often drawing from an enormous database of design checks. Though not all of these checks will apply to your specific circuit board — especially if you are working with experimental designs — they are important to evaluate and consider during the design process.

While DFM checks are common practice for numerous manufacturing companies, many don’t properly communicate discovered design issues with the engineering or design company. This can result in serious problems down the road. For example: say a design company sends their design to one manufacturing company for prototyping their circuit board and another for manufacturing the circuit board in quantity. The prototyping company runs a DFM check and finds a few design problems with the PCB design. They alter the design to remove these problems and proceed with the fabrication process, resulting in a perfectly functioning circuit board.

However, they do not communicate their design changes with the original design company. Thrilled with the prototype and unaware of the problems with the original design, the design company sends the original, unchanged design to the quantity PCB manufacturer. The quantity manufacturer does not run DFM checks on their quantity products, because they assume the design has already been perfected, so they simply fabricate the circuit boards with the design they have.

When the circuit boards are finished, many of the end products may be defective. This will often require remaking the defective products and revisiting the design, which are both expensive and labor-intensive tasks. In other words, the poor service of these two manufacturing companies results in a nightmare for both them and their client.

This is a catastrophe no company wants to deal with, which is why you need to look for a company with a thorough DFM check and a communicative, professional staff that will walk you through any issues that may arise with your original design. In short, you need MCL.

MCL’s experienced team of PCB professionals can take a look at your PCB files and run complete DFM checks on them, communicating the results with you so you stay in the loop! Submit your PCB files today for feedback on how to eliminate delays and quality issues before they happen.

 

Do you have questions about our services or your project? Contact us today to speak with MCL’s award-winning support team!

PCB Glossary

  • TG

    Glass transition temperature. The point at which rising temperatures cause the solid base laminate to start to exhibit soft, plastic-like symptoms. This is expressed in degrees Celsius (°C).