28 May 2022
WHEN PROVIDING safety-critical products, it’s key to understand the difference between quality and reliability. The former shows how a product performs its proper function, while reliability defines how well the product maintains its original level of quality over time through various conditions. As Ryan Pellow outlines, it’s not enough that a product simply works, but also about how many times it will continue to perform.
The quality of a given fire safety-related product can often be improved over time. However, the same cannot always be said of reliability, given that many key aspects are decided at the very earliest stages of the design phase. While often an ‘invisible’ element of many such products, the Printed Circuit Board (PCB) is the most critical component due to the fact that, if this fails, the product in question will not work.
As such, it’s imperative that the PCB should be considered in any discussions regarding reliability as any reliability issues will put the end product itself at risk. Quite simply, it’s important to make things right at the first attempt.
Creating reliable PCBs is about considering all of those aspects that can affect reliability as early as possible in the design process. If there are potential problems with the PCB design, this is undoubtedly the best time to tackle them. You cannot compare a PCB to any other component as its design has been tailored to a specific product and application. As a component, its role is critical. A good PCB design improves the reliability of the end product and lessens the risk of failure.
To ensure the reliability of the boards, it’s necessary to apply design guidelines based on what the PCB manufacturers can achieve in practice. In this way, we can show the customer the best layout of the board and, in essence, make sure that the customer receives the most cost-effective and reliable product that we can provide.
As technology advances, it follows that increasingly complex solutions are now beginning to emerge. Today, everything on the boards is incredibly small in scale and more technology can be added to them. The more technology that can be placed on a board, the more complex the construction and manufacturing procedures will become.
In terms of design features on which to focus, ensuring that the track and gap on the board is appropriate to the required copper thickness is very important. With the type of components we’re seeing today, there’s a need for a smaller track and gap, which means the use of less (ie thinner) copper.
Before going too far down the line with the design process, it’s vital that designers know exactly what’s achievable with the specified copper weight requirements. If changes are demanded in terms of the layer stack up – for example, if some high power areas are needed on the board – then the PCB should be designed to ensure that those high power sections are located within the inner layers. The outer layers need to be avoided. This is where the fine-pitch components are located. This approach makes the board much easier to produce in practice, which then in turn offers multiple benefits.
The higher the technology, the greater the effort required to design the board optimally for manufacturing. That’s the only way in which to make sure the boards will function reliably.
In high-technology fire safety products, there’s far less margin for error, with fewer tolerances applied in every single process. That’s exactly why all aspects of the PCB design need to be strengthened. A better design increases the reliability of the product and reduces the risk of failure.
Once the board design is completed, it’s not normally a case of being able to turn to the same factory and simply asking for the manufacture of a more advanced board than has gone before. The product developer has to make sure the manufacturer has the appropriate capability and competence needed to produce the kind of board required.
Although the manufacturing processes for a two-layer and High-Density Interconnect PCB are pretty similar to each other, it must be recognised that the technology is very different. To ensure reliability, the factory needs a higher level of understanding and control of the manufacturing process in its entirety.
The ‘final inspection’ is the penultimate step in the production process. Here, the PCB undergoes an ocular examination by NCAB-approved quality controllers using standard specification requirements. The board is compared to the Gerber file using AVI, which is actually faster than the human eye. Nevertheless, the whole process is still monitored by trained and experienced controllers.
At the same time, it’s always best to strive to design boards in a way that they can be manufactured reliably by as many factories as possible. In this way, the fire safety solution manufacturer can attain better lead times as well as an improved cost picture, while in parallel maintaining reliability demands.
Manufacturing PCBs can be seen as something of a challenging task when talk turns towards applying the very latest technology required by the component manufacturers. That’s why it’s important to avoid the hassle of doing things unnecessarily.
If it’s possible to avoid designs containing six or seven different layers of blind or buried vias to track out a BGA (ie component) and reduce it to a standard multilayer board, it’s Best Practice to do so. This does away with all of the extra drilling and plating processes and significantly reduces costs, while at the same time serving to improve the overall ‘manufacturability’ of the product itself.
If manufacturing options are as open as possible due to a smart design, this would also then allow production to be switched from one factory to another, which reduces the risks posed to the whole process. If one factory is underperforming or dealing with some kind of issue, production can be moved to another facility. PCB designs that limit the process to the use of a single factory are not advisable.
One factor behind achieving reliable circuit boards is to ensure that they meet the industry standard IPC requirements. The NCAB Group has taken this a step further and produced its own standard product specification.
At present, that specification comprises up to 103 different requirements and criteria that factories must follow when manufacturing for us. It’s very much a real-time document that’s being continuously built upon and improved in conjunction with feedback received from customers. Several of the requirements, in fact, are considerably tougher than those stated in IPC Class 2.
In the main, this is because PCBs cannot be treated like other components as there are so many levels to be taken into account. The industry standard IPC applies to many different types of products, requirements and performance levels. In the case of PCBs, it’s unable to cover everything, but for the product owner, an unreliable board involves huge risks. That’s why it’s well worth taking all of the aspects involved into due consideration.
Through all of the years that the NCAB Group has been supplying PCBs, it’s fair to say that we’ve built up a wealth of knowledge covering factors affecting the boards’ quality and reliability. That knowledge will involve such elements as material selection right through to hole copper thickness or the solder mask used in manufacturing, or even selecting the peelable mask that gives the best end result and so on. This is precisely the kind of knowledge that the NCAB Group has built-in as part of its standard requirements specification.
The specification covers specific areas where a higher degree of control is necessary. For example, the factories must use only approved brands for the materials that are employed in manufacture. For the plating of the hole wall, 25 μm thickness of the copper must be achieved, which exceeds IPC Class 2. No track welding or open circuit repairs are allowed if the boards are to be approved and there are requirements for factors such as cleanliness.
In point of fact, we also define clear cosmetic requirements for PCBs. Multiple scratches on the board can suggest accuracy and care issues during the manufacturing process, which could then affect reliability going forward.
The optimum way in which to communicate the demands that are to be placed on a factory is not always clear. For example, it may seem like a good idea to specify an exact material of a precise brand to ensure adequate control. It may sound like a contradiction, but in such a case, it might be safer to settle for an IPC standard (ie IPC 4101) and a number of approved brands. The factory will then be free to choose the material with which it has the greatest experience and which is also best suited to its own tried-and-trusted manufacturing processes.
Make no mistake: forcing a specific choice on to the factory can create problems, since such an action could well impact the reliability of its processes rather than if the factory had used a material with which the team is familiar.
While there may be many factors that impact reliability, a simple approach is always recommended. By being mindful of the impact the design has on reliability, ensuring that all requirements are clear within the data and, at the same time, maximising PCB partners’ strengths, all of this will contribute towards the realisation of a much more reliable end product.
There’s no doubt that wholly reliable PCBs are the key factor determining the overall quality of fire safety/life safety-critical products. That being so, it’s always best to begin any PCB-centric discussions early on.
Include all stakeholders in the discussions and make sure that you learn to trust your chosen partners to add value in their particular fields of expertise. Not only will this improve reliability, but it will also reduce your products’ time to market and, ultimately, save costs due to there being a reduced need for multiple revisions to be enacted at points somewhere down the line.
Ryan Pellow is Managing Director of the NCAB Group UK (www.ncabgroup.com)