I read a story once about an experiment to see if people could figure out the simplest solution to a problem. Volunteers were put into a jail cell and given an hour to figure out how to escape. From loosening door hinges to chipping the cement out around the window bars, all of these test subjects were convinced that they had found the best solution, if only they had a little more time to complete their efforts. Nobody tested to see if the door was actually locked, however. If they had, they would have discovered that they could have simply opened it and walked out.
It is human nature to overthink a solution to a problem, and in the world of printed circuit board manufacturing, you will find this same tendency. Some designers are concerned about whether or not their boards should be built to the highest standards of manufacturing. The Association Connecting Electronics Industries (known as IPC) publishes the industry standard for PCB manufacturing—IPC-A-610. The standard lists three classifications by which PCBs are to be built: Class 1, 2, and 3.
Most PCBs do not need to be built to the highest standard—Class 3. In fact, you may get better overall results by having them built to IPC-A-610 class 2 requirements instead. Let’s take a closer look at the differences between these three classifications and how they will affect you in the manufacturing of your PCBs.
How IPC-A-610 Class 2 Requirements Compare to Other Classes
The first step in identifying which classification your PCB needs is to understand the differences between the classes and their requirements:
- Class 1: General electronic products. Class 1 has the least stringent manufacturing requirements. It is intended for inexpensive applications that do not expect product longevity and require low assembly costs. An example of this would be a toy or other lower-level consumer products.
- Class 2: Dedicated service electronic products. Class 2 has more rigorous manufacturing requirements and includes most industrial products and consumer appliances. It is intended for electronic assemblies in which extended reliability is the goal. This class will allow for some imperfections in the assembly that are fully functional electrically and mechanically, but may aesthetically look incorrect.
- Class 3: High-performance electronic products. Class 3 has the most stringent manufacturing requirements of the three classes. It is intended for products whose performance is critical, such as military, aerospace, and automotive applications. It is also used when the operating environment of the PCB is particularly harsh. This standard demands that everything is perfect in the assembly, including how it looks visibly. This level of perfection, however, comes at a higher cost to ensure that all of the requirements are met.
Although class 3 boards are often used in aerospace applications, class 3 is not exclusive to just that industry. Class 3 requirements for circuit board manufacturing is applied to any board that is going to be used in a harsh environment where its long-term ability to reliably function is mission-critical. This applies to anywhere the PCB is subjected to a harsh environment, such as in the engine compartment of your car.
On the other hand, most PCBs will not be operating in an environment like that. Consumer products, such as computers or other appliances, as well as industrial devices, are the normal applications targeted for class 2 requirements.
The Advantages of Class 2 PCB Manufacturing
Since most PCBs fit into the realm of class 2 manufacturing requirements, trying to build them at class 3 requirements will result in a lot of unnecessary expense. A few advantages to manufacturing a class 2 PCB when that is all that’s required include:
- Design: To meet class 3 requirements, the board will have to be designed to tighter tolerances and specifications, which include annular ring sizes. To achieve the class 3 requirement of a minimum of 1 mil annular ring width without any breakout, you will have to increase your pad diameters. This will reduce your routing channels and could affect your placement. A class 2 board, however, allows for some breakout of the hole from the pad provided that the minimum lateral spacing is maintained. This gives you the flexibility of using smaller pad diameters, which will increase your space for placement and routing.
- Manufacturing: To achieve class 3 perfection during assembly, the manufacturer will often slow down some of their processes, causing more time and expense. The amount of solder required to fill the barrel for thru-hole leads is 75% in a class 3 board as opposed to 50% in a class 2 board, and it’s a more complicated process. Surface mount parts must be aligned perfectly without any off-centered parts in a class 3 board. In a class 2 board, the surface mount parts can be slightly off the center of the pad as long as there isn’t any impact to the electrical or mechanical function of the board.
- Inspection: Class 3 boards must pass more rigid requirements. Therefore, the inspection times increase, which results in additional costs. The tighter tolerances on annular rings, perfectly aligned surface mount parts, and thru-hole leads filled to 75% must all be inspected and verified.
Class 2 requirements are the standard for the majority of products that demand high reliability, such as network equipment, industrial electronics, and many electronic medical devices. Not only is the production line much quicker for a class 2 board, the requirements are well-known and easier to work with for manufacturers, which also increases the throughput of the manufacturing process.
Finding an Assembly Partner for Your Class 2 PCB
It is not unusual for PCB designers to think they need their boards to be manufactured to class 3 requirements to get a high-quality, reliable product. However, the truth is that boards built to class 2 requirements will often result in the quality and reliability that is needed. Class 2 is the appropriate choice in most applications and will meet the performance expectations that designers are looking for.
Any minor issues, such as slightly misaligned surface mount parts, are completely acceptable for class 2 boards. Often, attempting to “fix” them is unnecessary and poses too much of a risk to the integrity of the finished product.
There is also another important point that should be made for class 2 manufacturing: Contract manufacturers that have well-controlled assembly processes will produce class 2 circuit boards that are built to the higher-quality levels that approach class 3 boards, especially in their surface mount technology processes. This doesn’t mean that they are building class 3 boards, just that their processes are such that they are exceeding the standards of the class 2 requirements.
Designers who are looking for a high level of quality require a CM that is skilled and experienced in their manufacturing processes. Partnering with a CM who can provide you with this will help you realize the benefits of this higher quality in your class 2 boards.