Are designers overclassifying their boards?
IPC performance specifications have three classes. Historically, Class 1 has been used for consumer products with shorter lifespans, and where failure is an inconvenience. Class 2 has been selected for commercial and industrial applications where longer uninterrupted service is desired. Class 3 was designated in those cases where performance anytime, anywhere is critical; continuous service without interruption is required. Examples of this are aerospace and defense applications, as well as life-supporting medical equipment. Historically, mil spec jobs dominated, and the commercial and industrial customers followed their lead.
Now it is the reverse. Today, the commercial world is where most of the technical advancement happens, and aerospace and defense users are the followers, looking for data to prove reliability. Most of the medical diagnostic and commercial electronics are now Class 2, and the volume is much higher than that of Class 3. However, many designers gravitate toward Class 2 or 3, often choosing Class 3 as the default. After all, who wants to say it is OK if their product fails?
Keep in mind the limits set in the specifications are a result of multiple inputs over the past few decades. Much of Class 3 has its origins in military specifications dating prior to 2000. Fabricators, assemblers, suppliers, OEMs and end-users have collaborated to refine requirements and limits based on experience and new designs. They developed the limits of the classes based on good judgment, with the intent to maximize product lifespan? Specification committees have an obligation to stay conservative, so that if you select a specific class, you can be comfortable you will get the performance expected. At the same time, processes and materials improve, so performance of features at certain levels gets better.
Over time, the trend to Class 2 and 3 has increased, even among commercial and industrial users. Beyond the high-reliability needs, these users have customers with escalating expectations. Commercial performance contracts and litigation risk make it difficult for designers to specify a performance level that seems to permit failure.
As features get smaller, however, many designs are not compatible with Class 3 requirements. Hole-to-pad size relationships continue to shrink, robbing annular ring. HDI processing requiring wrap-and-cap plating makes etching features on those layers more difficult. Compromises must be made between plating limits and lines and spaces.
What should a designer do? For starters, keep in mind the PCB manufacturer does not have three different processing lines called Class 1, 2 and 3. Boards are processed the sameway in the factory. Differences between classes are acceptability limits and test frequency. The biggest differences are between Class 1 and 2. In many cases, Class 2 and 3 have the same requirements. In fact, the IPC default is Class 2, if not defined by the customer. Since most products seem to be procured to Class 2 and 3, I will concentrate on the differences between them.
Where are the key differences that may impact performance and long-term reliability? First is copper plating thickness. As class goes up, minimum copper plating thickness increases, but only in select cases. For copper plating, it is limited to wrap-and-cap plating and buried via cores. Class 2 also permits limited copper voiding, while Class 3 does not.
Annular ring is the next big one. Class 3 requires full 360° annular ring, while Class 2 permits 90° of breakout. This is probably the biggest area of concern for most designers. Annular ring breakout brings the risk of open nets.
Annular ring and wrap plating seem to be the primary drivers to calling out Class 2 when Class 3 was originally desired. The manufacturer may be the one that asks for relief after performing a DfM review of the data package.
Often in the case of annular ring, the design does not support meeting Class 3 annular ring, and there is no room to increase pad size and no practical means to reduce hole size, either due to bit size availability or, in some cases, because the through-hole aspect ratio may go too high, causing other risks. In these cases, Class 2 annular ring allowance is a lower performance risk than other options.
When specifying Class 2 annular ring, be sure to have fillets or teardrops on all pad-to-trace transitions. This will improve annular ring at the transition where it is most critical. It is good design practice anyway. It improves the finished etched feature with a smoother profile. It also provides mechanical stress relief by eliminating a stress riser of an abrupt pad to trace width change. It also reduces “etch traps” caused by acute angles between the pad and trace. When it comes to wrap-and-cap plating, adding the extra plating to achieve Class 3 is not difficult. The challenge lies in the print-and-etch process that follows. Typically, for wrap-and-cap, it is an HDI design, and the etch pattern is dense and lines and spaces are small. Add wrap-and-cap plating on top of the base copper and any plated through-hole copper can result in an overall copper thickness exceeding 2 mils or more. This makes etching fine lines and spaces very difficult. In some of these cases, allowing Class 2 plating rather than Class 3 will provide enough process relief to make etching possible and consistent. Your supplier may even suggest starting with a thinner base copper as well to increase the margin for success.
Most of the rest of the differences are primarily cosmetic in nature, or related to sample size for coupons or parts inspected at lot acceptance. When it comes to test frequency, lot size plays a big role. The specifications use the C=0 sampling plan. Not until lot size exceeds 90 units will a big disparity begin to be seen in the sampling plan.
If you select Class 2, how can you mitigate the differences? For starters, 100% electrical testing of all boards produced verifies electrical continuity and isolation.
Additional stress screening is another option. This may include IST, HATS or other thermal cycling, followed by electrical testing to confirm performance after stress. In this case, you select performance testing rather than attribute evaluation.
Use the smallest drill size you and your supplier are comfortable with to maximize annular ring. Consider your wrap-and-cap plating needs. Optimize artwork routing; even adjustments of 0.5 mils can make a big difference when features get tight.
Both Class 2 and 3 have legitimate uses. In some cases, you may need to consider Class 2 when you originally planned for Class 3. Review the differences and determine if Class 2 can work in your application. You can also specify one class, but then give direction to follow another class for specific attributes. In the end, aligning end-user needs with the appropriate class requirements will ensure a product that functions as intended in its environment.
is senior field applications engineer at TTM Technologies (ttm.com), vice chairman of the IPC Flexible Circuits Committee and co-chair of the IPC-6013 Qualification and Performance Specification for Flexible Printed Boards Subcommittee;