The updated rigid-flex specification overhauls copper thickness requirements.

As we start a new year, it’s a good time to review what changed in 2021. In the flex world, the IPC Flexible Circuits Performance Subcommittee worked through the pandemic and released a new revision to IPC-6013. Revision E was released in September, replacing an amended Revision D from April 2018. Some updates and changes are subtle, while others are significant. Many changes attempt to increase clarity.

Let’s start at the finish – final finish, that is. Tin, silver, and ENIG/ENEPIG will not have minimum thicknesses in IPC-6013. Instead, we are defaulting to the new IPC-4552/4553/4554/4556 specs for thickness and sampling frequency. This avoids unintended differences or conflicts as the finish specs are updated.

Often questions arise related to the rigid-to-flex transition – and what is delamination versus non-lamination? In paragraph 3.3.1.3, we added an explanation about what’s happening at the transition and a new Figure 3-1B to provide a more visual explanation of what is acceptable and rejectable.

Flex circuits have always been more prone to questions about foreign material or entrapped particles. Unlike rigid boards, flex circuits are more transparent, making cosmetic anomalies more evident. Once noticed, disposition is required. We expanded Section 3.3.2.4 to provide more clarity on acceptability, including prepreg resin that may deposit on the external surfaces of flex regions of rigid-flex.

All the pieces that add up to the right fit.

“I am developing a flexible circuit for my application and will soon be ready for prototypes, followed by production a few months later. A lot of flexible circuit suppliers are out there. How do I know if a vendor is reputable and will meet my needs?”

Many variables must be considered when picking a flexible circuit supplier. Do your homework and find a vendor that is a good fit for the project. It is advisable to also select a vendor that will support your program from prototype through production. Multiple vendors could build to the same Gerber files and overall specifications, but the end-product could have differences due to processing and material variations between suppliers. Switching fabricators midstream can introduce significant risk at a critical time between prototype and production. Following are the items I recommend learning about a vendor before making your sourcing decision:

Circuit application/performance class. This is more about the IPC performance class rather than specific application, but mil-aero, implantable medical devices, and so on generally are specified as IPC Class 3, while most everything else is Class 2. IPC Class 3 is the highest reliability and overall performance class and is usually specified when the product is used in a life-critical application. Class 3 product typically requires more stringent processing controls, QA, and documentation. Suppliers that primarily serve Class 3 users typically “stay in their lane” and build all products to Class 3 performance level regardless of the requirement.

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First differentiate between rigid-flex and true flex.

As is often the case with flex circuits, knowing which solder mask to use on flexible circuits is somewhat of a trick question, one with several answers. The decision boils down to circuit construction and design intent.

To start, there are several ways to insulate circuits in the flex world. These include solder mask, coverlay and coverfilm. In most cases, the designer may simply note solder mask per IPC-SM-840 and leave the rest to the fabricator. This allows the fabricator to use the proper mask in the proper setting.

When making a design decision, first differentiate between rigid-flex and true flex circuits.

Let’s cover the easiest one first: rigid-flex. Typically, a rigid-flex construction will have solder mask applied to the external rigid layers to insulate all external traces, as well as define surface mount or BGA pads. It may also provide mask dams between pads to reduce the potential of solder shorts at assembly. This solder mask usually is classified under IPC-SM-840 as a type H solder mask, which denotes a high-reliability solder mask. These are the most common solder masks. Normally green in color, they can be modified for other colors, as desired. It is worth noting that if the color deviates from the as-formulated green option, there may be feature resolution and web size tradeoffs. This is because the additives used to change the color impact how the mask material absorbs light energy during the imaging process. As a result, the fabricator may need to ask for some relief for other colors.

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A little information goes a long way – but can carry added cost.

“My company has traditionally specified the finished thickness for each flex printed circuit (FPC) layer, and total thickness. This is because it’s understood some material layer thicknesses (i.e., adhesives) change during the manufacturing process due to compression and curing. As a purchaser of FPCs, we are less concerned with the initial raw material thickness than the finished thickness.

“We have received feedback, however, that the FPC market in general specifies the raw material thickness used in FPC fabrication, and not finished thickness. The assertion was nearly all customers purchasing FPCs follow this rule to minimize miscommunication. Is this common practice?”

Answer: The level of detail we see on customer drawings is all over the map, but the majority of customers that do specify individual materials will indicate the raw material thicknesses and then the overall finished circuit thickness.

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