The IPC slash sheets simplify communicating design intent.
It is common to see a drawing with a stackup that identifies all the materials used to build a board. The designer selects foil and dielectric types and thicknesses, and in many cases may even call out glass weaves and specific brands. And why not? It is their design, and it is natural to want to control it. As a designer, it makes sense to be as clear and as complete as possible, right?
Specifying a particular material is required in some instances to get a particular performance attribute. In many cases, however, performance levels can be achieved with a variety of material selections.
It is common to want to call out a specific prepreg because it has been modeled to achieve certain signal integrity performance. Often several materials, however, can accomplish the same result. Sometimes the manufacturer can offer a solution that helps manage cost. In other cases, the manufacturer may need to use a different glass weave/resin combination to achieve a good, void-free lamination. This can impact dielectric thickness between layers and the resultant impedance lines.
IPC has a series of material specifications that categorizes materials into specific buckets using slash sheets to differentiate among materials and their performance characteristics. Notably, the rigid laminate and prepreg specification, IPC-4101, has many slash sheets covering polyimide and epoxy resins, and more. In many cases, the designer will be best served by defining materials based on slash sheets and not specific brands. The reasons for this are numerous.
Most manufacturers have qualified a series of materials. The manufacturing partner might not have qualified a certain specified brand. The designer will then be facing a drawing change, or require the fabricator to use a new material set and validate its process for the first time on the designer’s part.
If the part requires UL certification, the manufacturer may not have submitted that specific material or combination of materials to UL as part of its UL listing. Again, this might mean a drawing change, or charges to have the particular construction tested and approved by UL.
Rigid-flex throws another wrench in the works. Rigid-flex uses no-flow prepreg in order to control resin flow and keep it from the flexible regions of the part. As a result, options are limited, as not all types and brands of prepreg are offered as no-flow. Calling out one of those materials will drive an exception request and drawing change. Also, with no-flow prepreg, many are limited to just a few weaves, most commonly 106 or 1080 types. The manufacturer will have a series of no-flow prepreg materials it has qualified and characterized. Take advantage of its experience here.
For example, even polyimide prepreg is not that simple. IPC-4101 slash sheets for polyimide laminate and prepreg really number just three: /40, /41 and /42. It may be quite common that a manufacturer’s preferred rigid laminate is qualified to one slash sheet and its no-flow prepreg certified to another.
As the name implies, no-flow prepreg does not flow as standard-flow prepreg does. As a result, the manufacturer has different rules for the amount of prepreg needed between layers, depending on foil thickness and copper coverage on each layer. This can drive use of prepreg sheets that might be a bit thicker than what the designer might expect, or maybe multiple thinner sheets.
Some rigid-flex boards have more than one type of prepreg in the same stackup. This is especially true when using HDI fanout layers in combination with microvias. These designs are typically sequentially laminated, using no-flow prepreg in the first lamination but possibly standard-flow for subsequent laminations. This can open the design to more possibilities for the outer laminations.
Additionally for rigid-flex, some prepreg materials laminate at higher temperatures than others. This can have a damaging effect on some lower-temperature-rated materials in the stackup, such as the coverlay and flexible bondply. Sequential laminations can magnify this effect with multiple high-temperature excursions.
Where possible, then, use of IPC slash sheets is recommended to signal design intent and leave some room for the manufacturer to navigate the design and fabrication. If it helps to soothe concerns, ask the manufacturer to share its final stackup to confirm the board will be built to match the drawing.
Once the circuit is qualified, it becomes a fabrication of record. There should be no issues with lack of design control moving forward. Good manufacturers will not change material selection without the designer’s prior knowledge and approval. •
Nick Koop is director of flex technology 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; This email address is being protected from spambots. You need JavaScript enabled to view it.. Co-“Flexpert” Mark Finstad is director of engineering at Flexible Circuit Technologies (flexiblecircuit.com); This email address is being protected from spambots. You need JavaScript enabled to view it.. They are speaking in October at PCB West.
PCB West: The leading technical conference and exhibition for electronics engineers. Coming Oct. 4-7 to the Santa Clara (CA) Convention Center. pcbwest.com
