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42 DESIGN007 MAGAZINE I MARCH 2023 Feature Article by Alun Morgan VENTEC INTERNATIONAL GROUP At a recent IPC Laminate/Prepreg Materi- als Subcommittee meeting, I made two obser- vations: It was ver y well-chaired and man- aged, and despite a full attendance of very knowledgeable materials experts, none could remember the rationale for several of the specifi- cation sheet descrip- tions—slash sheets— that comprise IPC- 4101E, "Specification for Base Materials for Rigid and Multilayer Printed Boards." e thought natu- rally followed that if a comprehensive c r o s s - s e c t i o n o f m a t e r i a l s e x p e r t s struggled with this, what hope do PCB d e s i g n e r s h av e o f making sense of it? It was a revelation to learn at a previous materials subcom- mittee that the headings for each specification sheet do not form part of the specification. ese headings include reference definitions for the material, including any reinforcements, resin systems, flame retardants, and fillers that may be used, as well as its other known identi- fications and the glass transition temperature. However, the requirements for the material are Material Matters and Tribal Knowledge solely defined by the ensuing specific line items within the specification sheets. Bearing in mind that the specification sheets are organized according to the reinforcement type, resin system, and/or construction—although these do not form part of the specification require- ments—one could be forgiven for won- dering what value this classification system has. Hypothetically, it would be possible for a materials man- ufacturer supplying a glass-reinforced polyimide laminate m a t e r i a l , w h i c h w o u l d o r d i n a r i l y be classified as IPC- 4101/40, as an IPC- 4101/126 mater ial which would ordi- nar ily be classified as an epoxy woven- glass material. ere would be nothing to stop this from happening if the material satisfies the specification sheet requirements. Clearly absurd, but possible under the classification system. So how do PCB designers specify materials? Perhaps unsurprisingly, many specify what- ever material worked last time on a similar