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Design007-Sept2020

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108 DESIGN007 MAGAZINE I SEPTEMBER 2020 Documenting Your Flex Circuit Design Flex007 Article by Tony Plemel FLEXIBLE CIRCUIT TECHNOLOGIES As a flex circuit applications engineer, when I receive an RFQ, the first thing I do is look at the customer's data and review their manu- facturing notes. Quite often, I find notes that supersede IPC specifications in manufacturing documents, as customers often believe these added notes and associated specifications will make the circuit more robust. However, these non-standard IPC manufacturing specifica- tions/notes can wreak havoc on the manufac- turing process and can actually lead to a less robust circuit. For example, a customer will sometimes specify additional copper plating, believing it will result in a more reliable circuit. In reality, that type of requirement can make the circuit less reliable, more difficult to manufacture, and more expensive. When manufacturing yields go down, the price goes up! In taking a deeper dive into manufacturing notes and the potential issues that they can create, let's use a three-layer multilayer flexible circuit as an example. The first note on a manu- facturing print is usually "Manufacture to IPC- 6013, Class 2, Type 3." This note should always be included; I cannot stress that enough! Unfortunately, in the continued review of the documentation, I often find one or more additional conflicting notes further down in the manufacturing notes that overrule IPC- 6013 specifications. Copper Plating One example would be "Minimum copper plating shall be 0.0015"." This note supersedes the IPC-6013 specification in Table 1. PCB designers who are not well-versed in flex cir- cuit manufacturing may not know that exceed- ing IPC-6013 of 984 µin (0.000984") can cause the circuit to be less reliable and possibly cause problems later in the manufacturing pro- cess. Having a specified requirement this large (0.000516" thicker) will require the plating line at the factory to plate more than 0.0015" to ensure the minimum plating is 0.0015" thick. Also, if there are impedance-controlled traces on the outer layers, we then need POP plating (pads only or button plating). Plating thicker than IPC-6013 recommendations can cause the POP pads to poke through the dry film resist during the roll or vacuum lamination, imag- ing, and developing process. The exposed bump will then result in pitted etching on the surface of the POP ring, causing rejected parts. Additionally, this excess plating thickness will then require a thicker cover coat adhe- sive layer to compensate for the thicker plated bump. The extra adhesive thickness needed to encapsulate the bump will result in more adhe- sive flow around the pad, causing annular ring issues and also making the circuit more rigid. On a Type 3 (three layers or more) circuit with 1/2-oz. copper plated to standard IPC recommended panel plating or POP plating, the cover-lay typically only requires 0.002"' of adhesive. However, with the 0.0015" mini- mum plating thicknesses, the cover-lay may need 0.003" adhesive. Additionally, when Table 1: IPC-6013 copper plating requirements.

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