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84 FLEX007 MAGAZINE I JANUARY 2019 It is important to note the following: 1. Drilling: Use a laser to drill round holes for future use. It is not necessary to drill through since conduction is not needed. 2. Image transfer and etching: Use film and UV light to transfer the test circuit to FPC sensitizer film, and form the circuit on copper foil after developing and etching. 3. CVL attachment: Use laser cutting to make a pattern, and attach CVL to the etched FCCL substrate to protect the circuit and insulate. 4. Surface treatment: Plate gold onto connection points via ENIG (electroless nickel/immersion gold) to prevent the copper surface from being oxidized. Signal IL depends on the impedance of the signal leading trace, and this impedance var- ies with the line thickness, width, and pitch, as well as the material and thickness of the dielectric layer. To compare the loss perfor- mance of various materials under high fre- quency, we designed a standard test circuit layout which has the same impedance for all materials. Therefore, all parameters unrelated to the material are excluded and IL will be dependent only on the material. The IL test circuit layout is designed as follows: Use differential microstrip to configure as in Figure 3. As opposed to a single signal line, the differential signal line has better resistance to noise and suppression of electromagnetic interference and increases the precision of IL measurement. D1 and D2 are a pair of differ- ential signal test circuit traces or lines with the same width (W) and thickness (T), and length. Adjust W and pitch (S) of the test lines to accommodate various dielectric materials, dielectric thickness and copper thickness, so as to make the impedance of the test wires on all materials have the same value of 100 ohms. Test wires for IL are shown in Figure 4. Each working panel (WPNL) of a 250-mm width contains five pairs of test wires. The finished product as shown in Figure 5 is waiting for IL measurement. Measure IL for the 10-inch line before for the 1-inch trace. The IL difference is then an accurate number reflecting the IL of a trace of nine inches in length because the effect of the connection point on IL mea- surement has thus been minimized. Since IL is proportional to wire length, the IL per inch wire can be derived by dividing this number by nine. After measuring impedance and IL for Figure 3: Configuration of microstrip test circuit traces. Figure 4: Layout of test lines to measure insertion loss.