SMT processing practices demonstrate that the imposition size of FPC should not exceed optimal dimensions. For small devices containing only capacitors, resistors, and inductors, the standard imposition size is 250mm*200mm. However, when ICs, headphones, motors, USB ports, or connectors with numerous pins are involved, it is advisable to limit the imposition size to 250mm*150mm. Larger boards experience greater expansion and contraction, which impacts FPC stability during SMT processing and solder paste printing—wherein the FPC will not receive pad printing. Efficiency and production yield in SMT processing necessitate careful consideration of the following issues:
1.1. Plate size and the number of impositions must optimize overall production efficiency.
1.2. Design specifications for positioning holes and optical identification points on the FPC.
Positioning holes (SMT positioning holes) and optical alignment recognition points (SMT alignment recognition points) on the FPC must meet stringent accuracy requirements for solder paste printing and component placement. These elements directly influence the quality of FPC assembly. Positioning discrepancies should be within 0.1mm, ensuring uniformity across all boards.
1.3. Selection of reinforcement board for FPC structure is crucial in evaluation. The number of layers, substrate structure, material selection, thickness requirements of the reinforcement board, and connection point processes are integral to assessing the SMT process. They significantly influence FPC SMT assembly yield and device reliability post-soldering.
1. Requirements for surface treatment processes of FPC device pads:
2.1. Electroless Nickel Electroless Gold (ENEG) process:
2.2. ENIG technology for nickel-gold plating. The impact of abnormal nickel-gold pad conditions on SMT processing and soldering can be understood through analyses of abnormal soldering due to ENIG pads in “Modern Surface Mount Technology” and applicable rework methods.
2.3. Organic Solderability Preservatives (OSP) process:
These three processes are common treatments for FPC pads, each with distinct advantages and disadvantages. The choice of pad surface treatment critically determines soldering reliability. For instance, nickel-gold coatings are prone to “black pad” formation, directly affecting SMT solderability. OSP processes demand high precision in FPC production pre-SMT due to susceptibility to oxidation, which is detrimental to soldering. Detailed studies can be found in the December 2012 issue of “Modern Surface Mount Technology” on the solderability of different BGA surface treatments.
3. Solder mask design requirements for FPC pads:
The solder mask on FPCs, akin to PCBs’ “green solder mask,” is applied using either of two methods: a polyimide film as the material with windowing achieved through drilling and die punching, or a screen-printed process similar to PCBs. Regardless of the method chosen, for SMD/SMC device pads, uniformity and symmetry are essential. Pads must not be covered by solder mask to prevent reduction in solderable area during SMT, ensuring post-soldering reliability.
There are two primary approaches to designing pads with windows in SMD/SMC technology: SMD (Solder Mask Defined) and NSMD (Non-Solder Mask Defined).
4. The design rationality of SMD/SMC pads impacts SMT processing:
4.1. Deviations from customer-specified window openings in device pad design can lead to SMT process defects.
4.2. Alignment between FPC flexible printed circuit board pads and SMD/SMC device soldering pads affects SMT and soldering:
4.2.1. Smaller FPC pad sizes compared to SMD/SMC device solder feet prevent effective soldering.
4.2.2. Excessive inner distances between FPC pads prevent proper solder foot alignment, causing SMT failure.
4.3. Compatibility of SMD/SMC device solder feet with corresponding FPC pads:
During FPC design, alignment between SMD/SMC device solder feet and corresponding FPC pads must be ensured. For example, if the FPC requires 0402 device mounting but the pad design is based on 0603 specifications, soldering will fail in SMT. Adjustments to either device specifications or FPC pad designs are necessary.
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This revision aims to clarify and streamline the original text, improving readability and coherence.
