Printed Circuit Design & Fab - July 2008 - (Page 40)

SoLder Mask Environmentally Friendly Digital inkjEt SolDEr MaSk The advantages of inkjet processing provide the PCB fabricator with a new tool to improve product quality and reduce waste. by Yehuda david and dr. Yifat Bareket The traditional solder mask process requires expensive capital equipment and a major amount of production floor space, in both the wet process area as well as in the clean-room environment of the “yellow room.” Because of the number of steps, traditional liquid photoimageable solder mask processes involve a high number of skilled employees. The total process involves the use of a number of chemicals (developer) and the manufacture and maintenance of photo tools used to image the solder mask. While today’s solder mask materials are fine tuned, quite often the results demonstrate limited performance, long cycle time and a major contributor to the total “cost of goods” in the profit and loss statement of many PCB companies. The expected benefits from digital inkjet solder mask printing as shown by comparative tests with other systems are numerous and significant, and may include: ■ flexibility with balancing line throughput ■ higher registration accuracy and resolution ■ ability of printing robust solder dams between fine pads ■ no solder mask residue inside holes ■ adjustable, selective coating thick40 ness for conductors, laminate and power areas ■ labor, photo tools cost savings, reduced cycle time and material savings ■ reduced chemical waste and waste treatment costs Inkjet digital printing allows significant simplification in the solder mask process. The one-step process also has the potential to increase final yield in both PCB fabrication and assembly. Handling (and handling related) defects are minimized in the PCB fabrication process, and “selective additive printing” technologies minimize shorts and improve yield in the assembly process. Zero Clearance Technology Using a traditional solder mask process with a medium pitch density SMT component, most PCB manufacturers are able to produce solder dams (used to prevent shorting during the reflow operation) with sufficient clearance between adjacent pads. However, in the case of a high-density SMT component, many PCB manufacturers cannot produce the required solder dams. One limiting factor is that there is insufficient registration accuracy between the film and the PCB in the traditional solder mask process. Additionally, with many processes there is a resulting solder mask undercut after exposure and development, which reduces adhesion of the mask so that solder dams cannot stand up to further processing. In these cases, where a solder dam is needed for advanced high-pitch components, the industry is often forced to do without, simply because the traditional processes are not robust enough to generate reliable 3 mil or thinner solder mask dams. These problems associated with traditional processes can be avoided by digital inkjet printing, with each panel going through an individualized registration process between the CAM data and the panel prior to printing, facilitating accurate solder mask deposition, as seen in Figure 1 and Figure 2. A Selective Printing Method Holes. With a traditional process, the solder mask stage is applied as a nonselective coating. Typically, a doublesided screen coater or spray coater applies the solder mask material to cover the entire panel. During this process, not only are the board surfaces coated with solder mask, but the holes are partially or completely filled JULY 2008 PRINTED CIRCUIT DESIGN & FAB

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Printed Circuit Design & Fab - July 2008

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