Printed Circuit Design & Fab - April 2008 - (Page 36)

INNERLAYER FABRICATION Troubleshooting the INNERLAYER PROCESS Identifying and maintaining proper process controls is key to improving innerlayer yields. by BETTY XIE AND SIMON LEE An improved awareness of the relationship between key innerlayer process controls and defect modes can help improve both product yields and overall quality. Whether the process is designed to produce ultra-fine line product or more mainstream innerlayers, each of the steps within the manufacturing process can influence both quality and yield. Identifying and maintaining the proper controls and procedures for each process step is a critical first step to maintain high quality while meeting cost targets. This review of the issues associated with the liquid-resist based innerlayer process will provide an overview of the critical issues. the later resist coating and exposure processes. Using clearly defined work practices to eliminate debris at the source is the most effective approach to avoiding problems in later process steps. Scratch and burr defects are more easily formed on RTF foil, so particular care should be taken when handling these materials. Surface Pre-Cleaning Proper pre-cleaning is the key to achieving good adhesion between resist and copper. The typical process flow employs a cleaning step, followed by a copper microetch and acid rinse. Usual process controls including cleaning uniformity, using such tests as water break, Material Preparation and Handling and maintaining bath concentrations and microetch rates The quality of the copper surface over which the resist is being based on measurements made at least once per shift are always applied has a primary impact on the process defect level. In generappropriate. More sophisticated techniques such as contact al, while liquid resists have a better ability to cover surface defects, angle measurement may be useful for troubleshooting purposes, dents, scratches or burrs on the copper surface, the surface condibut are less suitable for routine use. Since tion can still lead to formation of defects variations in copper foil grain structure after etching (predominantly opens). can affect microetch rate, it is important to The key to avoiding such issues is to ensure that coupons used to test etch rate be aware of the different points at which are representative of the product being run the copper surface can be damaged. in the line. Control of the process starts with clear Cases are occasionally encountered acceptance criteria for incoming copper when the preparation of a new cleaner clad laminate. This should be combined bath led to a significant improvement in with appropriate levels of sampling and resist adhesion, even though the water inspection to confirm that quality targets break test showed no indication of insufare being maintained. ficient cleaning. When qualifying a line, Each time clad laminate is handled, data can be obtained to establish a there is another opportunity for creation defined dump schedule for the cleaner, of surface damage. Both procedures and based on resist adhesion as a function of equipment maintenance schedules for cleaner bath age or throughput. sheet cutting and edge cleaning operaWhether the microetch used is a persultions should be examined to ensure that FIGURE 2. Development of resist fate or peroxide based material, creation of surface quality is maintained. coated over a water-soluble marker a consistently rough surface morphology is Debris formed in these operations pattern, allowing clear identification of required. Troubleshooting can be done by is a primary source of contamination in developer breakpoint. 36 PRINTED CIRCUIT DESIGN & FAB APRIL 2008

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Printed Circuit Design & Fab - April 2008 Contents Our Line Market Watch Around the World Happenings ROI Tip Jar Interconnect Strategies Positive Plating Improving Fabrication Yields by Design Solve Design Problems with Signal Integrity Optimization Laser Direct Imaging Made Easy Troubleshooting the Innerlayer Process SIPS Give More to Moore Off the Shelf Marketplace Ad Index BGA Bulletin

Printed Circuit Design & Fab - April 2008

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