Circuits Assembly - August 2008 - (Page 24)

PWB Procurement Figure 1. Poorly calibrated flying probe tester can miss board test locations, plus leave unsightly markings. Figure 2. Uncalibrated silkscreen equipment can result in the creation of a nonconductive paste or covering on an SM pad, causing current blockage. Figure 3. A defect occurs when part of a trace is clipped by poor process control during fabrication. • What kind of aspect ratio can it handle (e.g., 10:1, 20:1, 25:1)? • What is the maximum layer count it can process? • Does it offer all processes? If not, which processes are performed in-house and which are contracted out? • How efficient is first-article inspection for prototypes and production PCBs? A lack of proper equipment calibration poses another slew of potential problems. Keeping equipment calibrated per OEM specifications is critical to maintaining proper tolerances. Some machines require calibration every three months, others semiannually, and still others each year, depending on use, the number of mechanical parts involved in the process and other considerations. Simply put, if it is not calibrated, chances of inducing errors increase, resulting in an out-of-spec product. An example is a poorly calibrated probe tester. Let’s say it tests an extremely fine surface mount pad, but hasn’t been calibrated in a year. As a result, the probes might miss predetermined points on the surface mount pads, thereby failing to test the critical test points and locations, not to mention making unsightly markings at those locations (Figure 1). Consistent quality and reliability is the key goal of the audit. Select a fabricator that’s compatible with the PWB type and complexity. For instance, if the product calls for 16 to 20 layer boards, and the fabricator is capable of producing 10 to 14 layers, it might not be a good candidate for the higher-layer counts. Among the top considerations is whether a fabricator offers all the critical processes needed to properly fabricate a complex board. If not, which ones are performed in-house 24 and which are subcontracted? Processes normally subcontracted by mediocre fabricators include silver-filled via, laser drilling, gold immersion plating, and sometimes testing. As a result, the fabricator loses some control and, occasionally, incurs longer delivery times since the subcontractor generally sets the pace for performing those processes. Take, for example, a 48-hr. quickturn immersion gold plating job. A plating subcontractor can take half a day for plating, but the fabricator has just two days to complete the entire PWB. Consequently, 20 to 30% of time has been lost outsourcing, whereas that particular process could have been accomplished in-house in 5% of the time. Local or Offshore? The main question relating to production PCBs is whether a job can satisfactorily be performed domestically, or whether it must be shipped offshore. If subcontracted to Asia, for example, certain checks and balances should be implemented: • Is there US (or Western) representation? • Are incoming board inspectors at EMS companies alerted to overseas PCB shipments? • Does the fabricator provide required material certifications, time domain reflexive (TDR) test and other relevant certifications? • Are board cross-sections available to ensure product quality? • Is there a backup plan with multiple, reputable and qualified fabricators? • Do you have a thorough understanding of the fabricator’s outsourcing practices and subcontractors? When contracting to a fabricator overseas, it’s ideal to find out if this shop has local US representation. If so, vendor-customer communications will be enhanced, which increases the chances a PWB order will be properly and efficiently fabricated. Dealing directly with individuals in other countries and languages increases the chances for misunderstandings, delays, obstacles and product problems, and results in communicating with vendors at odd (night) hours. However, with or without US representation, an offshore fabricator can be a good candidate if it has excellent customer references. In cases like these, an OEM or EMS provider cautiously can begin working with that fabricator, and then proceed with an increasing level of confidence as results dictate. The best of all worlds, however, is to conduct an onsite audit when using an overseas fabricator and follow the questions above. A major aspect of such an audit is determining a fabricator’s process strengths, capabilities, capacity issues and outsourcing practices, especially for production PWBs at offshore locations like China and Taiwan. Some fabricators are known for outsourcing large numbers of processes. When a process subcontractor is used, extra care should be taken to maintain and verify board quality from that supplier. For instance, if process after process is subcontracted without intermediate checks and balances in the middle, errors introduced at early stages could be magnified during latter stages of fabrication. When multiple fabrication processes are subcontracted, so many personnel are involved that it is virtually impossible to detect root causes of such problems. Plus, in most cases, none of the several subcontractors will assume responsibility for problems. Continued on pg. 42 Circuits Assembly AUGUST 2008 circuitsassembly.com http://circuitsassembly.com

Table of Contents Feed for the Digital Edition of Circuits Assembly - August 2008

Circuits Assembly - August 2008 Contents Caveat Lector Industry News Market Watch Talking Heads Screen Printing Better Manufacturing Auditing a Fabricatior Cutting Machine Programming Time Simultaneous Acoustic Imaging and Surface Mapping Tech Tips Soldering Test and Inspection Process Doctor Pb-Free Lessons Learned Getting Lean Component Advances Product Spotlight Ad Index Assembly Insider Techincal Abstracts

Circuits Assembly - August 2008

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.