Auditing a Fabricator Print E-mail
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Written by Zulki Khan   
Thursday, 31 July 2008 19:00

Guidelines for prototypes and production boards overlap, but for offshore production the rule is caveat emptor.

PWB procurement can be categorized as either procuring prototype or production boards or both. But the common denominator is both require assurances the fabricator has the capability and capacity to deliver the required quantities on time and with high quality. A PWB order ranging from a few units to a few thousand can be placed with a highly capable and reliable vendor offering excellent pricing. However, if that vendor has capacity issues, meeting OEM product deadlines will create major challenges. Therefore, it is prudent to qualify multiple vendors as a Plan B to avoid PWB purchasing conditions that might arise when relying on a sole supplier.

Each case, whether prototype or production, involves distinctly different buying patterns and requirements, and with them, issues OEM purchasers should be aware of. Different parameters are associated with prototype and production PWBs. On one hand, prototypes are smaller in quantity and require faster turnaround, and not all potential problems may be resolved prior to product development. Conversely, production PWBs demand all evident and not-so-evident issues be resolved at the outset. And for production boards, turnaround time is less an issue than for prototypes, yet pricing moves from second-tier status to paramount.

However, there’s much more to consider than meets the eye. Therefore, OEM purchasers must navigate a tricky, often problematic and challenging path to ensure delivery of high-quality, high-reliability, fully functional PWBs. And a fabricator’s equipment, capability and expertise are foremost considerations.

It’s important to perform a detailed audit to determine the age of fabricator’s equipment, especially when procuring prototypes. This is important because newer prototypes push technology limits and mandate faster and better equipment. Outdated equipment can lead to a large assortment of costly problems for leading-edge PCB prototypes, particularly those using the latest components such as fine-pitch BGAs and CSPs. For example, a leading-edge board can require an inordinately high number of vias, especially if used in a handheld product consisting of densely populated components. PCB complexity of this level, for instance, demands advanced drills capable of holes of 0.006" and less with utmost precision to avoid half-moons and drill-to-pad misregistrations. The list below shows a number of key questions to ask the prospective fabricator:

  • Does it handle cutting-edge technology (e.g., 0.001"-0.002" lines and spaces)? Does it use UL-approved materials? Mixed material?
  • How new is its equipment?
  • How often is the equipment calibrated?
  • Is the company certified for special cases, such as plating heavy (e.g., 4 oz.) copper?
  • Is it certified by UL, and for what parameters and specifications?
  • Can it effectively perform flying probe and bed-of-nails testing?
  • Can it perform via-in-pad technology?
  • How fine are its drilling capabilities (e.g., 0.003" laser drilling)?
  • 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).

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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 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.

Therefore, it’s important to have process-oriented controls and checks and balances, as well as disciplined corrective actions. Process traceability is critical for locating and correcting even the most difficult problems as well. Take, for example, a short between power and ground planes in a 16-layer board. Unless proper process traceability is in place, it is difficult to identify and correct the problem, especially if the board has been assembled.

In situations like this, an inordinate amount of time and money is spent because assembled components add a layer of complexity to problem identification, testing and debugging. On the other hand, such issues would have been easier – and less expensive – to troubleshoot provided advanced AOI had been deployed. Advanced AOI can be used to locate opens, shorts and registration issues when innerlayers are constructed during fabrication. For example, when equipment used to print silkscreen on a board is not calibrated, it can result in creating a non-conductive paste or covering on a surface mount pad, creating a current blockage (Figure 2).

Image

First-article inspection for prototypes and production is the best way to uncover problematic areas in these and other instances. Equally important, it is an excellent indicator of how efficient a fabricator’s processes are. High first-article yields – say 90%-plus – indicate processes are well defined and controlled. Low first-article yields speak loudly that a fabricator’s equipment, personnel and practices are out of kilter. First-article problems can stem from poorly calibrated equipment, callous or untrained personnel, lack of preventive maintenance and a host of other reasons.

Therefore, to avoid PWB procurement issues, it is prudent to institute a well thought-out policy and associated purchasing practices and, at the same time, anticipate a host of adverse purchasing conditions. The worst-case scenario never happens, unless, of course, it does. For example, an undetected minor board error can cascade into a complete PCB order remake. For example (Figure 3), a defect results when part of a trace is clipped by poor process control during fabrication. At times like this, a remake can involve orders ranging from 500 to 5,000 boards. Besides lost time, extra shipping costs cannot be recovered if the board is fabricated overseas. Further, returning boards to an overseas fabricator involves more time and hassle because a laundry list of certifications must be provided to customs officials in these countries.

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Zulki Khan is president and founder, NexLogic Technologies Inc. (nexlogic.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

 

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