Technical Whitepaper NexLogic Technologies, Inc. 5 Key Success Factors For Medical Electronics Printed Circuit Boards

Transcription

1 Technical Whitepaper NexLogic Technologies, Inc. 5 Key Success Factors For Medical Electronics Printed Circuit Boards Author: Zulki Khan, Founder & President NexLogic Technologies Inc. Contact: NexLogic Technologies, Inc Brokaw Road info@nexlogic.com (408) July 2, 2012 NexLogic Copyright 2012 Page 1

2 5 Key Success Factors For Medical Electronics PCBs The five success factors for medical electronics PCBs are: Quality Robustness Repeatability Traceability Reliability Today, medical electronics OEMs are relying on advanced technologies to make their newer products and systems more efficient and meet the demands of their respective markets. They depend on their contract manufacturers (CMs) and EMS Providers to assure them of these top five success factors. In particular, quality, repeatability and reliability must be at the heart of welldisciplined rules and procedures at design/layout, fabrication, and assembly. On top of this, stringent compliance with ISO 13485:2003 further strengthens these requirements, and puts in place vital assurances for producing effective, reliable medical electronics PCBs. ISO is a specific Standard defined as a comprehensive management system for the design and manufacture of medical devices. Similar in some instances to other ISO standards, attention is placed on continuous improvement in process and quality systems. Also, the Standard places special emphasis on cleanliness procedures as a medical product undergoes various development and production stages. The basic difference between regular ISO 9001:2008 and ISO 13485:2003 is that the latter Standard requires process and design control, records retention, NexLogic Proprietary Page 2

3 accountability, and most important of all, traceability. Traceability is the most significant aspect for the medical electronics manufacturing community. It means contract manufacturers (CMs) and EMS Providers must have a well-organized and robust system for tracking purposes over an extended number of years. According to the Standard, PCBs must be serialized and allocated in batches. In other words, medical electronics products being built can be traced according to their associated lot and batch codes. Plus, at the assembly and manufacturing stage, components populating a particular PCB can be traced when a problem surfaces with that component or set of components. In cases like this, a certified ISO CM or EMS Provider must have a disciplined, well-documented paper trail that points to a particular supplier associated with that bad component or set of components. Also when a medical electronics product is recalled for any reason -- whether for failure, FDA compliance requirements, product upgrading, or any other -- the CM or EMS Provider must have a clearly defined set of procedures for traceability to lot boards, batch boards and date code. Plus, a bad component must be quickly identified without incurring any time-consuming obstacles, as well as its particular supplier, and that component must be precisely linked to a particular batch, lot, and date code. PCB DESIGN/LAYOUT More so than ever before, ISO plays a crucial role in medical electronics as previous desktop or cart-carried systems become increasingly portable and newer, innovative devices are handheld and wireless. NexLogic Proprietary Page 3

4 Consequently, Bluetooth and RF technologies are being applied more and more to cordless, portable, and wireless medical electronics products. At the PCB design/layout stage, this means, among other things, that component placement must be exact and assure signals are clean. Also, in earlier generation larger products, their metal chassis could be used as ground shields and to radiate heat. But that becomes extremely difficult with portable handheld medical electronics devices since they use plastic and light body materials. As a result, thermal analysis and performance are taking a front row seat for these handheld medical devices. This requires a newer set of calculations, and they need to be performed based on new device packaging. But regardless whether a medical electronics PCB is small or larger, other important aspects must be factored in at PCB design layout. At the top of this list are the following: Checks and balances Component selection and cross referencing when needed Split planes Complete fabrication and assembly drawings Test coverage on the board Checks and balances are categorized as major and minor because even the smallest mistake can evolve into big problems. Big and small problematic areas can surface during assembly, but when they are latent problems, they happen out in the field, posing even greater costly issues, especially at customer locations. To take care of this matter, it s always a good idea to buddy up with an assembly tech associate to double check all the key areas of a layout. The new NexLogic Proprietary Page 4

5 perspective those extra set of eyes provides can double check proper component footprints and polarities, ensuring a silk screen is properly associated with a particular component, and pin numbers and sequencing are accurate for complex components. When it comes to component selection at the design phase, OEMs specify components and related part. At this point, it s best that the experienced PCB design work hand in glove with OEM designers to screen the bill of materials or BOM. This way, they can reach consensus on using components with correct tolerances and availability is not a problem. For example, sometimes, components with 2% 5% tolerances should be used rather than those with 10% tolerances. There s extra cost involved, however the lower tolerances assist in maintaining higher quality and reliability. Part of the selection process at layout is carefully selecting an alternate component. The OEM may have chosen a particular field programmable gate array or FPGA. However, now it s not available to comply with the design budget. The burden falls on the PCB designer to search out substitute components to replace the one the OEM specified. Criteria for finding this alternative component include performance history reviews, careful datasheet study, and investigating whether or not it has higher failure rates than usual. As for split planes, Fig. 1 shows how power and ground planes are properly split to cut down on noise and crosstalk. Also, it s important to remember that low signal-to-noise ratios or SNR are unacceptable, especially in medical electronics PCB designs. SNR disrupts signals, meaning that a critical medical electronics system can misread a patient s symptoms and deliver an inaccurate diagnosis. Noise generating high frequency devices also pose a corollary to SNR. These particular devices must be kept away from high-speed NexLogic Proprietary Page 5

6 digital signals. If not, the resulting unwanted noise adversely affects those highspeed digital signals. Figure 1: Split Planes The solution in cases like this is to use as many ground layers as possible. The objective of these extra ground layers is to suppress noise and manage SNR at a desirable level. The tradeoff is a slightly higher cost. But those extra dollars are in effect a major investment in long- term quality and reliability. That means medical electronics products with extra ground layers don t incur failures in the field as much as those with a lower number of ground layers. Meanwhile, the experienced PCB designer must develop and produce complete and highly detailed fabrication and assembly drawings. These drawings are vital to avoid costly guesses and mistakes when a medical electronics PCB is fabricated and subsequently assembled. Assurances are made as to revision or REV levels, and they are clearly noted on all documents. If changes occur in the original design, the REV is rolled, meaning a specific portion of all layout documentation is properly released. When REV levels are NexLogic Proprietary Page 6

7 correctly assigned to fabrication and assembly drawings, engineering change orders (ECOs), and other associated documents, document control and their related departments operate efficiently. TESTING The savvy PCB designer knows in his or her heart that an ample number of assigned test points on the medical electronics PCB ensures design-for-test (DFT). Like they say, the more, the better, and when it comes to a medical electronics PCB, it couldn t be truer. The larger number of test points on the board, the higher percentage of PCB testability. The target is 80 to 90% and sometimes it s even better to get beyond 90% coverage such as flying probe testing, Fig. 2. But as was stated earlier, medical electronics PCBs are shrinking to accommodate emerging handheld portable/wireless products. In situations like those, the experienced PCB designer takes on these new challenges and discovers methods to manipulate board area versus design criteria to get those necessary test points on those smaller boards. NexLogic Proprietary Page 7

8 Figure 2: Test point coverage tested at flying probe Also, when you have the medical electronics OEM and its EMS Provider or CM working in close cooperation, their engineering teams can more expertly define a battery of tests to significantly increase a medical electronics product s reliability in the field. One has to take into consideration the inordinate cost of downtime for such medical systems as x-ray or heart scanning. Also, consider there may be liabilities when those systems fail and result in big costs to OEM and its healthcare customer. In-circuit testing, as shown in Fig. 3, is used to minimize those costly issues associated with mature medical electronics boards. In effect, ICT ensures accurate assembly and manufacturing of medical electronics PCBs. This tester transmits a signal throughout a PCB to verify all active components are correctly operating. Once a component failure is detected, it is flagged and replaced so that the product can then be re-tested. NexLogic Proprietary Page 8

9 Figure 3: Agilent ICT-3070 Once ICT is performed, functional testing might be mandatory for a particular medical electronics product. In this case, the design should be tested on the bench and in the lab before it is sent to mass production. Sometimes, a PCB is exposed over time to a battery of tests. Also, a PCB undergoing R&D should be run through different humidity and temperature cycles. Additionally it should undergo a different battery of tests to assure a robust product build. A medical electronics PCB may also require special testing other than functional and flying probe. Those could be burn in testing that subjects PCBs or fully functional subsystems to different temperature cycles. By doing so, it ensures that the complete simulation of the operating environment is a medical facility is simulated. NexLogic Proprietary Page 9

10 Here, boards are subjected to a 24 to 48 hour test cycle of anywhere from -40 C to 85 C temperature cycling. In effect, this process accelerates the aging process of the circuitry to make sure the product fully operates over time. Some tests are designed so that one year worth of circuit life is simulated by a 24 to 48 hour testing cycle. FABRICATION As stated earlier, quality is of paramount importance for medical electronics PCBs, therefore, inexperience, shortcuts, and undisciplined engineering are highly unacceptable. In this regard, PCB fabrication house selection requires ISO and/or mil spec certification. In fact, it is ideal to process medical electronics PCB using mil spec certification because it is a higher certification compared to those for medical electronics. To further add to quality, incoming QC should be maintained when fabricated boards arrive at the CM s location. Included are checking such things as hole tolerances, cutout verifications, board dimensions, and surface finishes, among others. As for board finishes, a CM s design and assembly engineering should have a good handle on various trade-offs and characteristics dealing with HASL, ENIG, and OSP to keep from making assembly errors. QC should also look into test coupons, board cross-sections, TDR reports, and material certification before PCBs are released on the floor for assembly. NexLogic Proprietary Page 10

11 Correct impedance control calculations must be carefully implemented at fabrication to obtain desired results. Otherwise, unwanted signal noise leads to erroneous results in a medical electronics system. Moreover, there are proper procedures and processes at the fabrication level that must be strictly followed. Proper baking cycles must be performed with no shortcuts taken. Special consideration needs to be given when mixed materials are used. For instance, careful thought must go toward mixing FR4 and Rogers layers because issues could arise when these materials are laminated together due to the different temperatures profiles for lamination cycles. ASSEMBLY The correct thermal profile is critical, especially if the medical electronics product is a lead-free. With RoHS exemptions ended for medical products, OEMs have transitioned to lead-free. But certain precautions still dominate assembly. A particular profile must be correctly designed and implemented for a given medical electronics PCB. It must be remembered that thermal profiles are not created equal. One size does not fit all. Further, the CM or EMS provider must exercise special expertise when hybrid products containing leaded and lead-free components are subjected to re-flow. If care is not applied, an entire PCB project can be damaged and those boards must be scraped. Developing a correct thermal profile, as noted above, together with properly defining a stencil design and solder paste dispensing make up the big three aspects for medical electronics PCBs at the assembly stage. When this NexLogic Proprietary Page 11

12 major trio is implemented, about three quarters of the potential issues during rework and touch-up rework phases don t occur. The medical electronics OEM can be further assured of achieving quality and reliability by the advanced systems and equipment the CM or EMS Provider has on the assembly and manufacturing floor. The more automation, the less the human error creeps in. For example, as shown in Fig. 4, x-ray provides the OEM assurances all joints are proper. AOI is used to make sure of consistency. The first article inspection or FAI makes inspection and QC more reliable, repeatable, and faster. Figure 4: X-Ray Machine Aside from assembly automation, the experienced CM or EMS Provider places a high priority on regularly checking its assembly processes and procedures. The objective is to maintain assembly strengths at the highest levels possible at all times to efficiently produce quality medical products. Plus, those processes are made repeatable with minimal effort so that at QC stages, there are no surprises. NexLogic Proprietary Page 12

13 ISO Binds It All Together Aside from traceability discussed above, risk management and approved supplier list or AVL are two other major ISO entities. ISO places considerable emphasis on risk management activities or design transfer activities during and after a medical product development cycle. That alone differentiates it from less stringent and demanding ISO standards relating to commercial applications. Medical product risk management is classified two ways. First, it deals with identification, assessment, and prioritization of risk that could be associated with these specific products. Secondly, coordinated and economic resources are applied to minimize, monitor, and control the impact of unfortunate events that might otherwise occur if procedures aren t put in place and exercised. Risk Management involves having such procedures in place as an internal audit checklist and a periodically generated internal audit report. Typically, this occurs once every six months when an internal audit is conducted. Also, once a year, there is a surveillance audit with ISO re-certification every three years. Failure Mode and Effect Analysis or FMEA is another aspect of risk management. In effect, it is a tool to document risk management s requirements. With FMEA, medical electronics OEMs can follow their ISO product through assembly and manufacture, allowing them to periodically review it to assure risk management is under control and intact. AVL calls for maintaining an updated record of all suppliers. Plus, it demands that those suppliers be ISO certified so that a CM or EMS NexLogic Proprietary Page 13

14 Provider can effectively manufacturer ISO based PCBs for medical electronics OEMs and have the necessary component and device tracking data for efficient traceability. In summary, it s important to know that ISO deals with five specific aspects when a medical product is designed and assembled: Identifies, characterizes, assesses impending damaging threats. Assesses vulnerability of assets based on identified threats. Determines amount of incurred risk. Identifies ways to reduce risk. Prioritizes risk reduction based on specific strategies. Moreover, the Standard calls for specific requirements dealing with inspection documentation, process validation for sterile medical devices, and verification of how effective preventive and corrective actions are. ISO also determines the sequence and interaction of processes involving medical product design and assembly. For each medical device model, an EMS provider, CM, or OEM is expected to maintain a file either containing or identifying all documents that define product specifications and quality management system requirements. These documents define the complete manufacturing processes. If applicable, medical device installation and servicing processes are included as well. In effect, ISO establishes a documented procedure for a feedback system that provides early warnings of quality problems and input into corrective and preventive actions before subjecting a medical product or PCB into production. For further information and a Free DFM Analysis on Your PCB in Medical Devices call us at NexLogic ( ), us at info@nexlogic.com or visit our website:. NexLogic Proprietary Page 14