The assembly of medical devices continues to increase in complexity due to newer, more advanced electronics technologies. The current trend toward portable wireless and handheld devices, in particular, is playing an important role in this area. Compact medical devices like these generally rely on such sophisticated electronics as digital signal processors, field-programmable gate arrays, and radio frequency and mixed-signal chips. Making printed circuit board (PCB) assembly even more challenging is the fact that these chips are often packaged in fine-pitch ball-grid arrays (BGAs) or chip-scale packages (CSPs) with pin counts ranging from 256 to 1,052 and more, and with pitches of as little as 1 millimeter or even 0.5 millimeter in newer products.
Not surprisingly, higher component and solder joint counts coupled with increasing board densities create a higher probability of defects. This, in turn, leads to lower yields at final product assembly. It is crucial, therefore, that carefully planned and implemented design-for-assembly (DFA) and design-for-test (DFT) procedures be deployed to ensure greater efficiencies and device reliability. The latter concern is especially critical in the medical field where products must be more reliable than in the industrial or commercial sectors.
First Article and Quality Control
The two key aspects of an effective DFA program are OEM approval of a successfully designed first article, and subsequent creation of quality control steps at each stage of that product's assembly process. An OEM-approved first-article PCB is vital because it answers all questions relating to manufacturing, assembly and test. It is the "proof of concept" before a PCB order goes into production, and should not be omitted even if the production run is as small as 10 to 50 units.A sound first article ensures that the transition into production volumes will be smooth, with few, or no questions left unanswered. In addition, it helps boost reliability by making sure component values, orientation and polarity are correct, and that all power supply, cable and mechanical considerations are addressed before the product is shipped. It also ensures all thermal profiles are correctly defined-a critical aspect of assembly. Finally, a sound first article gives an OEM a chance to test and debug a board before the rest of the components going into a product are built and shipped.
By creating the first article with final assembly in mind, manufacturers can shorten the product launch cycle, minimize development cost and ensure a smooth transition into production from the prototype stage. Unfortunately, product-engineering teams often accelerate design and development cycles without taking these kinds of considerations into account-an approach that can lead to disaster at the final production stage. This happens because prototype designers are more interested in quickly getting a product on a test bench to perform proof-of-concept validation, as opposed to emphasizing DFA considerations for long-term product reliability. Remember that there are different considerations for prototype assembly and production assembly. Each requires a specific design skill set and equipment. At the prototype level, engineers are far more concerned with circuitry validity and board functionalitythan they are with the testability and manufacturing guidelines that are necessary for smooth and flawless production.
With regard to quality control (QC), design engineers need to plan for and insert QC steps even as they are producing the first article. For example, an early and vital quality control step at the component-placement stage is verifying that all necessary components are correctly placed on the PCB before it goes to the reflow oven. It is crucial that this work be performed expeditiously. Solder paste has a limited shelf life of 4 to 6 hours. Ideally, corrections should be performed within 1 to 2 hours to maintain solder paste integrity.
When setting these standards, the first article PCB is used as a kind of "golden board" against which every subsequent PCB is inspected and tested. If an automatic optical inspection machine is used during assembly, production engineers can use images of the golden board as the standard by which to inspect all other boards.
Another elementary quality control procedure is verifying that adequate solder paste has been correctly dispensed on each surface mount pad. Again, the first article board can serve as a guide.
Increasingly, design and production engineers are focusing on extra-fine-pitch components like BGAs and CPSs. When PCBs are populated with fine-pitch components, dialing in the perfect thermal profile on the reflow oven is critical. In this instance, quality control involves two actions. First, engineers need to create the perfect thermal profile. Second, components need to undergo X-ray inspection to ensure all BGA solder balls are correctly collapsed and soldered on the PCB.
Dialing in the perfect reflow profile depends on a number of factors, including PCB component density, board thickness and the number of ground planes. A perfect thermal profile also depends on board size and the type of substrate.
Some assemblies include components on the board's bottom side that need to be protected during wave soldering. In these instances, one possible QC solution is to use a fixture that protects those particular components during wave soldering. Doing so means these components can be placed at the same time the rest of the board is assembled. Manually placing these temperature-sensitive components after wave soldering not only increases production times, but can create errors.
After a PCB goes through wave soldering, the next step is to inspect the assembly to ensure there is no bridging on through-hole components. This step ensures that solder is flowing as it should, that the conveyer speed of the wave solder machine is adequate, and that the board is not being subjected to excessive heat. If a board has a number of analog components and grounds, engineers may need to set a slower conveyer speed to allow the solder to adhere to the pins of through-hole components.
Bear in mind that odd-shaped boards, like round, T-shaped and triangular boards, cannot be run on a surface mount pick-and-place machine and therefore require an additional quality control step. There are two options at this stage. One is to perform hand placement, which slows down the process and introduces a high probability of human error. The other, preferred option involves creating a square or rectangular fixture, which can be placed on the pick-and-place line and processed in the same way as a conventionally shaped board.
DFA Considerations
When thinking about DFA and DFT, engineers should use just one side of the board for component placement whenever possible. This speeds up component placement and reduces component test time during in-circuit test and flying probe testing. It also reduces rework and debugging times.With regard to DFT, engineers need to place adequate test points across the board for complete access and coverage during flying probe, in-circuit test and direct probing. Engineers also need to pay attention to issues like height and slot dimension tolerances to eliminate any possible confusion at the component placement, testing and debugging stages.
At fabrication, the OEM or electronic manufacturing service (EMS) provider should use panelization whenever possible, as opposed to fabricating one board at a time. This is especially important with smaller boards of 30 square inches or less. By taking the panelization route, fabrication and assembly processes can be performed faster and cheaper.
In addition, engineers should specify proven, time-tested components whenever possible to reduce debug and test times and make the product more reliable. Good DFA practices require using commonly available components with proven specifications and a solid track record in other system applications.
Of course, OEMs may still decide to include special product features requiring custom components. In cases like these, OEMs need to take care not to become overly dependent on just one or two suppliers, because doing so severely violates good manufacturing practice. Remember that these suppliers' production times depend on their capacities and production levels. Delivery times can easily fall behind, which may cause delays in OEM production schedules and shipments. Custom components also run the risk of causing problems in the field, because they haven't necessarily been exposed to the test of time and applications.
Careful monitoring of equipment-related features and physical tolerances represents another DFA consideration. An example is a machine used to drill holes in the PCB during fabrication. Drill bits have a tendency to wander. Therefore, design engineers need to ensure production processes will maintain tight tolerances, especially when drilling smaller holes.
Perhaps one of the more critical DFA considerations comes when creating mixed-signal designs employing both digital and analog technologies on the same board. Analog components need to be clustered together in a module and digital components must be clustered together in another module. Their respective power and ground planes must be correctly placed to ensure clean, solid signal transmission.
Clock shielding is particularly crucial in medical electronics products to avoid corrupted signals and maintain precise clock speed. In addition, a well-thought-out DFA strategy should reduce signal-to-noise ratios, correctly define power and ground planes according to component placement, and shield all high-speed digital signals. Failure to accommodate these and other mixed signal considerations can result in unacceptable noise levels, which may then require expensive product redesigns, costing more time, engineering resources and money.
As a side note, while medical electronics OEMs are exempt from the European Union's Restrictions of Hazardous Substance (RoHS) and lead-free compliance laws, it is highly probable that they may have to abide by lead-free restrictions in the near future. This will result in the need to use a host of different PCB materials, surface finishes, solders and components. Prudent OEMs will start now to engage with lead-free-savy EMS providers to be ready to transition products from eutectic to lead-free solder.
Real-world Dilemmas
As a long-time EMS provider, NexLogic Technologies has had the opportunity to work with customers at many different stages in the design process. This, in turn, has allowed us to experience firsthand the benefits of DFA.In one case, NexLogic produced a 16-layer PCB populated with multiple BGAs for an OEM that makes networking gear. The resulting prototype underwent the necessary DFA and QC steps, complied with OEM specifications and successfully operated in the field. However, OEM engineers later learned they could not adequately test the BGA section because their specifications called for tinting the vias both at the top and bottom side of the BGA. This, in turn, prevented test engineers from correctly probing and testing the BGA. To solve the problem, the company decided to tint the vias on just the component side to provide access to bottom side BGAs for testing purposes.
Another customer created an electronics subsystem for a cancer cell imager that was not producing a clean enough digital signal for doctors to obtain a detailed view of the cells. Closer examination revealed that a host of subsystem issues was responsible for the problem: Mixed signal circuitry distorted digital signals; stringent subsystem fabrication was lacking; and there were minimal test procedures for achieving the reliability the product demanded. At one point, a DFA team determined that the subsystem's analog signal circuitry was not correctly separated and shielded from digital ICs, thus creating noise and distorting digital signals. The solution involved correctly splitting the analog and digital power and their corresponding ground planes. Engineers also incorporated changes that shielded the digital signals from noisy analog circuitry to minimize or eliminate distortion.
Yet another medical OEM customer was found to be using an obsolete 10-year-old memory module. It was highly inefficient for storing data and contributed to slower digital signals. The customer eventually replaced the old eight-chip module with a more advanced, faster and powerful two-chip memory module to substantially increase digital signal speeds.
Multiple Team Interaction
Ultimately, successful DFA relies on multiple team interaction if it is to succeed. The different design, fabrication, assembly and procurement teams assigned to a given project must precisely tune into a project's specifications. Design engineers must be fully aware of certain assembly aspects, and likewise, assembly engineers must understand a project's design limitations in detail. These teams need to know the particular areas that make assembly and testing go smoothly. They also need to anticipate problems that can arise during production and in the field, with an eye toward resolving those problems as far in advance as possible.Working in tandem, the different teams can address most issues and questions at root-cause levels, before they become set in stone. For example, if the design team is specifying a component that has an extended lead-time of, say, 12 weeks or more, the procurement team, working in parallel with designers, can flag this issue and suggest a replacement component that is more readily available.
The result is a competitive edge for the OEM in the form of faster product delivery times, fewer procurement headaches, reduced rework and easier field servicing. In fact, DFA can save an OEM 10 percent to 30 percent in pick-and-place, testing and debugging times-which translates into major cost savings. Simply put, a poorly designed medical product, or one that hasn't been developed using DFA techniques, will incur more design and assembly rework time, therefore incurring greater cost in assembly. Conversely, DFA techniques based on standard, proven components and practices will lower costs and make the process more efficient. Products can go smoothly from design to fabrication, assembly and testing. This will reduce time to market as well as make the product more robust and reliable.
