|Highly automated production floor. |
Finishing the layout of a printed circuit board (PCB) might have led to cases of bad nerves, sweating, and sleeplessness 20 years ago. But even with looming customer deadlines for contracted PCBs, today's tools such as PCB design software can make this task considerably more straightforward than 20 years ago. Designing and manufacturing the perfect circuit board requires harmony among PCB designer, PCB fabricator, and PCB assembler. The circuitry must be designed to provide a functional product, while the layout must encourage manufacturability. If these forces work against each other, the end result can be a nonworking circuit board, or an assembly process that is so expensive, it will be priced too high to meet the competition.
A PCB designer must ensure that proper circuitry is being supplied, with complete connections for all SMT component lands, holes, and traces. Via holes must be properly placed, with hole sizes matching lead specifications for components from different manufacturers. SMT landing points must match the manufacturer's recommended layout, and all pitch and spacing must match the dimensions of the component packages being used in the PCB.
The Perfect Handoff
Creating a perfect circuit board requires proper handoff from a circuit designer to an assembler. A bare board is not functional until it is fully populated. Often, a PCB fabricator will provide a design-for-manufacturability (DFM) report which will generally verify complete connectivity within the PCB. An assembly house can also provide a DFM prior to production, specifically comparing footprints of parts to a PCB layout. Another report completed after production, often referred to as a post-assembly report (PAR), can also be offered by an assembly house. This report summarizes all production-related issues discovered during an assembly process, along with recommendations for changes.
What makes a perfect circuit board? For one thing, the use of as much SMT as possible, within reason. Headers or connectors that must undergo the repeated stress of plugging and unplugging will require the mechanical strength of thruhole solder joints. But SMT joints are much preferred over thruhole interconnections. There are many reasons for this, with the speed and efficiency possible with automated pick-and-place possibly the main reasons, with performance that greatly outperforms manual and thruhole automated installations.
Edge Clearance and Elbow Room
When possible, circuit parts should be kept away from the outer edges of a PCB. Components near the edge have a greater risk of being damaged from the depaneling process, or from being sheared off on a holding rack or conveyor fingers. Based on experience, at least 2mm edge clearance is recommended for components mounted on a circuit board.
Naturally there are times when connectors must protrude over the edge of a circuit board. In such cases, a routed panel design with tabs is recommended. An automated depaneling machine can remove the tabs easily. Manually snapping the tabs, even with mousebites present, creates risks of haloing along the edges, and a goal is to have no components within 2mm of any circuit-board edge.
|The perfect board may be incredibly complex in design. |
Conductive anodic filament (CAF) is becoming a huge problem as PCBs and electronic components continue to shrink in size. With a problematic design coupled with a substandard material weave, ionic impurities aided by moisture content can create a pathway between two conductors. This pathway can be from hole to hole or even from layer to layer in a circuit design. The smaller the distance between hole walls, the more likely it is the problem will occur. Without CAF-resistant material preventing the migration, CAF may occur and lead to product failure.
Connector leads with a 50mil pitch have a much greater chance of suffering CAF effects than connector leads with a 100mil pitch. By maintaining a certain distance between anodes and cathodes, for example, a designer can reduce the possibility of CAF effects. These effects increase with smaller distances between anodes and cathodes combined with inferior material weave. Also, applying excessive heat from an assembler will increase the likelihood of failure.
Fiducials are the lifeblood of component placement. They are instrumental in proper registration for automated part placement. Too often, they are excluded from a PCB layout, causing headaches for pick-and-place programmers. When possible, fiducials can be added to a PCB's rub rails, but often rub rails are not required I a design. As a general guideline, a minimum of 2 (preferably 3) fiducials are included on each side of a PCB, in the outermost corners of the board. The PCB should include a keep-away area, located away from nearby SMT lands or holes. Although they can be created in a variety of shapes, a circle of 1 to 1.5mm is recommended as fiducial. A "perfect" PCB has three 1mm fiducials on each side in the outermost corners.
Mask dams are also extremely important in the PCB manufacturing process. Lack of a proper solder mask between SMT pads raises the potential for solder shorts and costly rework. Since the pitch on PCBs is becoming smaller and smaller for many parts with miniaturization, a lack of space between pads makes it difficult to include masking material in a PCB assembly. However, solder resist is necessary to greatly reduce possibilities for shorting. Typically, most PCB houses can maintain a mask dam as small as 3mils, so many are reducing the mask keep-out area to zero, creating as much space for the dams as possible. The "perfect board" has a minimum mask dam dimension of 8mils.
For improved reliability and performance with thruhole components, especially those with dual row headers, they should be oriented so that they run parallel to the wave solder conveyer, since the risk of solder shorts increases dramatically when connectors or headers run sideways over the wave solder conveyer. Often connectors are oriented in both directions, forcing a PCB assembly-house to creatively find ways to prevent shorts. This can involve trimming the leads prior to installation, or even designing a tool to run the circuit board at a 45° angle over the wave. When designing the layout of the PCB, or the entire panel, special thought must be given to the method of thruhole soldering.
BTW, Inc. has extensive experience in bridging the gap between PCB fabricators and end customers for those PCBs. The company uses an overhauled NPI process, twice gathering all of its department heads to review and analyze their process before a product reaches the production floor. Immediately before release, these department heads scrutinize the paperless work instructions that the production staff will use, encouraging a smooth handoff between engineering, documentation, and production. During this time, a customer can clarify any discrepancies that were discovered. An NPI process engineer has open communication with the board house to design an optimum circuit array, which can help lay the groundwork for a smooth assembly process.
Low Volume, High Volume
Although BTW specializes in lower volume and high-mix electronic assemblies, the company has the capacity for high-volume orders as well. With twin SMT lines capable of placing 40,000 SMT components per hour, high volume is well within the company's capabilities. With customers in the communications, utilities, medical, and transportation fields, the company's range of customers has helped the firm earn the recognition of one of the "Top 25 Veteran Owned Businesses in Minnesota."
With over 20 years of outstanding service, BTW, Inc. offers a wide range of manufacturing services, including PCB and mechanical assembly, clean-room manufacturing, global supply-chain management, conformal coating, test development, and third-party logistics. By designing partnerships, BTW has worked with clients from concept through final production. With a highly experienced engineering and management staff, the company is capable of guiding a customer through the entire process from design through production, and is well suited to manufacture even the most complex circuit designs.
BTW, Inc. provides a DFM report during the quoting phase and a post-assembly review (PAR) after the initial production order. This PAR can include recommendations for a more cost-effective design for future redesigns. Receiving feedback from the initial design phase to the final post-assembly step, many customers have taken advantage of this benefit, streamlined the PCB design, and kept labor costs to a minimum. All this is performed in pursuit of the perfect printed circuit board.
Contact: BTW, Inc., 11551 Eagle St. NW, Suite 3, Coon Rapids, MN 55448 763-767-4625 fax: 763-767-4635 E-mail: firstname.lastname@example.org Web: http://www.btw-inc.com