PCB design is vital to the efficacy of innumerable electronic products. A reliable product and marketplace success are the ultimate rewards for careful consideration of all design issues. Choosing an appropriate board level enclosure is just one element of successful design efforts, and the choice must be carefully considered along with other crucial issues including the operating environment, the overall volume of the product to be manufactured, the assembly method to be used, the planned test and inspection methodology, and PCB and equipment layout.All too often, decisions on RFI enclosures, like power supply choices, are left until the end of the design process creating a situation that makes their addition more difficult to accommodate. As a result, the enclosures may interfere mechanically with other areas of the design.
THE 3 Ds—DESIGN, DEVELOP, DRAW
An efficacious mindset underlying the design and development of PCB level cans and systems can be summed up in three crucial steps: design, develop, and draw. Active communication and consultation between the enclosure customer and the design team are crucial. Look for an enclosure manufacturer that will provide initial design guidance, ongoing advice, site visits, prototyping, sample production, finish and thickness options, forming, assembly, and reevaluation for cost reduction. Cost containment is certainly crucial to the profitable marketing of a finished product. A structured effort combining a detailed design plan and customer input can result in the desirable goal of arriving at “defined costs for defined ends.”FORMAT CHOICES
When choosing the format of the enclosure to be used, several factors must be considered. Exactly what is being shielded? What is the exact nature of the “culprit” that necessitates shielding? Will the customer require access to the components inside of the enclosure for rework, testing, inspection, or adjustment after the enclosure has been placed on the PCB? Will the enclosure be through-hole or surface mounted? What is the expected volume of production, and will that volume justify the costs of machine placement? How many areas of circuitry will require shielding, either individually or from each other? Are individual enclosures or multi-cavity enclosures the best choice in this application? Is the final product likely to undergo shock, vibration, or package-drop testing?SHIELDING FORMATS
Careful consideration of the questions mentioned above will assist in the choice of the most appropriate, cost-effective shielding format for a particular application. Among four-sided enclosures, there are a number of choices, each suited to varying application requirements. A four-sided enclosure with a spring-fingered lid consists of a four-sided fence with an array of pins on the PCB edge that are used to solder the fence to the PCB using hand soldering, wave soldering, or pin-in-hole reflow soldering. A spring-fingered lid is generally used with this type of enclosure. It is the best form of removable lid if the fence height is sufficient to accommodate the fingers. Spring fingers may be offered in varying sizes such as standard height or low profile. An internal spring finger is another option in situations where there is insufficient clearance on the outside of the fence to allow for external fingers. Also, external and internal fingers can be mixed as long as they are in the same format on opposing faces. Another enclosure option is a four-sided surface-mount enclosure with spring fingers. This type of enclosure is the same as the normal four-side enclosure, but lacks the fixing pins. It is usually seam-soldered to a continuous track on the PCB. When gauging the height of the fence in relation to the length of the spring fingers, a designer must factor in the thickness of the solder fillet at the base of the fence. An alternative to the even continuous seam-soldered fence is to castellate the PCB board edge of the fence. This choice reduces the amount of solder used to attach the fence to the board and also provides clearance for tracks to cross the fence boundary without resorting to specific track clearance mouse holes or multi-layer PCBs. If the fences are to be machine placed onto the PCB, a pick-and-place target may be needed. This would probably be combined with a punch-and-press format fence (Figure 1).
Figure 1. Punch-and-press format fence with pick-and-place target.Four-sided PCB enclosures are also available with plain, folded lids. See Figure 2. This type of lid is generally less costly to produce, especially during the development phase. The only drawback to this design is that there is no guarantee of a connection between the lid and the fence apart from where the lid-hold-down tags are placed. Any gaps in the connection could have an effect on the EMC performance of the enclosure. These lid hold-down tags can be either the fold-down type or the twist type illustrated in Figures 2 and 3 and blowups. Both options are suitable for up to five lid removals and replacements. 
Figure 2. Four-sided can with plain folded lid.
Figure 3. Plain folded lid with twist tag.For applications for which a very low profile fence and lid are needed, an enclosure with a pip-lock lid can be used. Pips in the side walls of the lid lock into slots on the fence side walls. This design choice comes with fence heights as low as 1.5 mm. As with the tag-and-slot lid, there is no guarantee of connection between fence and lid except at the locations secured by the pip. Also, the more pips along any wall, the more difficult it becomes to remove the lid after placement for necessary repairs or adjustments.Some designers prefer to place the lid and fence as a unified set using the pick-and-place facilities of a surface-mount assembly line. Lids are removed only if rework to the components inside the enclosure is required. Electing this option means that an array of holes must be made in the lid to allow the reflow heat into the enclosure to solder the electronic components inside to the PCB as shown in Figure 4. Unfortunately, these holes may degrade the EMC performance of the enclosure by up to 20 dBs. 
Figure 4. Five-sided enclosure with butted corners and reflow heat holes.Where the enclosure can be placed after testing, or when the PCB production yield is to be quite high, the more cost effective option is the five-sided can. This choice can be supplied with soldering pins, spot welded corners, or butted corners, or it can be fashioned with reflow heat holes for machine placement. By far the most cost effective five-sided enclosure for development and low volume production is the bend-line-formed five-sided enclosure. It is delivered in flat sheets with tags. See Figure 5. These are folded and formed into the required shape by the user as they are placed onto the PCB. 
Figure 5. Five-sided enclosure delivered flat for self-forming. SHIELDING MATERIALS
For most RF shielding nearly any base material such as copper, brass, stainless steel, aluminum, or nickel silver will work as a shield. In PCB mounting where components are soldered to the board, plating is used on most materials other than nickel silver. Traditionally, bright tin plating had been used; however, with the implementation of the RoHS Directive on hazardous substances, the PCB assembly lines have switched to non-lead solder, which has a reflow temperature equal to or in excess of the melting point of bright tin. Consequently, there has been a general change to the use of nickel silver. Alternative plating finishes such as silver or gold can be used, but clearly these can be cost prohibitive. At low frequencies where the interference is usually magnetic, the use of specialist materials such as Mu metal or radio metal are more common, although some shielding can be provided by the use of thicker steel or phosphor bronze. The frequency limitation of thin metal fabricated shielding is usually between 3 to 5 GHz, beyond which two effects may limit shielding effectiveness or its usefulness. Because of the effect of stray capacitance between the enclosure and the electronic components on the PCB, any minor movement in the metal of the enclosure could cause microphony to occur. At these frequencies, shielding usually takes the machine-from-solid form, a choice that overcomes this effect. Another very high frequency effect occurs when the cavity of the enclosure becomes a fraction of a waveguide, possibly at harmonics of the circuit operating frequency. This effect causes the enclosure to act as a cavity resonator rather than as an EMI shield. This effect can be partially overcome by the inclusion of absorbing material inside of the enclosure and by the careful choice of enclosure size. DESIGN FOR MANUFACTURE AND ASSEMBLY
One of the key considerations in enclosure design is understanding the production volume of the ultimate component or product. This judgment will determine the eventual method of manufacture—and to some degree—the enclosure format. As was noted above in discussion of the fence-and-lid designs versus the five-sided can, clearly the manufacture of a one-piece component will be less expensive than creating two pieces that must be put together to form the shield. The production method chosen will also have an impact on component cost. For example, consider the comparative costs of photo chemical machining (PCM) versus punch-and-press progression tooling or some “halfway” hybrid of the two processes. Will the components be hand or machine placed? If machine placement is an option, then a pick-and-place target may be required as most place machines use a vacuum suction head to pick up the components. There are some machines that use pincer type pickup systems, but these are still fairly uncommon. For machine placement, the co-planarity of the PCB edge of the fence of a five-sided can will need to be 0.1 mm or better to ensure that the can will sit in the solder paste when placed and while going through the reflow oven. Machine placement generally requires some form of special packaging, either tape-and-reel or waffle trays. Still, not all surface-mount assembly lines can handle waffle trays; and for larger enclosures, wide tape feeders can slow down the placement process since they will take up several feeder slots that could be used for a number of smaller electronic components. For straightforward hand-placed fences in small to medium volume, the usual production method is PCM strip-line assembly–in which the fence is made from a long single strip of metal, with part etched bend lines in each corner. These are then hand-formed into a rectangle or whatever shape is required. They are then spot-welded or seam-soldered in one corner to form the final permanent shape. One of the benefits of this method is that the maximum number of components can be obtained from a single sheet of material. For fences that will ultimately go into a punch-and-press, a fold-down format similar to a five-sided can is used. The bulk of the material is removed from the top surface, leaving either web corners or an all around web. These fences also lend themselves to the inclusion of a pick-and-place target. See Figure 5. Additionally, a five-sided can for machine placement will require heat holes and an area clear of these holes to provide the pick-and-place target. ACCESS TO THE COMPONENTS WITHIN THE ENCLOSURE
Where practical, access to the components inside the enclosure is achieved by lifting one of the various types of lids described above away from the enclosure fence. In situations where access is required only on a very small percentage of a high-volume, high-yield line, perhaps for corrective rework, a five-sided enclosure with a rework access area is one possible solution. As shown in Figure 6, an area of the top face is surrounded by a series of postage stamp type perforations, possibly with a part etched line joining them, to allow for easy removal using small long-nosed pliers.
Figure 6. Five-sided enclosure with part-etched perforations for access.Following the corrective action, the exposed area can be re-sealed using either a spring-fingered lid or a plain folded lid touch-soldered to the remaining frame left on the PCB. This process eliminates the difficult task of attempting to remove a complete enclosure. Such an attempt would likely result in damage to the PCB, or worse still, the disposal of complete units as preferable to undertaking the painstaking repair process. Given the escalating complexity and cost of high volume products and the implementation of environmental directives such as WEEE, a rework access area delineated with perforations is an option worth serious consideration. Finally, in cases in which there are a number of separate areas of the PCB that must be shielded either from extraneous interference, or from each other to avoid crosstalk problems, the option of using a multi-cavity labyrinths arises. COMPONENT MANUFACTURING METHODS
A number of methods are used to manufacture shielding enclosures from metal including photo chemical machining (PCM), laser cutting, punch-and-press, or some hybrid of these processes. The method chosen would depend on the technical requirements of the enclosure, the ultimate volume of production, and the component cost constraints imposed on the project. PCM is actually the same process as that used to manufacture bare circuit boards; the one distinction is that it starts with a sheet of metal instead of a metal-clad insulator. The process consists of creating a flat form of the product to be produced. Using CAD, etch and bend allowances are added before two photo tools, one for each side of the metal, are plotted. Where the process is etching the profile of a product—the two tools are the same. When bend lines, logos, or connector or aperture details are required on one side of the material, the two tools are different. The sheet of metal, pre-coated with photo resist, is then exposed to UV light through the photo tools. The sheet is then developed, and the unwanted resist removed leaving clear outlines of where the material is to be etched away. The PCM process has several advantages. The cost of tooling and tool modification is low, and turnaround times are fast. Bend lines can be etched so as to create precise angles—e.g., 135, 90, or 45 degrees. See Figure 7. The process is burr and stress free, and magnetic and other material processes are unaffected by the process. Complex designs are easy to tool, and the inclusion of apertures, track clearance holes, logos, and other details has no cost effect on the ultimate product—an advantage that allows mechanical designers to be as creative as they wish. 
Figure 7. Etched fold lines.An alternative to PCM is to laser profile the enclosures. This choice is not cost effective for volume work on small cans, and precision bend lines are difficult to achieve. Still, lasers come into their own for larger enclosures manufactured from thicker material, such as 19-inch rack housings. Toggle or fly presses can be used for simple one-sided forming; however, for forming multiple sides at the same time and for cutting material, a power press would be required. A small press can be manually operated with one die set. Alternately, a very large press will be machine operated and can hold a much larger, multi-stage tooling. A die set consists of a set of (male) punches and (female) dies, which, when pressed together may form a hole in the material or may reform the material in some desired manner. The punches and dies are removable; the punch is temporarily attached to the end of a ram during the punching process as the ram moves up and down in a vertically linear motion. Although the materials used for screening enclosures are relatively thin, larger high-power presses are needed because of the number of progression stages of press tooling used to create the complex structures used in electronics. These progression tools require a large tool bed and are, as such, more powerful than would normally be required for the thin material being handled.PCM VS. PUNCH-AND-PRESS
Switching from PCM to hard tooling is usually based on financial considerations. If the use of hard tooling is being considered, the product and enclosure design must be fully fixed before embarking on hard tooling because even minor changes can be very expensive. The critical switchover point where the decision is made to use punch-and-press is impelled by enclosure complexity. Complex products necessitate higher tooling expenditures so the ultimate volume of product production and sales must be great enough to justify this option. One of the most cost effective materials currently in use for volume production is pre-tin plated mild steel, but this material choice is suitable only in instances where bare steel edges will not be an issue after placement on the PCB. If the product may be subjected to any form of harsh environment, this choice risks the possibility of corrosion occurring. Also, with the introduction of non-lead solder on volume assembly lines, more designers are switching to nickel silver as an alternative to bright tin-plated steel. HYBRID MANUFACTURE
Fortunately, enclosure production is not simply a matter of choosing between the less expensive and the costly. For some designs, using PCM profiling followed by one-hit forming will improve the cost effectiveness of production without any commitment to full progression tooling. Another viable option for cost effective, high volume production would be separate blanking and forming press tools, a two-step solution that also bypasses the very high costs of progression tooling. PCB DESIGN
The rule of thumb for determining PCB track width should be a minimum of three times’ material thickness; this “formula” ensures an even fillet of solder on either side of the can wall and allows for any placement tolerance. Some designers prefer a continuous track for the shielding fixture—a choice that results in a continuous seam, solder-joined to the PCB (apart from any track clearance holes). Others prefer a castellated edge to the enclosure resulting in an intermittently “broken” track for soldering the can to the PCB. The latter choice enables signal or other tracks to cross the enclosure boundary on the surface of the PCB. On multi-layer PCB designs, common practice is to make the outer layers of the PCB RF ground and to confine the signal tracking to the internal layers. ASSEMBLY
Today, the assembly of RFI shielding cans onto the PCB is an increasingly important factor in the quality of finished assemblies. Often, “pressed” (or stressed) cans will not sit flush against the PCB surface. Several methods are currently used to overcome this problem. One of these is preheating and placing weights on the cans during reflow, but this technique can be problematic. Weighted cans can affect the reflow characteristic of the assembly and are also likely to lead to problems with joint integrity during cooling and use. Shielding cans manufactured using PCM do not exhibit these severe co-planarity problems since PCM does not induce stress into the material. Once plated and assembled, the seating faces on all four sides of a PCM-produced can will achieve the flatness tolerances required. Another aid in overcoming any co-planarity problems is the use of selected thickness, printed solder deposits. On one printing pass, creating solder deposits of increased height for the shielding can and other larger components may assist in eliminating problems associated with component co-planarity. This technique provides stronger solder fillets that increase mechanical security and may prevent problems associated with voided fillets. Often solder deposits are printed in isolated blocks, but the solder does not reflow sufficiently and can create voids and blowholes in the sealing fillet that require reworking. One obvious solution is to ensure that the solder paste deposits do not have these large interrupts that cause these troublesome effects in the first place. It is possible to create “integral” solder deposits that assist in reflowing without voids. Replacement of the solid metal tags within the solder paste stencil with suitable mesh aperture patterns that join the isolated deposit apertures permits a sufficient admixture of solder material and flux within the paste (Figure 8). In turn, this “improved” paste helps create a condition of surface tension that ensures that solder fill flows over all areas creating an even fillet of solder all around the base of the can. 
Figure 8. Paste deposits for screening cans.Initially both the PCB and screening can possess good solderable finishes. The addition of printed solder paste along with the heat applied during reflow often encourages the migration of solder up the can walls away from the PCB. The result is sometimes less than aesthetic and may not provide adequate shielding or the mechanical strength required in the solder fillet. A unique solution to this problem of solder fillet migration is the “reflow Plimsoll line” (RPL). In this instance, a Plimsoll line is found not on the hull of a ship, but on the can wall. Specifically, an interruption in the plating finish on the can wall allows the solder only minimal upward migration and ensures that fillet strength and volume are maximized. It also avoids the problems associated with the formation of blow holes, gaps, and an untidy appearance of the can wall finish. Another problem with the traditional approach, including webs between adjacent apertures as seen in Figure 9, is that it often results in isolated paste deposits that do not always deliver integral solder fillets and that may need rework. An enhanced stencil design as shown in Figure 10, includes metal mesh that ensures paste deposits on the entire can footprint on the PCB; and the presence of both flux and paste contributes to an effective reflow process. 
Figure 9. Stencil layout for screening can deposit with components that require screening within footprint.
Figure 10. Enhanced stencil layout for screening can deposit for components that require screening within footprint.Yet another issue that is common when placing enclosures or other large components on PCBs is to place sufficient solder paste on the surface of the PCB to accommodate fine pitch components both within and outside the enclosure. This requirement can be achieved by using a multi-level stencil, with the volume of solder paste dispensed on the board being a function of the stencil thickness as well as the aperture size. See Figure 11. 
Figure 11. Conventional single thickness paste deposits. Selective multi-height paste deposits can be achieved using multi-level stencils.CONCLUSION
The ubiquity of electronics and radio applications in our everyday lives, alongside the changing directives from various regulatory bodies, means there is a greater need than ever to consider the radiated interference between individual equipment and adjacent circuits on PCBs. The shielding of this interference needs to be considered alongside all of the other design aspects of a product and preferably at an early stage, to avoid costly modifications to PCB layouts or equipment re-designs following initial EMC testing. Other issues to consider include product testing, handling of the shielding in production, other regulatory directives, such as RoHS compliance, and cost. Alan Warner, the author of ‘The World of EMI/RFI Enclosures,’ offers over 40 years of experience in the electronics industry, has a degree in Electronics and Systems Engineering and is a member of the Institute of Electrical and Electronics Engineers (IEEE) and Institute of Sales and Marketing Management (ISMM).Alan is now retired from Tecan but still acts as a consultant. Through his company AW Services, he now offers EMC training—in particular for fixed installations.This article is an extract from Tecan’s recent publication, ‘The World of EMI/RFI Enclosures’—which covers all aspects of the design, prototyping and manufacture of off-the-shelf and custom shielding solutions, from cans to complex multi-cavity and totally-enclosed structures. The guide is available from Tecan—contact [email protected] for further details.
