Shielding at PCB Level

Perancea’s Peter Hendrick in association with Keith Armstrong of Cherry Clough Consultants take a grass roots look

Shielding, perhaps the most common technique for controlling EMC - Electro-Magnetic Compatibility - is deceptively simple! Metal enclosures keep harmful interference out and also unwanted emissions in. Perancea’s Peter Hendrick in association with Keith Armstrong of Cherry Clough Consultants take a grass roots look.

As with most simple concepts, practical EMC shielding requires considerable engineering skill to give good results. However, at product enclosure level, it can be complex and expensive, adding to both material and assembly costs. Added to that, effective enclosure shielding usually requires substantial filtering of all cables which pass out of the product, right at the point where they penetrate the enclosure shield.

Tackling an EMC solution cost effectively and conclusively, shielding at PCB level is both simple and low in cost. Board level shielding solutions can be used to achieve an EMC performance that would be far more difficult and costly at product enclosure level.

At board level, small shielding cans can be applied selectively to the problem areas of a circuit, directly tackling components with emissions that are too high or are too sensitive.

In effect, PCB shielding cans are simple metal boxes that are easily soldered in place, much like any other board level component. Tackling EMC problems at this level, keeps component and assembly costs low and the appearance and ease of maintenance of the product are not compromised. Solutions such as Perancea’s fingered lids increase the number of points of contact ­between the box and the lid for improved performance.

With circuit board tracking and wiring shielded inside a can as a possible solution, the contained tracks and wires are consequently very short and thus limited as aerials radiating RFI or EMI. Furthermore, they may not need much, if any, filtering to reduce overall emissions to an acceptable level.

Where immunity is a prerequisite and the very problem to be resolved, board level shielding will require the same performance from cable filters as for enclosure-level shielding. However, as the filters can easily be fitted to the circuit board instead of an enclosure shield, component and assembly costs are minimised as a result.

Although board level shielding using simple metal cans is much easier than shielding at enclosure level, there are still important design issues to be considered. These are discussed briefly as follows:

Ground Plane

In operation, a PCB shielding can must be soldered to a good ground plane which lies underneath all the components that require shielding to create a complete six-sided metallic enclosure – a mini Faraday cage. The ground plane, as discussed later, must not have any significant apertures in it.

Can Material

Shields work by reflecting and absorbing electromagnetic fields. Good conductivity as exhibited for example by copper, is required for reflection and this is especially important above 100MHz. High permeability, as exhibited by steel and nickel, is required for absorption which is especially important below 100MHz where the interfering source is closer than:

3 x 10⁸  metres  where F is the highest problem frequency.

  2pF

In general, tin plated steel is the best choice for shielding below 100MHz whilst tin plated copper is best above 200MHz. Tin plating makes for easy soldering. As aluminium is not easily soldered to a ground plane, it is not generally used. However, a wide range of materials are generally available for shielding, including brass, nickel silver and stainless steel.

Thicker materials give better shielding and 0.2mm steel is thick enough to give 40dB shielding effectiveness above 500kHz – assuming there are no significant apertures. (see below)

Apertures

The real world performance of all shielding and ground planes is always compromised by apertures such as holes for adjustments, indicators, wires, construction seams and the gaps between a shielding can’s ground plane connections. And it does not matter how narrow those apertures are! The effect of an aperture depends on the frequencies to be attenuated and the shielding performance required. As a rough guide, for a single aperture:

L=1.5 x 10¹¹

     F x SE

Where

L   = the longest dimension of the aperture in mm

SE     = the shielding effectiveness required, as a number and not in dB

F   = the frequency at which the above SE is required

This formula indicates that for a shield with a single aperture to achieve a 40dB (100 fold) SE at 150MHz, the longest dimension of the aperture must be less than 10mm.

Normally, a safety factor of at least 6dB should be allowed when using this formula. Multiple apertures of the same size worsen SE by 3dB for every doubling in their number. In rare circumstances SE can be very much more than this equation suggests. So, the best aperture is no aperture!

Making the best use of PCB Shielding Cans

The lowest cost shielding cans are sheetmetal fabricated rectangular boxes, so it is prudent to arrange the components that need to be shielded inside a rectangular area on the PCB, providing sufficient clearance for the walls of the can.

Arranging the PCB to have a ground plane on the same side as the shielding can allows its edges to be soldered all around to eliminate any apertures in this area.

Realising all PCB tracks and wires in and out of a shielded area as correctly-terminated transmission lines is best practice but if this is not possible designs should allow for ferrite beads or more complex filters to be fitted where the tracks and wires cross the boundary of the shielding area.

To help derive shielding solutions, Perancea has developed its expertise in fabrication to produce a wealth of options for effective shielding solutions including PCB mounting screens and cans, locking pins, screening strips, thin gauge materials and solder anti-migration.

Screens & Cans

Mounting screens provide effective EMI/RFI shielding for high EMC performance in demanding and sensitive applications. The new Perancea metal designs are low in cost, precision manufactured and feature a range of sizes and options including a new and unique locking pin mechanism designed to keep the enclosure in place during reflow, wave and hand soldering operations. The mounting screens, as a board level solution, are more economical than full enclosure screening.

Enclosure styles include PCB frames and cans with or without fingered lids. Removable fingered lids provide easy access to the enclosure contents plus high attenuation in service.  Standard or locking mounting pins can be specified and internal dividers can be provided for discreet screening. Additional features include PCB locations, holes, brackets and other sheet metal produced features to accommodate customer requirements. Cans are produced with all the features of Perancea frames plus butt jointed corners that can easily be soldered if required.

Locking Pins

Specially designed to retain an enclosure securely in place on the PCB during reflow soldering, Locking Pins – exclusive to Perancea – ensure full contact between the frame or can edge and the solder paste. An extremely reliable solder joint with optimum screening is therefore achieved.

The pin design is such that only minimal force is required to place the enclosure on the board making it ideally suited to pick and place applications. This unique concept in PCB mounting also has hand soldering advantages as the operator is ensured that the enclosure is held in position during soldering, giving increased reliability and the potential for higher production rates. This new Locking Pin can be offered as a standard feature on all sizes of PCB mounting frames and cans.

Screening Strips

Metal screening strips are ideal for segregating sensitive devices and assemblies on circuit boards. The strips can be arranged to ‘fence off’ selective and even complex area shapes on a PCB, either eliminating or minimising interference between different parts of the circuit, providing a highly versatile and economical board level solution compared to full enclosure screening.

Screening strips are available in a range of standard heights to suit standard enclosures. They come in 232mm lengths and are easily cut to required length, bent and soldered to suit individual applications, enabling from the simple to even labyrinthine solutions for multi-segmented areas to fully optimise discrete screening.

Available standard heights, when mounted on a PCB, are 10, 15, 20, 25, 35 and 50mm. PCB location pins as standard are 1mm wide by 2.5mm long. Perancea solutions can also be supplied with locking pin options as well as custom solutions including dividers and pre-fitted labyrinth screening.

Thin gauge material capabilities

With a thin gauge materials capability down to 0.2mm based on extensive CNC technology, Perancea is able to provide a highly competitive alternative solution to chemical etching. Based on the significant advantages of its ready formed boxes and enclosures, solutions are shipped ready for instant use rather than as flat sheet forms that require folding as normally associated with chemically etched parts.

With expertly designed in-house tooling, Perancea now offers high quality, thin, burr free components for high reliability solutions that are not usually associated with this highly cost effective production method. The complete range of Perancea standard sizes and styles as well as custom designs are now available to special order in thinner materials. There’s also a choice in finishes including hot tin dip to BS EN 29453 BS219 grade K,  bright acid tin & electro-tin to BS 1872, nickel plate to BS EN 12540, electroless nickel to DEF 03-5 Iss.B and zinc  to BS EN 12329 & 12330.

Solder anti-migration barrier

Perancea has developed an effective yet low cost solution for the prevention of solder creepage up the sides of its PCB mounting screening enclosures during automated and reflow soldering processes.

If solder migration is an important consideration in your application, the company can provide a solder/tin free barrier that will prevent solder wicking up the side of the can or frame.  The barrier essentially provides a physical break in the plating so that solder will only flow to the height of the line and not beyond it thus ensuring good quality solder joints.

The linear barrier is set around the circumference of the enclosure at the optimum height from the PCB to suit individual applications. It is nominally 0.15mm wide with a depth commensurate with the surface finish.

Perancea is a highly experienced mechanical engineering company specialising in the production of RFI/EMI shielding and other enclosures for the electronics and associated industries. The company, with its expertise, is also able to provide custom shielding and enclosure services to meet the needs of the broad range of application requirements from mechanical to electrical and electronics engineering applications.

Peter Hendrick is the Managing Director of Perancea and can be contacted on tel: +44 (0)20 8365 2520

Keith Armstrong is a Partner with Cherry Clough Consultants - http://www.cherryclough.com/