PCB Blog - Analysis of PCB board level shielding cavity and system design development

Printed circuit boards,referred to as printed board,english abbreviation PCB board (printed circuit board) or PWB (printed wiring board),with insulating board as the base material,cut into a certain size,its At least one conductive pattern is attached to it, and holes (such as component holes, fastening holes, metallized holes, etc.) are arranged to replace the chassis of the previous electronic components and realize the interconnection between electronic components. Because this board is made by electronic printing,it is called a "printed" circuit board. It is inaccurate to call a "printed circuit board" a "printed circuit" because there are no "printed components" on the printed circuit board but only wiring. Mounting methods used, testing and inspection methods planned, and layout of printed circuit boards and components are key issues. The design of the printed circuit board is based on the circuit schematic diagram to realize the functions required by the circuit designer. The design of the printed circuit board mainly refers to the layout design, which needs to consider various factors such as the layout of external connections, the optimal layout of internal electronic components, the optimal layout of metal connections and through holes, electromagnetic protection, and heat dissipation. The layout design can save production costs and achieve good circuit performance and heat dissipation performance. Simple layout design can be realized by hand, and complex layout design needs to be realized by computer-aided design (CAD). Just like power supply selection, the decision to choose an RFI shielding cavity is often made during the design process, which often leaves insufficient space to add a shielding cavity, causing the cavity to physically affect other areas of the design.

3DS - Design, Develop, Draw
The design and development of PCB board-level shielding cavities and systems can be boiled down to three key steps: design, develop, and draw. Active communication and consultation between cavity users and cavity design teams is critical. Look for a cavity manufacturer who can provide initial design guidance, usage advice, site visits, prototyping, sample production, paint and thickness selection, machining, assembly, and re-evaluation for cost savings. Costs must be restrained to obtain market profitability of the product. Structural design combined with detailed design and customer input may achieve the desired goal of "achieving the desired result with limited cost".

Form choice
When choosing the type of cavity to use, a number of factors must be considered. What exactly is being blocked? What is the exact nature of shielding interference sources? After installing the cavity on the PCB, does the customer still need to open the cavity for modification, testing, inspection or adjustment? Does this yield match the cost of machine placement? Which loop areas need to be shielded or separated from other areas? Should one cavity or multiple cavities be used in this application? Will the final product undergo impact testing, vibration testing or packaging drop testing?

shielded form
For a particular application, careful consideration of the above issues can help to select an appropriate, economical form of shielding. Different four-sided shielding cavities can be selected for different application requirements. Fingerspring covers are an option among removable covers if the fence height is high enough to accommodate the finger springs. If there is not enough space outside the fence for external finger springs, internal finger springs should be used. Also, a mix of external and internal finger springs having the same form on opposite sides is possible. Surface mount tetrahedral cavities with finger springs are another option for shielded cavities. This type of cavity is the same as the ordinary tetrahedral cavity except that there are no fixed pins. Seam welding is often used to solder it to the PCB along a continuous trace. This may necessitate the use of a stamped form of fence construction. The four-sided PCB board cavity can also use a flat folding cover, as shown in Figure 2. This type of cover is less expensive to produce, especially during the development phase. The disadvantage of this design is that there is no guarantee of an effective connection between the cover and the fence, except where the cover is used to hold the sticker. Any gaps in the connection will affect the EMC performance of the cavity. This cover fixing strip can be either folded or wrapped, as shown in Figures 2 and 3. Both types of stickers can be used for more than 5 movements and replacements of the lid.

Flat Folding Lid
Snap-on lids can be used when the actual application requires the fence and lid to have a low profile. The nubs on the side walls of the cover are inserted into the small slits in the side walls of the fence. This design choice can reduce the height of the fence to 1.5mm. As with the choice of stickers and slot covers, this design does not guarantee an effective connection between the fence and the cover unless a small piece is used to secure the position. Some designers prefer to use the placement equipment of a surface mount line to integrate the cover and fence. Only open the cover when reworking the components in the cavity. Choosing this design meant that an array of small holes had to be left in the lid so that heat could enter the cavity to solder the electronics inside to the PCB, as shown in Figure 4. Unfortunately, these small holes will reduce the shielding performance of the cavity by about 20dB. When the cavity is installed after the test, or the output of the PCB board is large, it is more cost-effective to choose the five-sided cavity. This option can be achieved by soldering pins, spot soldering or butt soldering, and can also be machined through thermal reflow holes. Up to now, the cost-effective way to develop a five-sided cavity and to produce small quantities of it is to choose a curved five-sided cavity. As shown in Figure 5, it is done by adding logos to the plate. When mounting them on a PCB, the user simply folds them into the desired shape.

shielding material
For most RF shielding, the shield can be made from almost any base material such as copper, brass, stainless steel, aluminum or nickel brass. The mounting process for soldering components to the PCB is more electroplated than nickel brass. Traditionally, bright tin plating has been used. However, with the implementation of the RoHS directive on hazardous substances, the PCB production line has been changed to lead-free soldering. The interference at low frequencies is generally caused by the magnetic field. Although a thicker steel plate or phosphor bronze is sometimes used to make the shielding cavity, more special materials such as Mu metal or radio frequency materials are used to make the shielding cavity. The frequency limit of shielding cavity made of metal film is generally 3~5GHz. If this frequency range is exceeded, two effects will limit the shielding effectiveness or its effectiveness. Due to the distributed capacitance between the cavity and the electronic components on the PCB, any slight movement within the cavity metal will produce a microphonic effect. In this frequency band, the shield is usually machined in solid form, which overcomes the above effects. Possibly at harmonic frequencies of the loop operating frequency, the cavity of the cavity becomes part of the waveguide, where another high frequency effect occurs. This effect causes the cavity to behave more like a resonator than a shield. This effect can be avoided by adding absorbing materials to the cavity or by carefully choosing the cavity size.

Production and Assembly Design
A key factor in cavity design is knowing the throughput of the final part or product. This judgment will determine the choice of production method and, to some extent, the choice of shielding form. As discussed above with the fence-lid design and the five-sided cavity, it is clear that it is much cheaper to produce a single piece than to combine two pieces to form a shield. The chosen production method also affects component cost. For example, compare the cost of photochemical machine (PCM) versus stamping or a combination of the two methods. Are the components installed by hand or by machine? If machine mounting is used, since most machines use a vacuum tip to pick up the components, a placement target is required. While there are machines that use a pliers-type system to pick up components, this type of machine is uncommon. For machine installation, the coplanarity of the edge fence of the PCB board is required to be more than 0.1mm to ensure that the cavity is on the solder paste when installing or entering the reflow oven.

Machined General Shielding Materials
For most RF shielding, the shield can be made from almost any base material such as copper, brass, stainless steel, aluminum or nickel brass. The mounting process for soldering components to the PCB is more electroplated than nickel brass. The interference at low frequencies is generally caused by the magnetic field. Although a thicker steel plate or phosphor bronze is sometimes used to make the shielding cavity, more special materials such as Mu metal or radio frequency materials are used to make the shielding cavity. The frequency limit of shielding cavity made of metal film is generally 3~5GHz. If this frequency range is exceeded, two effects will limit the shielding effectiveness or its effectiveness. Due to the distributed capacitance between the cavity and the electronic components on the PCB board, any slight movement within the cavity metal will produce a microphonic effect.