PCB Frame and Cover EMI Shields

The quality of design and manufacturing of electronic devices has evolved to such a point that customers expect near perfection and will not accept failure out of the box or an unreliable device. Competition in the global economy is also continuously driving quality standards higher and profit margins tighter and tighter. At the same time, technology is increasing exponentially, PCB component density is increasing, and component size is shrinking. PCB EMI RF shields are specialized components integrated into PCB designs to mitigate the effects of EMI/RF.

All of these are great for the consumer but add tremendous pressure to manufacturers. This has led to the evolution of PCB frame and cover EMI shields. Historically, PCB EMI shields were simple cans that were soldered in place during the reflow operation. A major downfall of this approach is the inability to inspect and rework components prior to going into testing or even the finished device. PCB frame and lid style EMI shields came about to overcome this issue.

With a frame and lid PCB EMI RF shield, the frame or fence is soldered to the board during reflow. The area enclosed by the frame is open for inspection and rework before the lid or cover is attached to the frame to complete the faraday cage. Thus, you have a PCB EMI RF shield that enables inspection and rework.

Shortcomings of PCB frame and lid EMI shields

However, there are several drawbacks to this approach that can be overcome by SnapShot board level EMI shielding technology. Those shortcomings of frame and lid board level EMI shields are:

• Co-Planarity: The frame needs to meet stringent co-planarity specifications in order be reliably soldered to the PCB.
• Cross Members: The frames typically have cross members in order to allow placement by pick and place machines. These cross members obstruct inspection and rework and must be removed manually as a post-reflow process step.
• Obstruction: Although the frame and cover PCB EMI RF shield approach does allow for inspection and rework of most components, the components and solder joints adjacent to the frame or fence are still obstructed.
• PCB Space: The frame requires solder pads that can be 2 or 3mm in width, which consumes excess valuable PCB real estate.
• Reliability: In many cases the lid or cover of a frame and lid EMI shield system is attached by mating dimples in the frame and cover. These dimples can be 5 – 10mm or even greater apart. This creates susceptibility to EMI/RF leakage. The cover or lid can also shift during shock and vibration which results in variability in performance of shielding effectiveness leading to an unreliable device.

How Snapshot offers better PCB Shielding over Traditional frame and lid EMI shields

SnapShot board level EMI shield technology offers all of the advantages of a PCB frame and lid EMI shield and overcomes all of the challenges.

• Co-Planarity: SnapShot EMI shields are flexible and do not require ANY co-planarity specification. The SnapShot board level shield will flex and conform to the planarity of the PCB.
• Cross Members: SnapShot does not require cross-members.
• Obstruction: The solder sphere attachment technique results in an unobstructed view and access to all of the components and solder joints, including those located around the perimeter of the circuit to be shielded.
• PCB Space: Using small diameter solder spheres as the attachment mechanism between the SnapShot EMI shield and the ground plane of the PCB requires 50% less trace width
• Reliability: SnapShot board level EMI shields are connected to the ground plane every 2mm around the circuit being shielded. This results in very high shielding effectiveness compared to frame and lid PCB EMI shields. Also, the lightweight and flexible SnapShot EMI shield will not shift during shock and vibration and will perform the same on day 1,000 as it did on day 1.

In summary, SnapShot board level EMI shields offer:

• Completely unobstructed access for inspection, including automated optical inspection, and rework
• Greater than 50% savings in trace width requirements
• 20-25 dB greater shielding effectiveness
• Reliable and consistent performance in the most demanding applications