Push-fit shield

Details Push-fit shield Push-fit shield

2001-04-27

2003-04-22

Reichard, Dean A. (Department: 2831)

Electricity: conductors and insulators

Anti-inductive structures

Conductor transposition

C361S816000, C361S818000

Reexamination Certificate

active

06552261

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to surface mountable EMI/RFI shields and, more particularly, to two-piece shields of the type having removable push-fit or snap-fit lids.
Modern electronic equipment includes electrical components and circuits mounted on a substrate that are sensitive to electromagnetic interference (EMI) and radio frequency interference (RFI). Such EMI/RFI interference may originate from internal sources within the electronic equipment or from external EMI/RFI interference sources. Interference can cause degradation or complete loss of important signals, rendering the electronic equipment inefficient or inoperable. Accordingly, the circuits (sometimes referred to as RF modules or transceiver circuits) require EMI/RFI shielding in order to function properly. The shielding reduces interference not only from external sources, but also from various functional blocks within the RF module. One type of prior art surface mountable shield is a five-sided metal enclosure, known as a can, that is mounted by automated equipment on the PCB (printed circuit board) and fits over the shielded components. The can is soldered to the board at the same time as are the electronic components. However, repairing components and fixng other problems covered by a soldered can shield is impossible without removing the shield. Removing a soldered shield is an expensive and time consuming task that can cause additional damage to the assembly and/or possibly remove the cause of the original fault leading to no-trouble-found defects. The access problem with soldered can shields can be avoided by using shields that can be opened when repair work is necessary.
Such openable shields are known and in the past have included cut-to-open shields (such as shown in U.S. Pat. Nos. 5,354,951; 5,614,694 and 5,365,410), and shields with snap-on or push-fit removable lids (such as shown in U.S. Pat. Nos. 5,895,884; 5,844,784; and 5,495,399). The two-piece shields comprise a frame or base member and a lid that is intended to provide secure mechanical locking and excellent electrical connection to the frame. Heretofore, mechanical locking of lid and frame has been accomplished by use of dimples and receiving slots provided on the frame and lid sidewalls (e.g. U.S. Pat. No. 5,895,884), or oppositely flared interlocking fingers (e.g. U.S. Pat. No. 5,354,951). It is desirable that the frame of a two piece shield be surface mountable by automated equipment both with the lid affixed and, alternatively, without the lid affixed. Such requirement necessitates that the frame have a surface near its center of gravity so it can be handled by vacuum robotic pick and place equipment with the cover removed. U.S. Pat. No. 5,495,399 discloses an example of such a frame.
With surface mount technology, shields are attached, typically, via soldering to grounded traces positioned both on the PCB substrate and around the electrical circuits generating (or requiring protection from) the interference as well as around the electrical circuits that are susceptible to interference. Oftentimes, the shields must be attached in close adjacency. The traces (which are typically comprised of gold-plated copper trace) are fabricated using known bonding and plating techniques during construction of the substrate, which typically comprises printed circuit board material, such as polyamide or epoxy-based flame retardant industrial fiberglass (G10-FR4). Generally, the traces are segmented, but in some applications continuous traces are employed. The plurality of traces are electrically coupled to a ground plane. The traces are generally no less than 1.00 millimeter wide (or 3½ times the shield wall material thickness) so as to ensure an effective metallurgical connection between the plurality of the contact points of shields and the plurality of traces. Traces for adjacent shields are separated from one another by at least 0.26 millimeter of solder mask barrier or bare substrate material for simple can shields. For removable cover shields the tracings must be separated by at least 1.0 millimeter to accommodate shield cover wall material thick nesses and assembly tolerances.
Initially, the substrate is subjected to a screening process that deposits a predetermined amount of solder paste on the plurality of traces. To ensure secure attachment, the amount of solder (and the size of the plurality of traces) should be sufficient to allow solder to “wick” or adhere on both sides of each of the plurality of contacts of shields during reflow. Generally, the shield assembly is reflow heated up to a temperature that is sufficient to melt the solder paste to a liquidus state. The liquidus solder wicks up on both sides of the shield wall and forms an effective metallurgical interconnection therebetween.
Shields are generally fabricated, using known progressive metal stamping, forming or slide tool techniques, from 0.05 millimeters to a 0.30 millimeters thick sheet stock of a nickel-silver alloy, a tin-plated steel, or other suitable electrically conductive and non-magnetic material. The side portions of the prior art shields are then folded along fold lines into position based on the maximum height of the portion of the transceiver circuit that is to be shielded. Depending on the type of components comprising this portion of the transceiver circuit, the height of the side portions might be less than 3.0 millimeters. However, two piece shields with push fit lids having side walls of the general type shown in U.S. Pat. Nos. 5,895,884; 5,844,784; and 5,495,399 have required a minimum vertical space of about 2.25 millimeters because there must be a spacing of a minimum of 0.5 millimeters between the PCB and the bottom edge of each lid side wall to prevent the lid from being soldered during reflow heating.
Shields typically include a plurality of holes or apertures to facilitate reflow heating interiorly of the shield, to enable cooling of the covered circuit elements during use, and to permit visual inspection of the portions of the transceiver circuit therebeneath. Such holes are generally sufficiently small (one-eighth wavelength or less at the highest frequency for which shielding is necessary) to prevent passage of interfering EFI or RFI. The size of the holes of shields can be varied based on the sensitivity of the portion of the transceiver circuit therebeneath. For more sensitive circuitry, the diameter of the holes are made smaller. Distal separations between the plurality of contacts and openings between the bottom edge periphery of shields and the skipped ones of the plurality of traces are similarly constrained.
In the known construction of two piece shields there can be a poor contact between the shield lid and the shield frame due to tolerance build up (tolerance stacking). Then, at high frequencies the lid or a part of it will rise to an impedance and begin to radiate or to receive radiation. The protective effect of the EMC enclosure or a part of it is then lost.
Two piece openable shields of known construction also suffer the drawback of requiring usage of a new lid each time the lid is removed due to actual (or perceived possible) deformation of the removed lid occasioned during the removal process. Such deformation has come to be expected by reason of excessive deflection of the lid sides during removal and/or by reason of line workers using improper techniques of lid removal to save time (e.g., using the worker's thumbnail to pry off the lid). Additionally, designs relying on interlocking fingers or dimples have posed design and production difficulties. In particular, it is difficult to hold tolerances and, consequently, the locking capabilities and forces.
Typically, shields are made from sheet metal approximately 0.20 millimeters in thickness. Tin plated CRS is a common material. Shields for cellular phones are typically applied to circuit boards using surface mount processes (e.g., vacuum pick and place) and must meet rigid quality control standards. They also must be produced in large quantity at v

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