Electricity: conductors and insulators – Anti-inductive structures – Conductor transposition
Reexamination Certificate
1999-02-09
2001-01-16
Kincaid, Kristine (Department: 2831)
Electricity: conductors and insulators
Anti-inductive structures
Conductor transposition
Reexamination Certificate
active
06175077
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to shielding and, more particularly, to shielding devices for shielding electronic components or circuitry mounted on, or embedded in, printed circuit boards.
BACKGROUND OF THE INVENTION
Printed circuit boards (PCBs) are widely used electronic substrates in the electronics and telecommunications industry. PCBs generally include one or more layers of an insulating substrate (e.g., plastic) on which an electrical circuit is formed by depositing a predetermined pattern of a conducting metal (e.g., copper) for connecting various electronic components (e.g., semiconductors) which are mounted on or etched in the PCB layers. Many of these electrical circuits include components which operate at a high radio frequency (RF). The RF emissions from these components can interfere with the proper operation of other components or circuits in the vicinity of the PCB. Thus, it may be important to block or shield these RF emissions in order to prevent RF interference.
Conventionally, a metal “shield can,” generally having the shape of an inverted open box, can be utilized to cover an electronic component or a specific portion of circuitry in or on a PCB which is causing RF interference. Shield cans are usually applied to the appropriate section of a PCB either by soldering or forced galvanic engagement. For proper and effective RF shielding, it is conventionally desired that a shield can uniformly contact a PCB. There may be little or no shielding of RF energy if gaps exist between the shield can and the PCB.
A conventional shield can
10
, illustrated in
FIG. 1A
, includes a cover
12
and a plurality of side walls
14
formed from sheet metal. For rigidity, a lip
16
is formed around an upper portion
14
a
of each wall and a pair of cross members
18
a
,
18
b
are provided, as illustrated. The illustrated shield can
10
also includes a “pick-up-point”
17
located at the intersection of the cross members
18
a
,
18
b
that facilitates the placement of the side walls
14
by surface mount equipment. The cover
12
is attached at a later point in the manufacturing process.
To mount the illustrated shield can
10
on a PCB
11
, a lower end portion
14
b
of each wall
14
is placed into a solder paste deposit
20
on a mounting pad
22
, as illustrated in FIG.
1
B. Heat is then applied to cause the solder paste
20
to reflow which produces the illustrated solder joints (also referred to as “fillets”)
24
in FIG.
1
C. When cooled, each fillet
24
can provide a bond between a mounting pad
22
and a respective wall
14
. The cover
12
is configured to be secured to the upper portions
14
a
of the walls
14
to enclose a portion of a PCB. Unfortunately, the lip
16
and cross members
18
a
,
18
b
can hinder inspection and re-work performed on soldered components under these features, and/or removal of the shield can
10
at a later time.
For a shield can wall formed from sheet metal to be properly soldered to a PCB, it is typically important that the sheet metal be substantially flat, both initially when the side wall is formed, and during reflow operations. Unfortunately, heat from reflow operations may cause sheet metal to warp. This may result in gaps between a shield can wall and a PCB which may threaten the effectiveness of the shield can in blocking RF emissions. The presence of gaps may require costly re-work to properly solder a wall to a PCB.
A conventional molded shield can
10
′ that is often preferred over shield cans formed from sheet metal is illustrated in FIG.
2
A. The illustrated molded shield can
10
′ includes a cover
12
′ and a plurality of molded side walls
14
′, and a pickup point
17
′ for surface mount equipment. As illustrated in
FIG. 2B
, each side wall
14
′ of the molded shield can
10
′ has a thickness T
2
greater than a thickness T
1
of the sheet metal walls
14
of the shield can
10
illustrated in FIGS.
1
A-
1
C (i.e., T
2
>T
1
).
To mount the molded shield can
10
′ of
FIG. 2A
on a PCB, a lower end portion
14
b
′ of each wall
14
′ is placed within solder paste
20
on a mounting pad
22
, as illustrated in FIG.
2
B. Heat is then applied to cause the solder paste
20
to reflow which produces the illustrated solder fillets
24
in FIG.
2
C. When cooled, each fillet
24
can provide a bond between a mounting pad
22
and a respective wall
14
′ . The cover
12
′ is configured to be secured to the upper portions
14
a
′ of the walls
14
′ to enclose a portion of a PCB
11
.
By increasing the thickness of each wall
14
′, the need for a lip and cross members to provide rigidity can be eliminated. As a result, any additional re-work can be performed without hindrance therefrom. Molded shield cans are also preferred because a mold can be cost-effectively prepared for complex shapes and configurations that may be otherwise difficult into which to form sheet metal.
Unfortunately, molded shield cans having thicker walls may utilize significantly more PCB space than shield cans with walls formed from sheet metal. For electronic devices, such as radiotelephones and other communication devices where PCB space is limited, molded shield cans having walls thicker than sheet metal may be impractical or may compromise design/component spacing.
SUMMARY OF THE INVENTION
In view of the above, it is therefore an object of the present invention to facilitate the use of molded shield cans within electronic devices, such as radiotelephones, where PCB space is limited.
It is another object of the present invention to facilitate reducing the width of mounting pads that are used to secure shield can walls to a PCB.
It is yet another object of the present invention to facilitate the use of shield cans within electronic devices, such as radiotelephones, that do not require reinforcing members.
These and other objects of the present invention are provided by an electromagnetic shielding apparatus, such as a shield can, that includes tapered wall end portions having one or more cavities configured to receive a solder fillet therewithin. According to one embodiment of the present invention, a shield can may include an electrically conductive cover and a molded, electrically conductive frame. The frame includes a plurality of elongate metal walls. Each wall includes a pair of opposing, generally parallel side faces that terminate at a tapered end portion. Each tapered end portion is configured to be surface mounted to a respective conductive portion (e.g., mounting pad) of an electronic substrate via solder. The cover is configured to be removably secured to the frame to define an enclosure for shielding electronic components and/or circuitry.
The tapered end portion of each wall includes a pair of elongate end faces symmetrical about a plane that is generally parallel to, and equidistant between, the wall side faces. The end faces abut each other to form a tip. Each end face also abuts a respective edge of the respective side faces. The end faces define a pair of elongated cavities with respect to a conductive portion of the PCB. Each cavity is configured to receive solder therewithin for securing the wall to the conductive portion of the PCB.
According to another embodiment of the present invention, a shield can may include a top and a plurality of walls depending therefrom. Each wall includes a tapered end portion having one or more cavities configured to receive solder therewithin to secure the wall to a conductive portion of a PCB.
Shield cans incorporating the present invention can be advantageous because walls thereof can be formed from material thicker than sheet metal, and thus can be sufficiently rigid without requiring additional reinforcing members. Furthermore, the shape of a solder fillet contained within a tapered end portion cavity according to the present invention may not consume as much PCB surface space as conventional solder fillets. In addition, solder fillets produced via the present inventio
Lund Richard C.
Mendolia Gregory S.
Ericsson Inc.
Kincaid Kristine
Myers Bigel & Sibley & Sajovec
Ngo Hung V
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