Square body leadless electrical device

An electrical device designed for leadless surface mounting on a PC board has molding compound which surrounds the electrical component and is so shaped as to have an outer surface which defines a flat, so that when the device is placed on the board it will remain in proper location and not roll away. An appropriately shaped mold is provided to accommodate the shape the molding compound simultaneously for a plurality of devices.

The present invention relates to discrete electrical components and methods 
for fabricating same, with particular reference to a power rectifier with 
an improved construction which not only facilitates fabrication thereof 
but also facilitates the positioning of the device on a PC board and the 
making of leadless electrical connection thereto, and to a method for use 
in fabricating same. 
Molded axial lead power rectifiers are normally fabricated with soft copper 
wire termination leads extending from the ends thereof. Such leads are 
usually many times the length of the device itself. Because of this, the 
leads tend to deform during the manufacturing process, resulting in 
machine jamming and additional costs to restraighten the leads. In 
addition, the leads occupy a great deal of space, in fact, more space than 
the body of the device, reducing capacity of machinery an tooling. The 
care, attention, and special tooling required to handle the delicate 
leads, plus the scrap resulting from damaged leads, adds unnecessary costs 
to the device and limits the capacity of the manufacturing equipment. 
The latest technology for assembling printed circuit boards, where most 
power rectifiers are used, involves the use of leadless devices the outer 
surface of which makes contact with a contact pad on the surface of the 
printed circuit board, thus eliminating the need for threading wire leads 
through holes in that board. Initially that technology gained acceptance 
for the assembly of resistors and capacitors, but it was not until the 
invention disclosed in McCann U.S. U.S. Pat. No. 4,564,885 of Jan. 14, 
1986 entitled "Rectifier With Slug Construction and Mold For Fabricating 
Same", which patent is owned by the assignee of this application, that 
leadless device technology was applied to rectifiers, and the entire 
disclosure of that patent is here incorporated by reference. Briefly 
stated, the end parts of the devices, integral parts of those devices 
themselves, were effectively employed to define individual mold cavities 
into which a suitable insulating plastic compound was molded, the 
construction and method of manufacture not only giving rise to eminently 
usable devices, and particularly power rectifiers, but also permitting the 
simple and inexpensive fabrication of those devices by utilizing a single 
mold and molding operation to simultaneously produce a substantial number 
of finished devices. 
Leadless devices are so constructed that electrical connection thereto is 
effected at portions of the exposed outer surfaces of the devices, which 
portions are in physical and electrical contact with contact pads or the 
like on the printed circuitboards with which they are to be associated. 
Even if only one such device is to be used on a board, it is essential 
that it be accurately located with respect to the terminal pads on the 
board so that electrical connection to those pads can be made, and when, 
as is usual, a number of different leadless devices must become a part of 
a given printed circuit board assembly, the problem of location is 
exacerbated. To solve that problem it is necessary in the first instance 
that the devices be properly positioned on the board so that their 
external surfaces engage the proper terminal pads, but that is only the 
beginning of the problem. It is also essential that when a device is once 
properly positioned it remain in that position not only while other 
devices are being positioned on that board but also during the operations 
(e.g., soldering) required in order to permanently secure the devices in 
position and produce proper electrical connection. The end parts of 
leadless devices are in the form of discs and when, as is shown in the 
aforementioned U.S. Pat. No. 4,564,885, the outer surface of the molding 
compound has a cross-section which, like that of the end discs, is 
circular, the devices when initially placed in position on a board tend to 
roll from that position and hence present assembly problems. Cylindrically 
shaped devices of the type disclosed in the aforementioned patent have the 
advantages of easy manufacturability, ease of handling in manufacturing, 
low cost and high reliability. Because the cylindrical devices of the type 
disclosed in said patent can have the same internal construction as 
corresponding prior art devices with conventional leads, with respect to 
which the art has much experience in manufacture and use, such cylindrical 
devices are preferred over flat packages, which will not roll when placed 
on the printed circuit board but which do not match other electrical 
characteristics of axial devices because of their different internal 
construction, but prior to the present invention they have been 
susceptible to rolling, and hence mis-location, on circuit boards, and 
this constitutes a significant drawback to their use. 
The primary object of the present invention is to devise a basically 
cylindrical electrical device such as a power rectifier suitable for 
leadless electrical connection, with all of the advantages known to be 
present in that type of device, but without the "rollability" disadvantage 
of that type of device which presented problems in incorporation into 
printed circuit boards. 
It is a further object of the present invention to provide such a device 
which can be manufactured with the same ease, facility and inexpensiveness 
as the device of the aforementioned U.S. Pat. No. 4,564,885 but which is 
more readily and reliably incorporated into printed circuit board 
assemblies. 
It is yet another object of the present invention to devise a mold having 
all of the advantages of the mold of said U.S. Pat. No. 4,564,885 but 
which will produced improved products. 
In accordance with the above, when the molding compound is formed in place 
its outer surface is provided with one or more flats which can engage the 
surface of the board as the device is put into position and hence prevent 
the device from rolling. In its preferred form, and as here specifically 
disclosed, the outer surface of the molded body between the electrically 
conductive pieces of the active device defines a polygon within which the 
cylindrical outer surfaces of the end pieces are inscribed. The mold is 
appropriately shaped to that end. In a particularly preferred form that 
flat-defining portion of the molding compound does not extend over the 
entire axial distance between the conductive end pieces of the active 
device, the remainder of the molding compound filling the axial space 
between the end pieces having a cylindrical outer surface corresponding to 
that of the end pieces, thus making non-critical the axial location of the 
active devices in the mold prior to carrying out the molding operation.

As has been indicated, for purposes of illustration the instant invention 
is specifically disclosed in connection with the fabrication of leadless 
electrical power rectifiers. Typical such rectifiers, as here illustrated, 
comprise a substantially cylindrical central body generally designated 10 
and first and second substantially disc-shaped end parts 12 and 14. The 
end parts 12 and 14 have diameters which are larger than the diameter of 
the central body 10 such that they extend radially outwardly beyond the 
surface thereof. The outer surface of body 10 and the interior surfaces of 
the radially outwardly extending portions of end parts 12 and 14 define a 
circumferential body recess into which a molding compound 16 such as epoxy 
is received. Epoxy compound 16 is applied to the device utilizing a unique 
mold, a preferred structure of which is disclosed in detail below. 
Body 10 of the illustrated power rectifier includes a diffused silicon die 
18 sandwiched between two slugs 20, 22 preferably composed of molybdenum 
or the like which are brazed thereto. End parts 12 and 14 are preferably 
composed of copper and are, in turn, brazed to the exposed ends of slugs 
20 and 22. When electrical connection is made to the outer surfaces of the 
end parts 12 and 14, that in effect places the silicon die 18 in the 
circuit. 
Etching and the application of a glass slurry prior to the application of 
the epoxy compound 16, and the tin or solder coating of the end parts 12 
and 14, are also usually carried out, as pointed out in the aforementioned 
'885 patent. That patent discloses two alternative constructions for the 
power rectifier per se, which alternatives, as well as other 
modifications, are also applicable to this invention. 
The mold which is used for the formulation of the devices of the present 
invention is in many respects similar to that disclosed in the '885 
patent, and therefore, to the degree practical, the same reference 
numerals will be used as in that patent. The mold consists of a top half 
40 and a bottom half 42. Each mold half 40, 42 is provided with a 
plurality of spaced substantially parallel grooves 48a, 48b each having a 
similar cross-section as described more in detail below, such that when 
the mold halves are aligned and placed together, as in FIG. 3, a plurality 
of elongated cavities 48 are formed. Situated between each pair of 
cavities 48 is a conduit or runner groove 50a which, when mold halves 40 
and 42 are placed together, forms a conduit or runner 50 connected to each 
of the adjacent cavities 48 through a plurality of openings or gates 52 
which are situated at spaced intervals along each side of the conduit 50. 
These openings or gates 52 are so situated at spaced intervals that they 
will align with the compound-receiving recesses 48a and 48b of the devices 
when the devices are situated end-to-end within the cavities. 
The internal surfaces of the cavities 48a and 48b consist of alternating 
relatively short sections 60 and relatively long sections 62. The exposed 
surfaces of the section 60 are semi-cylindrical in shape and conform to 
the outer surfaces of the device end pieces 12 and 14. The sections 62 are 
defined by generally orthogonal planar bottom or top walls 62a and side 
walls 62b, with the walls 62a and 62b being essentially tangent to the 
cylindrical walls 60a of the sections 60 so that, when the top and bottom 
mold halves 40 and 42 are properly assembled, and as shown in FIG. 3, the 
cavities formed by the walls 62a and 62b have a polygonal peripheral shape 
(in this case a square) within which the cylinder defined by the outer 
surfaces of the end parts 12 and 14 is substantially inscribed, with the 
corners of the cavity defined by the sections 62 extending radially out 
beyond the outer surfaces of the end parts 12 and 14. The axial length of 
the section 62 is somewhat less than the distance between the facing 
surfaces of the end parts 12 and 14 of a given device, as may best be seen 
from FIG. 2, the axial length of the section 60 is somewhat greater than 
the combined thicknesses of the device end parts 12 and 14, and the 
spacing of the sections 60 and 62 is such as to correspond to the overall 
lengths of the devices, such that when a plurality of devices are abutted 
in line, with the end pieces 12 of one device abutting the end pieces 14 
of the next device, each pair of abutting end pieces 12, 14 will register 
with and be received within a mold section 60, the substantially 
cylindrical body portion 10 of the device being for the most part 
registering with and received within a section 62. When molding material 
such as epoxy is forced into the mold through the runner groove 50 and the 
gates 52, entering and filling the space between the inner surfaces of the 
mold and the outer surfaces of those portions of the device between the 
end parts 12 and 14, a device configured as in FIG. 4 results. The molding 
material 16 will not only fill the space between the end pieces 12 and 14 
but will also extend radially out therefrom at the corners 16a of the 
polygon defined by the flat outer surfaces 16b. As a result, when the 
device is placed in position on a circuit board one of those flat surfaces 
16b will rest on the upper surface of the board and hence the device will 
not roll. Because the flat outer surfaces 16b are substantially tangent to 
the cylindrical outer surfaces of the device end parts 12 and 14, those 
end parts will at the point of tangency physically engage the upper 
surface of the board on which the device rests, so that proper leadless 
electrical connection can be effectively accomplished. Because the axial 
end portions of the molding material, designated 16c in FIG. 4, are 
cylindrical in nature, corresponding to the shape of the device end pieces 
12 and 14, precise axial alignment of the devices within the mold prior to 
molding is not required and hence slight manufacturing variations in the 
lengths of those devices will not present any molding problems. 
It will be understood that while this invention has been here specifically 
described with respect to power rectifiers of a particular construction, 
the invention is of broader applicability. Moreover, if desired, the 
devices need not be used to take advantage of their leadless potentiality, 
but could have leads applied thereto if desired, and other variations may 
be made therein, all without departing from the scope of the present 
invention, as defined in the following claims.