Divided capacitor mounting pads

A split bonding pad provides a positive electrical indication of the proper electrical and mechanical connection of a capacitor to a printed circuit board by placing the electrical component terminal in series with electrical power supplied to a memory device while the electrical component remains in parallel connection with the memory device power lead. The proper mounting of the electrical component to the board becomes testable by testing continuity of a series circuit through the leads extending to the split bonding pad.

BACKGROUND OF THE INVENTION 
This invention relates generally to surface mounting electrical components 
on printed circuit boards and relates particularly to surface mounting 
electrical components on printed circuit boards under integrated circuits 
or other components while furnishing a positive indication of the proper 
soldering of the electrical components to the printed circuit boards. This 
indication occurs even though the electrical component rests substantially 
concealed from sight beneath the integrated circuit package or carrier. 
Present memory products such as large capacity--256K--dynamic read only 
memory devices (DRAMs) often come mounted in close proximity to one 
another on printed circuit boards. Surface mount technology uses gull wing 
of J-leads depending from the device package or carriers to connect the 
device terminals to pads formed on the board using reflow soldering. 
Decoupling capacitors, connected between the power supply leads of the 
devices, often come mounted on the boards with one capacitor reflow 
soldered to pads under each device. This obtains high density mounting of 
the memory devices and decoupling capacitors on the boards to reduce costs 
and reduce otherwise required mounting volume. The capacitor, having 
dimension of about 0.120.times.0.100.times.0.010 inches, extends into a 
well or cavity on the underside of each device package or carrier. With a 
number of leads, such as 18 for a 256K device, depending from the package 
periphery and soldering to the board, the decoupling capacitor 
substantially becomes enclosed and screened from sight rendering its 
visual inspection difficult, usually requiring a microscope. The close 
mounting of the packages further blocks the view of the capacitor under 
each package. In a J-lead package, the depending peripheral package 
material often extends to within 0.025 inch from the seating plane on the 
board. This leaves as little as 0.025 inch between the board and package 
and between the leads for visually inspecting the reflow soldering filets 
on the capacitor, which is only about 0.010 inch thick. 
An industry-wide problem exists because automated manufacturing equipment 
occasionally misplaces the decoupling capacitors from their pads for 
reflow soldering. This results in the decoupling capacitors occasionally 
not becoming soldered to their intended pads and remaining loose under the 
memory device and enclosed by the leads. If inspection fails to catch the 
loose capacitor or a capacitor wrongly aligned, the capacitor end 
terminals can touch the device leads and short them electrically. This 
renders the entire memory circuit card unuseable, and renders the entire 
system, in which the memory card is mounted, inoperative. Memory cards 
returned because of loose capacitors cost the customer and marker money 
and reflect poorly on the manufacturing process. Occasionally, an 
eventually loose capacitor is held out of position and away from the 
device leads temporarily by solder flux residue or rests away from the 
device leads during electrical tests, and accordingly passes inspection by 
the maker. Later, in use, the capacitor unpredictably becomes loose, 
shorts together two or more leads and stops the customer's machine. 
One solution tried operator visual inspection in addition to electrical 
testing. This increases cost and remains unreliable because of the small 
sized parts requiring a microscope and because of the restricted viewing 
space. Inspection with mechanized vision systems costs hundreds of 
thousands of dollars but provides only marginal improvement over manual 
optical inspection. The duplication of these mechanical vision systems and 
attendant maintenance costs and reduced serviceability of assembly 
equipment makes this approach to inspecting for failures unfeasible. 
Another solution used for stop gap protection vibrates or shakes all 
assemblies to verify that no decoupling capacitor is loose. This misses, 
however, capacitors mis-soldered or held temporarily by the solder flux. 
Alternate inspection procedures include x-raying all cards, which is 
prohibitively expensive. The x-ray does show a mis-aligned capacitor but 
lacks sufficient resolution to confirm an acceptable solder joint. 
While this problem has been described in conjunction with decoupling 
capacitors and DRAM devices, like problems exists with capacitors or other 
components mounted under other types of memory device, such as EEPROMS, 
EPROMS, ROMS and other integrated circuits. 
SUMMARY OF THE INVENTION 
The invention overcomes the described and other problems by placing one 
thermal of the decoupling capacitor in series with one power lead to the 
memory device. A misplaced or loose decoupling capacitor thus opens the 
power connection to the memory device, signaling a failure during the 
routine electrical test of the circuit card. This furnishes a positive 
test of the capacitor placement and adequacy of the reflow solder joint. 
It also eliminates further inspection. 
The terminal of the decoupling capacitor becomes placed in series with the 
power lead by splitting one of its bonding pads into two separate pads, 
electrically isolated from one another. One part connects to a lead 
sourcing power and the other part connects to the power lead of the memory 
device. The two parts are spaced from one another by about a certain 
distance and the terminal of the decoupling capacitor extents at least 
said certain distance. A properly placed or located capacitor mechanically 
bridges the two bonding pad parts with one terminal and proper solder 
joints between the terminal and the two bonding pad parts electrically 
bridges between the two parts to connect electrical power to the overlying 
memory device. An electrical test confirming power to the memory device 
confirms proper mechanical and electrical connection of the capacitor 
terminal to its pair of bonding pad parts. 
This arrangement confirms proper mounting of the decoupling capacitor 
during the normally required electrical test for the card and eliminates 
additional visual inspection of the capacitor, a double-saving. Placing 
one terminal of an electrical component in series with the power lead to 
an integrated circuit or other device also finds utility in components 
mounted on the board out from under the integrated circuit or other 
devices.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 1, printed circuit card assembly 10 comprises a printed circuit 
board 12 carrying eight memory devices 14 and eight decoupling capacitors 
16, only one of which decoupling capacitor is shown. Memory devices 14 and 
capacitors 16 mount or connect to printed circuit bonding pads to be 
described on the printed circuit board 12 using surface mount technology. 
This avoids drilling multiple holes through the circuit board for 
receiving the plural leads 18 of each memory device as previously has 
occurred. Capacitors 16 also mount or connect to the printed circuit board 
12 using surface mount technology also to avoid the numerous drilled and 
plated holes for mounting electrical components. 
Each decoupling capacitor 16 is quite small and extends into a pocket 
formed in the bottom surface of the memory devices 14 to avoid 
interference between the decoupling capacitors and the memory devices with 
the decoupling capacitors mounted underneath the memory devices. The 
relative sizes of the memory devices 14 and of the decoupling capacitors 
16, the proximity of the memory devices to the board 12 and the screening 
effect of the memory device terminal leads depicted in FIG. 1 convey some 
of the problem encountered in visually inspecting for proper placement and 
soldering of the decoupling capacitor 16 under the memory devices 14. 
In FIG. 2, printed circuit board 12 carries a plurality of device bonding 
pads 20, numbered from P1-P18. This numbering corresponds to the numbering 
of terminal leads for a standard plastic chip carrier package from such as 
Texas Instruments. Such a plastic chip carrier package often encloses a 
256 K dynamic random access memory (DRAM); thus the circuit card assembly 
10 of FIG. 1 can comprise a 256 K.times.8 memory array. 
Returning to FIG. 2, with the standard plastic chip carrier package 
definition, bonding pad P9 connects to the power source of Vdd while 
bonding pad P18 connects to the power source Vss. Printed circuit board 12 
further carries a pair of power leads 22 and 24 extending from a plated 
through hole 26 connected to a source of electrical power at Vdd. Lead 22 
extends from the plated through hole 26 to bonding pad P9 while lead 24 
extends from the plated through hole 26 to a capacitor bonding pad 28. 
Printed circuit board 12 further carriers power leads 30 and 32 extending 
from plated hole 34, which connects to a source electrical power at 
voltage Vss. Lead 32 extends to bonding pad P18 for the memory device and 
lead 30 extends to capacitor bonding pad 36. 
During assembly, a capacitor such as 16, depicted in solid lines, properly 
becomes placed on capacitor bonding pads 28 and 36 and is held in place by 
a solder paste, usually printed on the pads prior to assembly. A memory 
device also becomes placed on the printed circuit board 12 at bonding pads 
20, numbered P1-P18 and also is held in place by solder paste. When all 
the components have been located or placed on the circuit board 12, the 
card assembly 10 passes through a soldering device to melt the solder 
paste and form good mechanical and electrical contact between the leads of 
all the electrical components and their respective bonding pads. 
Capacitor 16 comprises a pair of end terminals 40 and 42 capping the ends; 
of a body portion 44. Most electrical components manufactured for surface 
mount technology have this same general arrangement of mounting terminals 
capping a central body. Metal terminal 40 thus becomes soldered to 
capacitor bonding pad 28 while metal terminal 42 becomes soldered to 
capacitor bonding pad 36. Body 44 comprises an insulating material 
enclosing the interior material providing the desired capacitance. 
Occasionally, a capacitor 16a, depicted in dashed line outline, becomes 
misplaced on the printed circuit board 12 and fails to be properly 
soldered at either of capacitor bonding pads 28 or 36. In such case, the 
terminal 40a can bridge between two bonding pads 20, such as the bonding 
pads 20 numbered P12 and P13, to cause an electrical short circuit between 
those two bonding pads. This results in the problem previously described. 
In FIG. 3, the invention furnishes a capacitor bonding pad 50 having two 
separate and electrically insulated parts 52 and 54 spaced from one 
another a certain distance A. Printed circuit board 12 carries one 
electrical power lead 56 extending from the plated through hole 26 to 
capacitor bonding pad part 54 and another lead extending from capacitor 
bonding pad part 52 up to bonding pad 20 number P9. The terminal 40 of 
capacitor 16 extends more than said certain distance A and is located or 
mounted mechanically and electrically to bridge the certain distance A 
between capacitor bonding pad parts 52 and 54. This places the terminal 40 
in electrical series connection with leads 54 and 58 providing electrical 
power to bonding pad 20 number P9 from plated through hole 26. This closes 
the electrical circuit between the plated through hole 26 and bonding pad 
20 number P9 confirming that the capacitor 16 rests properly mounted under 
the memory device 14. Any placement of the capacitor 16 other than 
bridging the space between bonding pad parts 52 and 54 opens the 
electrical power to bonding pad 20 number P9 providing a positive and 
ready test of a misplaced capacitor 16. 
Capacitor bonding pad 60 presents a similar arrangement with a bonding pad 
part 62 and a bonding pad part 64 that are however connected together 
electrically and mechanically by a bridging lead 66. This substantially 
presents the same type of bonding pad depicted in FIG. 2 at 36. 
Requiring only one connection between bonding pad parts 52 and 54 
positively defines that the capacitor is fixed in position soldered to the 
bonding pad parts 52 and 54. Thus capacitor terminal 42--instead of 
40--could extend between bonding pad parts 52 and 54 with capacitor 
terminal 40 spaced an equivalent distance toward bonding pad 20 number P9. 
While such a mounting keeps capacitor 16 from being connected between Vdd 
and Vss, it assures that the capacitor is secured to board 12. 
If the user required the decoupling capacitor 16 to be connected by 
terminal 40 to bonding pad parts 52 and 54 and for capacitor terminal 42 
to be bonded to bonding pad parts 62 and 64, the linking lead 66 could be 
opened and separate leads brought from plated hole 34 to one bonding pad 
part and a separate lead brought from the other bonding pad part to 
bonding pad 20 number P18. Thus an electrical open circuit between plated 
through hole 34 and bonding pad 20 number P18 would indicate the absence 
of the capacitor terminal 42 between bonding pad parts 62 and 64. 
Modifications and variations of the invention can be obtained in light of 
the above teachings while remaining within the scope of the appended 
claims. For example, the exact location and outline of the capacitor 
bonding pads and power leads extending to and from them can be changed as 
desired. Also, the bonding pad arrangement to place the terminal of an 
electrical component in series with power to another device can be used at 
locations in the printed circuit board other than under a memory device or 
other integrated circuit or other device. This obtains the advantage of 
eliminating visual testing while furnishing the positive indication of 
proper reflow soldering through a routine electrical test. While not fully 
explored, the split bonding pad part arrangement may provide a degree of 
self alignment of the electrical component through the surface tension of 
the solder wetting the entire length of the electrical component terminal 
cap. The invention also finds utility in locations where electrical 
components must be connected to leads other than those supplying 
electrical power, and where the leads or circuits are carried on 
substrates other than printed circuit boards.