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SOLDER PAD RECOMMENDATIONS FOR
SURFACE-MOUNT DEVICES
By Wm. P. Klein, P.E.
Cost and performance requirements continue to push the
packaging of electronic systems into smaller and smaller
spaces. At one time, the standard center-to-center pin spac-
ing was 100 mils (0.1") on through-hole parts (DIPs). The
advent of surface-mount devices (SMD) has brought pin
spacings that differ from one package series to the next. The
solder joint of pin-foot to printed circuit board must provide
the strength to hold the device in place. The close lead
spacings make lead-to-lead solder bridges more prevalent.
These factors increase the importance of an optimized printed
circuit board (PCB) design.
The criteria for a well designed solder joint is based on both
empirical data and reliability testing. Solder joint strength is
directly related to the total solder volume. An observable
solder fillet is evidence of proper wetting. Therefore, a
positive solder fillet is usually specified. A joint can be
described by the solder fillets formed between the device
pins and the PCB pads. Figure 1 shows the three fillets (toe,
heel, side).
A properly designed solder pad will minimize solder bridg-
ing while affording a strong and easily inspected joint. These
goals have conflicting dimensional requirements.
Factors to consider when determining the dimensions of the
solder pads include part dimension tolerances, PCB produc-
tion tolerances, and accuracy-of-placement tolerances. Fig-
ure 2 shows how placement accuracy can affect solder
bridge formation. The designer should also consider the
limitations of the soldering process. Boards designed for
wave soldering usually have slightly wider pads than those
designed for reflow techniques.
Two trade organizations provide industry standards. The Elec-
tronic Industries Association (EIA)
1
represents manufacturers
in all areas of the electronics industry. The EIA鈥檚 Joint Electron
Device Engineering Council (JEDEC)
2
establishes standard
package dimensions. The Institute for Interconnecting and
Packaging Electronic Circuits (IPC)
3
that has established
standards for printed circuit board design. The Surface Mount
Land Pattern Subcommittee of the Printed Board Design
Committee of IPC has developed standard pad dimensions for
the packages defined by the JEDEC committee. The IPC
document 鈥淪urface-Mount Design and Land Pattern Stan-
dard鈥?is designated IPC-SM-782.
To further assist the designer, the mathematical relationships
in the standard have been programmed in a spreadsheet
calculator. Access to this program is available at the IPC鈥檚
Toe
Heel
Side
FIGURE 1. Solder Joint Fillets.
漏
1998 Burr-Brown Corporation
AB-132B
1
Printed in U.S.A. April , 1999
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