PCB Pad Layout Recommendations

Solder pad design, solder application, and component placement are important elements of the soldering process. Excessive transfer of thermal or mechanical stresses to the MLC can result from oversized solder fillets. Nominal pad designs suitable for the solder reflow process are displayed in the interactive land pattern generator. These guidelines represent a starting point in Printed Circuit Board (PCB) design. For further information on the land pattern design, refer to the standard IPC-SM-782A “Surface Mount Design and Land Pattern Standard” published by the Institute for Interconnecting and Packaging Electronic Circuits (www.ipc.org).

Component Placement Considerations and Pitfalls
Solder pad design, solder application, and accuracy of component placement each must be considered when laying out circuits. This is especially true for RF and microwave circuits where incorrect or asymmetrical deployment of these parameters can lead to serious high frequency performance degradation. Designers beware. IPC-SM-782 is an excellent reference document for optimum soldering of components. However, blindly following this standard can lead to less than optimum high frequency performance and or high voltage issues. If you are not concerned with RF performance or high voltage requirements, Johanson recommends that designers follow this standard (IPC-SM-782). However, for those concerned with RF performance, we recommend a smaller land pattern (solder pad).

EIA (mm)/ Johanson Series

0402 (1005)/ R07 0603 (1608)/ R14 0805 (2012)/ R15 1111 (2828)/ S42 1206 (3216)/ R18 1210 (3224)/ S41 1812 (4532)/ S43 1808 (4664)/ R29 1825 (5728)/ S49 2211 (5878)/ R30 2220 (5751)/ S47

 Print

 Print

0805 / R15   Print

1111 / S42   Print

1206 / R18   Print

1210 / S41   Print

1812 / S43   Print

1808 / R29   Print

1825 / S49   Print

2211 / R30   Print

2220 / S47   Print

2225 / S48   Print

0.322
(8.1788)

0.262
(6.6548)

0.230 (5.842)

0.026(0.6604)

 

0.086
(2.1844)

0.086
(2.1844)

0.118 (2.9972)

0.290 (7.366)

0.160
(4.064)

2525 / S48E   Print

2.80
(7.112)

0.250
(6.350)

0.230 (5.842)

0.25(0.635)

 

0.050
(1.270)

0.050
(1.270)

0.300 (7.620)

0.400 (10.16)

0.150
(0.381)

3838 / S58   Print

0.434
(11.0236)

0.380
(9.65)

0.380 (9.65)

0.025(0.63)

 

0.090
(2.286)

0.090
(2.286)

0.266 (6.7564)

0.446 (11.3284)

0.170
(4.32)

Performance
For good performance, engineers designing PCBs should consider the following recommendations for Johanson Passives:

• Pad Size
  • Johanson recommends smaller pad size when compared to the IPC standard. This is done to achieve a compromise between the best RF performance and optimum solderability.
  • Large pad sizes like those recommended by the IPC-SM-782 standard will have RF impedance effects (which can be negative)
• Pad Symmetry
  • Symmetrical land patterns (pads) are recommended to achieve better RF performance and reduce the likelihood of tombstoning
• Solder Fillet
  • Solder fillet size can shift impedance at microwave frequencies. Johanson recommends smaller size fillets and consistent size fillets. Large wavelengths tend to be more forgiving with excess solder. Fillets at higher frequencies (very small wavelengths) are much more affected by poor soldering methods. Johanson recommends that the solder fillet be at the lower tolerances in order to minimize RF disruption.
  • Controlling the solder fillet to the low side is an effort to minimize negative impedance effects that occur at higher frequencies.
  • o Smaller solder fillets minimizes thermal and physical stresses

Safety Capacitors
Caution for Safety Caps layouts – Layouts should meet the 3mm (0.1182”) or 4mm (0.1575”) minimum creepage distance between pads as required in the UL or TUV specifications. Here the creepage distance is defined by the “C” dimension in the layout above.