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Manual S-20/S-23 HM/S-25 HM/F4/F5 HM /HS-50 Placement of 0402 components 03/2001 edition 73 10.3 Pad layout A good pad layout gre atly red uces the risk of tom bstoning . Use pads as shown in the drawi ng below , r ather…
Placement of 0402 components Manual S-20/S-23 HM/S-25 HM/F4/F5 HM/HS-50
03/2001 edition
72
10 Soldering
When small components are soldered, there is a risk of the tombstoning effect, i.e. the
components are pulled up vertically on one side, giving them the appearance tombstones.
Tombstoning can be caused by placement offset in the longitudinal direction, uneven heating of
the soldered connection or an unsuitable pad layout.
10.1 Basic principles of the soldering process
Both component connections and the solder paste deposit should be evenly heated, and reach
melting point at the same time. Commercially available solder (Sn63Pb37) melts at a fixed melting
point of 183 °C, and not within a melting range of 183 °C – 190 °C, for example, as is the case
with Sn60Pb40. If the two soldered connections do not reach melting temperature at the same
time, then the solder liquefies first on the hotter side. The resulting surface tension of the molten
solder then pulls the component up vertically like a tombstone.
This effect is more noticeable with capacitors than with resistors, since capacitors are easier to
pull vertically since they have a larger end surface.
10.2 Points to note during soldering
During soldering, be careful to distribute the air flow of a forced convection oven evenly over the
PCB - on both the underside and the topside - in order to minimize differences in temperature.
Manual S-20/S-23 HM/S-25 HM/F4/F5 HM/HS-50 Placement of 0402 components
03/2001 edition
73
10.3 Pad layout
A good pad layout greatly reduces the risk of tombstoning. Use pads as shown in the drawing
below, rather than square pads.
Fig. 10 - 1 Pad layout for 0402 components
Recommended transport direction
during wave soldering
Solder resist
Placement of 0402 components Manual S-20/S-23 HM/S-25 HM/F4/F5 HM/HS-50
03/2001 edition
74
10.4 Recommended pad layout
Å Reduce the size of the stencil aperture by 0.03 mm.
Å Increase the size of the solder resist by 0.05 mm.
Component A B C
0402 0.40 0.61 0.59
Standard 0402 0.43 0.45 0.51
Mini 0402 0.43 0.44 0.51