Datasheet

AD8158
Rev. B | Page 33 of 36
1.35mm × 1.35mm SQUARES
AT 1.65mm PITCH
COVERAGE: 68%
06646-151
(A) (B) (D)(C)
VIA
SOLDER
MASK
COPPE
R
PLATING
0
6646-152
Figure 51. Solder Mask Options for Thermal Vias: (A) Via Tenting from the
Top; (B) Via Tenting from the Bottom; (C)Via Plugging, Bottom; and (D) Via
Encroaching, Bottom
Figure 50. Typical Thermal Paddle Stencil Design
Large voids in the thermal paddle area should be avoided. To
control voids in the thermal paddle area, solder masking may
be required for thermal vias to prevent solder wicking inside
the via during reflow, thus displacing the solder away from the
interface between the package thermal paddle and thermal
paddle land on the PCB. There are several methods employed
for this purpose, such as via tenting (top or bottom side), using
dry film solder mask; via plugging with liquid photo-imagible
(LPI) solder mask from the bottom side; or via encroaching.
These options are depicted in Figure 51. In case of via tenting,
the solder mask diameter should be 100 microns larger than
the via diameter.
A stencil thickness of 0.125 mm is recommended for 0.4 mm and
0.5 mm pitch parts. The stencil thickness can be increased to
0.15 mm to 0.2 mm for coarser pitch parts. A laser-cut, stainless
steel stencil is recommended with electropolished trapezoidal
walls to improve the paste release. Because not enough space is
available underneath the part after reflow, it is recommended
that no clean Type 3 paste be used for mounting the LFCSP.
Inert atmosphere is also recommended during reflow.