NOVEMBER 2022 I DESIGN007 MAGAZINE 51
Lead-free PCBs require that the standard
HASL surface finish cannot be used. ere
is still a significant amount of discussion on
what the long-term surface finishes will be.
Currently, the immersion silver and OSP sur-
face finishes are the most prevalently specified
surface finishes for solderable PCBs. Immer-
sion tin is the prevalent surface finish for
press-fit backplanes. Please contact your PCB
fabricator for current information on where
industry specifications are heading.
Final Finishes
ImmSn (Immersion tin)
Immersion tin is a single layer metallic coat-
ing of 15.7 µin to 23.7 µin [0.4 to 0.6 µm] of
immersion deposited Sn.*
is process coats a thin layer of tin directly
on top of the copper surface. e tin produces
an extremely flat surface for mounting of sur-
face mount components with ultra-fine-pitch
devices. is also provides a thicker, uniform
surface that provides lubrication for press-fit
pins. ImmSn is inexpensive and typically used
for flat surface. ImmSn is OK for fine-pitch
devices with solder mask dams greater than
5 mils [127 µm].
ImmSn is the top choice for press fit pin
insertion, for the following reasons:
• Careful handling required
• Poor environmental resistance of
intermetallic
• Not great for multiple reflow cycles
*Per IPC-4554 Nominal pad size of 0.060" x
0.060" (1.5mm x 1.5mm)
Understanding the cost drivers in PCB fab-
rication and early engagement between the
designer and the fabricator are crucial ele-
ments that lead to cost-effective design suc-
cess. Following your fabricator's DFM guide-
lines is the first place to start. DESIGN007
Anaya Vardya is president and
CEO of American Standard
Circuits; co-author of The Printed
Circuit Designer's Guide to… Fun-
damentals of RF/Microwave PCBs
and The Printed Circuit Designer's
Guide to… Flex and Rigid-Flex
Fundamentals; and author of The Printed Circuit
Designer's Guide to… Thermal Management:
A Fabricator's Perspective. Visit I-007eBooks.com
to download these and other educational titles. He
also co-authored "Fundamentals of Printed Circuit
Board Technologies" and provides a discussion of
flex and rigid flex PCBs at RealTime with… American
Standard Circuits.
