|
What are the metallurgical and assembly process issues
to consider in the selection of a tin or nickel-palladium-gold
lead finishes for lead-free component packaging?
Pb-Free IC component packaging forces reconsideration
of almost every aspect of IC packaging. ‘Lead-free’ IC
components require that the component bill of materials
is free of lead and can tolerate lead-free board level
assembly. Most leaded IC components have historically
been plated with a Sn-Pb layer, of approximately 90%
Sn – 10% Pb. This lead finish obviously needed
to be replaced, and while preferred lead finishes have
emerged, the choices remain fairly wide. The most widely
endorsed lead-free lead finish is a plated tin layer,
and debate exists even with this finish.
Standard leadframes are supplied with a bare copper
surface, and spot-plated silver on surfaces that
will be wire
bonded (lead tips, and possibly on areas on the die
attach paddle). The lead-free component’s
tin leadframe finish is plated onto the bare
copper leads only after
all IC assembly and transfer molding has been completed.
Tin has long been known to develop microscopic protrusions,
or whiskers, over extended time periods. Historically
tin has been plated using three types of plating baths.
The first produces very shiny Sn, and is referred to
as full-luster tin. This tin possesses a relatively
high amount of grain refiners, resulting in the shiny
appearance.
This tin finish provides fairly good solderability
over extended storage life periods. A matte-Sn finish
generally
possesses very low levels of grain refiners, and will
not have a shiny appearance. This Sn finish has enhanced
solderability, but may not provide as long a solderable
storage life period as full luster tin. Another Sn
finish is referred to as half-luster, and this material
contains
a lower grain refiner content than full luster Sn,
and exhibits a solderability storage life about midway
between
that offered by full luster and matte tin. The stress
in the tin deposit is generally the highest in the
full luster tin, and the lowest in a coarse grain matte
tin
finish, so half luster and full luster finishes are
generally not considered to be suitable for IC component
lead finishes
due to concerns about tin whiskering.
Studies have also shown that the coarse Cu-Sn intermetallic
phase that forms at the interface between the tin and
copper alloy leadframe increases stress on the tin
layer and aggravates whisker formation. European IC
suppliers
have endorsed the use of a plated nickel layer under
the matte tin finish to further reduce risk of whisker
growth. While this approach may provide benefit, it
adds significantly to the leadframe cost, so it is
generally
not a requirement for matte tin finishes.
The Ni-Pd-Au lead finish is also widely accepted, and
is commonly the second choice in lead finishes from
many IC suppliers. This lead finish offers a highly
solderable,
durable lead surface, and provides good solderability
even after long-term storage. Unfortunately, the Ni-Pd-Au
finish is more expensive than other lead-free lead
finishes. Another benefit is that the non-oxidizing
surface does
not contaminate tester hardware, while Sn-Pb and Sn
finishes will transfer oxide to test contactors and
sockets, requiring
cleaning over time.
The Ni-Pd-Au lead finish is applied to the entire leadframe
surface by the leadframe supplier. Consequently, the
expensive Pd-Au materials coat non-interconnect areas,
as well as the areas to be wire bonded during component
assembly, and soldered during board level assembly.
There are also two other considerations in the selection
of
Ni-Pd-Au lead finishes. As a noble metal, it is more
difficult to achieve excellent adhesion between die
attach adhesives or mold compounds, and the gold surface
of
the leadframe. As a consequence, at the present time
the Moisture Stress Level (MSL) rating of a lead-free
component with the Ni-Pd-Au finish is generally lower
than that of a lead-free component with the matte tin
finish. The thickness and characteristics of the matte
tin plating is generally about the same as that of
the standard Sn-Pb finish, allowing the use of the
same leadframe
trim and form tooling. The Ni-Pd-Au stack thickness
is much less than that of the matte tin, and this may
require
a dedicated set of trim and form tooling for lead-free
packages with the Ni-Pd-Au finish.
|
