black pad reference

sha_apr

有时候客户会直接问到MI组的人，如何克服“black pad”的事情。
直接转给流程工程师也行，或者问他们拿到信息转给客户也行。
这里的信息供参考。

sha_apr

原来查看附件内容和下载一样都要扣“金钱”的，好像和“共享”有一点点违背 --- 我的确没有啥分享的，我现在是学习阶段，那又如何办呢，难道真的要我到处发水帖？哈哈
还有，自是自己要查看，也要扣“金钱”，这不是有点“扯”么？

sha_apr

还好，找到了原文件。帖出来给大家看看
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SEPTEMBER 2008  PRINTED CIRCUIT DESIGN & FAB 21
B e h in d  B la c k  P a d  
O V E R  T H E  P A S T  
ten years, composite
coatings of
electroless nickelphosphorous
and
immersion gold
(ENIG) have
become established
as the preferred
solderable surface
finish for high reliability
applications involving complex
circuit designs. The ENIG coating
has gained market share and demonstrates
versatility in a wide range of
component assembly methods including
solder fusing, wave soldering
and wire bonding, as well as the ease
with which it transitioned to lead free
assembly.
The ENIG finish provides a highly
solderable flat surface that does
not tarnish or discolor. It has a long
shelf life, and the precious metal topcoat
provides excellent electrical continuity.
The nickel serves as a barrier
against copper diffusion and prevents
copper contamination of the solder
during wave soldering and rework
operations.
About eight years ago, a major
OEM brought to the attention of the
industry a low level of interconnect
failures when ENIG was used as the
surface finish. The failure mode is
associated with a poorly formed joint
at the solder/nickel interface. When
the suspect joint is stressed, the connection
is easily broken leaving an
open circuit with dark corroded nickel,
commonly referred to as ªblack
pad.º
Initially, it was thought that the
cause was the formation of Au/Sn
intermetallic, but now it is understood
that gold is not part of the
intermetallic which is strictly Ni/Sn.
It is a phosphorous rich layer that
is a natural component of the Ni/Sn
solder joint.
Subsequent investigations have
shown that excessive nickel corrosion
during the immersion gold deposition
causes this condition, now commonly
referred to as black nickel or black
pad.
The immersion reaction by which
the gold displaces the nickel is a
displacement or corrosion reaction
that does not produce black pad or
soldering defects. So what is excessive
corrosion of the nickel and how does
it occur?
Under normal deposition conditions
the nickel deposit has an even
topography. This type of nickel
deposit will never produce a black
pad. It is only when an irregular
topography with distinct crevices
between the domains occurs, that
corrosion may initiate and cause
black pad.
Irregular topography can be
caused by a contaminated incoming
copper surface or inadequate
pre-treatment in the front end of the
ENIG line. Additionally, the nickel
bath itself could create the irregularity
during the course of deposition.
The nickel bath is constantly plating
and needs to be replenished to maintain.
This operation must be carefully
controlled to ensure the desired
outcome. A compromised deposit can
occur from by-product build up if the
bath is operated beyond its recommended
bath life. Another cause is a
higher than normal deposition rate
that can result from having a high
temperature and/or pH, operating
outside the recommended range. A
well-controlled nickel bath is the key
to the elimination of this defect.
Since the black pad occurs during
the gold deposition step, what is
the role of the gold bath, if any, in
creating the defect? Ideally, for every
two atoms of nickel metal oxidized to
nickel ion, one gold atom is reduced
to gold metal.
The nickel released into the gold
bath over time should follow the stoichometry
of the chemical reaction as
shown in EQUATION 1. If the amount
of nickel produced exceeds the calculated
value, the immersion gold bath
is labeled ªaggressive.º Aggressive
gold baths are more prone to producing
black pad on a compromised
nickel surface.
Ni   Ni++ + 2e
2Au + 2e   2Au+  


Equation 1
EQ. 1
Over time, two major developments
in ENIG deposition have
occurred. The first is the awareness
of the suppliers and manufacturers
of the criticality of the process
control in the ENIG line. Shops that
implement process control and are
ISO 9 0 0 0 certified stay clear of this
problem. This is a buyer beware
situation in that often you get what
you pay for.
The second major development
is the IP C -4 5 5 2 ENIG specification.
The document specifies 2 to 4 micro
inches of immersion gold. It is now
clear to all that the gold is only
there to protect the nickel until it is
soldered. These changes have gone
a long way toward eliminating the
occurrence of black pad.
L ike any other chemical process
in the manufacture of a P C B , if the
process is not controlled and run
to vendor specification, the results
lead to defective product. In today's
sophisticated electronic manufacture
process, it comes down to process
control as the only insurance for
continuous success. PCD&F



GEOR GE M IL AD  is  th e  n a tio n a l
a c c o u n ts  m a n a g e r  o f te c h n o lo g y  a t
U y e m u r a  In te r n a tio n a l C o r p o r a tio n  
(U IC ) a n d  c a n  b e  r e a c h e d  a t g m ila d @
u y e m u r a .c o m .
Pro c e s s  c o n tro l is  th e  o n ly  s o lu tio n  fo r c o n s is te n t s u c c e s s .
GEO RGE
MIL AD
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