This sentence describes the root of the problem. "The customer says he
is following the same
parameters as that for HASL boards for assembly".
When soldering to immersion silver, the silver is readily absorbed into
the solder, and you are forming an intermetallic directly to copper. You
are dissolving some of the copper into the molten solder.
When soldering to HASL, you are soldering to solder that has already
formed an intermetallic bond with the copper underneath. So you are
essentially just blending the solder that is already there with the
solder paste or wire solder, and slightly increasing the intermetallic
formation with the copper. In both of these situations you are forming
an intermetallic bond with copper. Copper's ability to dissolve into the
molten tin/lead solder is relatively good, compared to other metals.
However, when soldering to ENIG, the gold on top of the nickel is
readily absorbed into the solder, but you form your intermetallic bond
with the nickel base that is plated over the copper. The rate at which
nickel dissolves into molten tin/lead solder is much slower than copper.
Thus, when soldering to ENIG, the profile must be slightly hotter, and
for a slightly longer TALT in order to achieve enough nickel dissolved
into the tin/lead solder in order to form a good solder joint.
To ensure that the profile is adequate, thermocouples should be place
over several locations on the circuit board, and the board used for this
should be fully populated. Once the coolest spots on the board (and also
the hottest) are identified, the reflow profile must be such that the
coolest pads reach a temperature of at least 210 deg. C, and for a
minimum time above 183 deg. C of at least 45 seconds. This will ensure a
good IMF between the molten solder paste and the nickel base.
If this is done, but problems persist, it would point to other
plating-process-related issues such as brittle nickel, black pad, etc.
But understand that these can be present on the BGA pads as well as the
circuit board pads.
Many times, the issue is more of a design-related problem where the
particular configuration of the circuit board leads to large mismatches
between the CTE of the circuit board and the BGA or other large
components. Usually the compliant pins of the larger SMT parts alleviate
this difference in CTEs. With BGAs or leadless (castellated) components
there is much less compliancy. When you take a board of this type of
design and couple it with the brittle nature of solder joints formed
with nickel, the result is stress fractures. Understanding the causes of
the stresses is the key to taking steps to increase the reliability, ie,
reducing the stresses placed on the solder joints.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of mp3
Sent: Tuesday, July 24, 2007 7:31 PM
To: [log in to unmask]
Subject: [TN] Fwd: ENIG solderability
>Date: Tue Jul 24 09:50:21 PDT 2007
>From: [log in to unmask]
>To: technet <[log in to unmask]>
>Subject: ENIG solderability
>
>Dear Sir,
>
>We have been having a few complaints from our customers regarding
>problems wrt solderability, especially in SMD and BGA pads with ENIG
>finish. We have checked the solderability at our end as per IPC and we
>find it to be acceptable. The customer says he is following the same
>parameters as that for HASL boards for assembly. Can we get some
>details on the extra precautions wrt assy of ENIG boards
>
>Rgds
>
>Pradeep.M
>AGM - Customer Support
>Micropack Limited
>Plot#16, Jigani Industrial Area,
>Anekal Taluk, Bangalore - 562 106
>India
>Tel : 91 80 7825223 / 7825224
>
>
--
Pradeep.M
AGM - Customer Support
Micropack Limited
Plot#16, Jigani Industrial Area,
Anekal Taluk, Bangalore - 562 106
India
Tel : 91 80 7825223 / 7825224
-------------------------------
Micropack Ltd, Bangalore, India
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