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| 型號： | REPORT |
| 英文描述： | Report - Lead (Pb)-Free Packaging Strategy 2000-2003 |
| 中文描述： | 報告-無鉛（Pb）2000-2003免費包裝策略 |
| 文件頁數(shù)： | 12/16頁 |
| 文件大小： | 188K |
| 代理商： | REPORT |
processes used for connecting die in packages. The well-known IBM C4 process, of course, uses not only a Pb-
based solder but also a solder that contains mostly Pb. No acceptable alternatives to the Pb/3Sn eutectic (318°C
melting point) and related alloys used in this process have been identified. A good argument has been made that
C4 contributes only a very small portion of the Pb contained in present-day electronic equipment and C4
applications have been granted exemption in the current version of ROHS. Flip-chip processes that use solders
with lower Pb content (non-C4 assembly processes) would probably not be exempted from regulation. But even if
flip-chip-in-package applications are exempted from regulation, the solder used for the flip-chip connection would
have to be compatible with the elevated temperatures required by the board-level assembly process. There will be
a problem if the melting temperatures of the flip-chip solder, or the liquidus temperatures, are below the board
assembly temperature. For either of these cases, the flip-chip solder joint may melt or become soft during board
assembly, and the reliability of that joint may be reduced.
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8
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Packaging Issues Resulting from the Elimination of Pb in Solders
The issues for packaging that derive from a change to Pb-free solder assembly can be broken into several groups:
Metallurgical: selection of a compatible lead finish or solder ball alloy;
Temperature sensitivity: compatibility of existing package materials and construction with elevated board
assembly process temperatures;
Reliability: evaluation of the solder joint reliability of the new solder/lead finish system;
Cost: evaluation of the cost of converting manufacturing processes;
Schedule: managing the conversion to Pb-free processes.
The metallurgical issue of compatibility with the chosen material for board assembly is both an easy and a difficult
one for the component supplier. The easy part is that to a large degree customers will dictate the choice of
materials. The difficult part is that different customers may choose, for very logical reasons in each case, different
solder systems that they wish to use in their manufacturing. Selecting an alternate to Sn/37Pb solder is an engi-
neering problem for the OEM that does not have a unique solution. The component supplier may have the difficult
task of supporting multiple lead finishes or developing a package solution that is compatible with a variety of solder
systems. The choice is driven by external events that the component supplier may have little control or influence
over. Although industry organizations throughout the world are attempting to foster agreement to a single or limited
number of replacement alloys for board assembly, this effort is not yet complete.
For leaded packages the list of possible lead finishes is short. Plated tin (electrolytic or electroless), plated silver,
nickel/palladium (several variants), and nickel/gold have been suggested. Evaluation of these and other less likely
finishes have begun, but no clearly superior finish has been demonstrated. All have specific drawbacks: nickel/gold
and nickel/palladium may be too costly; tin has the problem of whisker growth; silver has the problem of
electromigration; etc. Any of these problems can be overcome, but successfully demonstrating the accomplishment
may require a substantial effort.
Lead Finish Issues
During the transition to use of non-Pb solders for board assembly, leaded packages with Sn/Pb finishes may
continue to be utilized. The solder finish on the package lead is thin, and it is only intended to ensure wettability of
the lead during assembly. The majority of the solder in the joint comes from solder paste screened on the board or
from solder plated on the board. So, the integrity of the solder joint is influenced only to a small extent by the lead
finish. However, with BGA-type packages the solder ball contributes most of the material in the joint. If a non-Pb
solder paste with a higher melting temperature is used on the board, then the Sn/Pb solder ball on the package
may melt during assembly before the solder paste melts. The lower melting Sn/Pb solder will alloy with the tin-rich
solder paste forming a non-eutectic alloy of indeterminate composition that will likely freeze before the solder paste
would have melted. In effect, a “cold” solder joint will be formed that may have low fatigue resistance. Thus, the Pb-
bearing solder balls on BGA packages will have to be replaced in most Pb-free applications.
For BGA-type packages not only the solder ball material but also the solder attach pad material on the package
substrate are of interest. Raw solder balls used in BGA package assembly can be manufactured of any desired
alloy. However, if the solder ball material is changed, then the interfacial characteristics at the substrate are likely to
change as well. Without a clear understanding of the metallurgy involved, predicting the results of a different
interface condition is difficult. Research organizations must be encouragement by industrial partners to obtain the
necessary metallurgical knowledge. Otherwise, the strength of the solder ball to substrate joint can only be
evaluated on an empirical basis.
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