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| 型號(hào)： | REPORT |
| 英文描述： | Report - Lead (Pb)-Free Packaging Strategy 2000-2003 |
| 中文描述： | 報(bào)告-無(wú)鉛（Pb）2000-2003免費(fèi)包裝策略 |
| 文件頁(yè)數(shù)： | 13/16頁(yè) |
| 文件大?。?/td> | 188K |
| 代理商： | REPORT |
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Similar considerations apply also to packages assembled with flip-chip processes. Although C4 assembly is likely
to be exempted from regulation, non-C4 processes (those using low Pb content solders) will be included in the
regulation. The solder used for the flip-chip joint needs to be compatible with the elevated process temperatures of
the new board assembly solders. Melting or significant softening of the flip-chip joint during board assembly is very
undesirable and may degrade the reliability of that joint. Thus, solders with melting points or solidus temperatures
below 260°C should not be used. Packages that are attached to boards only with sockets are not be effected by
board assembly processes, but may still need to avoid the use of Pb solders to ensure that the final product falls
within the scope of regulation. Any change in the flip-chip solder alloy may require a change in the barrier layer
metallization or the solder bump structure as well. The same issues apply to passive chip components used in
packages or assemblies; contact metallization and stability after high temperature reflows have to be evaluated.
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Assembly Issues
A potentially more difficult problem for the component supplier comes from an increase in the board assembly
processing temperature that comes with a change to Pb-free solders. Most replacement solder alloys have melting
or liquidus temperatures that are higher than Sn/37Pb by as much as 20°C. There are several factors that
determine the processing temperatures for Sn/Pb assembly, and they also determine the processing temperature
for any other solder. Current board assembly process temperatures are from 40°C to 50°C above the melting point
of 183°C for the eutectic Sn/Pb. This temperature difference is necessary to ensure that all components going
through a reflow furnace reach temperatures where the solder liquefies and wets the component leads. The
process temperature must account for variations in board size in a given manufacturing line, and for varying
thermal masses of components on the board, in order to satisfy throughput requirements. The process temperature
must also compensate for the fact that often the composition of material at the solder joint is not exactly at the
eutectic composition. Material contributed by the board finish, solder paste, and lead finish may all deviate by ±5
percent from the eutectic composition, and such deviation increases the melting (now liquidus) temperature. In an
actual assembly the liquidus temperature may be as much as 10°C above the stated melting point of 183°C
because of the variation (intentional or not) in solder composition. For non-Pb materials the same situation will
exist; manufacturing variation will result in compositions that deviate from the specified composition, and the actual
liquefaction temperatures will deviate as well. For all these reasons, processing temperatures up to 260°C can be
expected for alloys with melting points up to 220°C. And in order to maintain assembly throughput without adding
significantly to the size of reflow furnaces, heat-up and cool-down rates may be higher than those used at present.
It must be noted that for BGA packages, the above considerations apply to the ball attachment process as well.
Temperature Effects
Higher assembly process temperatures affect packages in several ways. If the package contains absorbed
moisture, then the elevated temperature of the reflow process and the ramp to that temperature can cause the
moisture to desorb from the package rapidly with a high risk of crack formation or fracture. This is the classic
“popcorn” effect. Current test methods to determine the susceptibility of packages to this problem specify peak
reflow temperatures of 225°C (or 240°C for thin packages). The tests are empirically derived, and there is no
damage at higher processing temperatures cannot be predicted.
In addition to the moisture-driven effects, higher temperatures can also degrade the package integrity by directly
effecting the adhesion at interfaces, and by stressing the structure as the temperature goes ever farther above the
glass transistion temperature (Tg) of the materials. Some of these effects can be modeled by use of finite element
analysis (FEA), but a thorough understanding of the material properties and the interfaces is necessary both to
construct adequate models and to interpret them. In the case where package functionally survives the non-Pb
assembly process (that is, the electrical performance of the device does not degrade as a result of the assembly
process), a reduction in the useful life of the component due to incipient defects initiated by the assembly process
would remain a concern. Reduction in adhesion may result in an earlier onset of catastrophic delamination;
development of harmful intermetallics at wirebond interfaces may be initiated; and harmful residual stresses may be
induced that lead to early thermo-mechanical failure. Mold compounds, die attach adhesives, underfill adhesives,
and substrate materials may all exhibit significant temperature-driven degradation; whether such degradation
occurs is very difficult to demonstrate except by extensive testing. Clearly, if it is necessary to develop or adopt new
materials to prevent such degradation, then the time required to implement a conversion to Pb-free manufacturing
may be dramatically increased.
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