Who's afraid of lead-free rework? The rules of rework change when dealing with lead-free assemblies.
The first, and most important, consideration is the higher temperatures required for lead-free alloys. In some cases, reflow temperatures may exceed 260[degrees]C. This fact should not pose a problem for most rework equipment, but extra care must be taken to prevent thermal damage to the board, components and conductors on the board. Dwell times at liquidous must be carefully monitored, since oxidation and degradation occur faster at higher temperatures. Solderability may also be compromised.
Another temperature-related concern is component "popcorning" due to moisture content--a potential problem, again, due to the higher temperatures required by lead-free alloys. Strictly observe specified pre-bake times and temperatures prior to rework. Also, be sure to observe correct storage procedures, particularly for moisture-sensitive components.
Another issue, perhaps a boon to soldering tool manufacturers, is contamination and compatibility. Quite a few lead-free alloys are available--many of them markedly different. Life was much simpler with tin/lead--an operator may have had six soldering tips, all wetted with tin/lead. Now, that same operator may need separate sets of tips for each individual alloy encountered. The alloys should not be mixed--a tip wetted with one alloy should not be used for another. Cross-contamination is a real concern, since the reliability of solder connections can be drastically altered by accidentally mixing small percentages of different metals or alloys. The same principle may also be applied to fluxes--certain fluxes are specifically designed to work with certain alloys to accommodate wettability issues or higher temperatures. Ensure that the proper flux is used for the alloy in question.
Ceretain pieces of rework equipment may not be capable of reaching the higher temperatures mandated by lead-free alloys, Virtually all hot air-based systems feature some type of digital control and display for setpoint temperature. However, the user should test the hot air output using a thermocouple or sensitive thermal measurement device to ensure that the reading is correct.
In addition to the need for accurate temperature control, lead-free rework systems often need the ability to develop and store, for recall, thermal profiles that mimic the original reflow profile. Most higher-end ball grid array (BGA) rework machines now include enhanced programming and operational features whereby programs may be created and edited, The rework cycle is automatically controlled by the machine and switched off after the cycle is complete. Recipes may be stored in the unit's controllers or in an external computer.
Thermal profiles may be developed with the help of feedback thermocouples or multiple sensors (sometimes up to six or more) and optimized with the aid of various software packages supplied by the rework equipment manufacturer. Data logging is a typical standard feature--allowing recipes to be transferred via the Internet. Higher temperatures may also require bottom-side preheating--for this reason, rework systems for lead free must be accurate and able to carry out complex recipes, including bottom-side preheating, and be programmable. Such systems should also possess sufficient heating power to perform the job without being forced to program dangerously high set points, a typical problem with underpowered units.
Operators Really Make the Difference
Operator training, as always, is essential--especially in lead-free rework. Operators should understand the differences in procedures, special concerns and care needed for any lead-free operation. In particular, these staff members should be trained to understand the appearance and acceptability requirements of lead-free solder joints. Many lead-free solder connections, depending on the alloy, are not necessarily shiny or cosmetically appealing.
As with any new technology or procedure, document everything--including materials, equipment and operator training. Attention to detail, extreme care and proper training will ensure reliable results in lead-free rework.
Jeff Ferry is president of Circuit Technology Center, Haverhill, MA; (978) 374-5000; www.circuittechctr.com.
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|Title Annotation:||Rework and Repair Depot|
|Date:||Nov 1, 2002|
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