Electronic manufacturers, as they implement lead-free repair and rework processes, are quickly recognizing higher reflow temperatures can quickly damage array packages.  To reduce damage to components, many companies are investigating suppliers of re-work equipment to validate their offerings while meeting lead-free re-work/repair requirements.

In the past, rework and repair equipment provided a basic process with results directly dependent on the skill sets of the operators.  High cost, delays in production thru-put, component damage and quality assurance concerns were direct subsets of early technology circuit board rework or repair procedures. Today, leading manufacturers of rework-repair equipment have developed and implemented advanced technologies to mitigate the issues of component damage. With lead-free solders as an added variable, reflow temperatures are higher, time above the higher reflow temperatures are different, appearance of the joint is considerably different, and the need for process control is even greater than for eutectic solders.  Reworking array package components using lead-free solder follows similar steps to leaded components with eutectic solder: establish and validate the thermal profile, remove the failed component, clean and prepare the site, prepare and place the new component, reflow, and inspect. As found in the assembly process, precisely controlled convection heating, not radiation, achieves a repeatable process.

Many different lead-free compositions exist; the most common are based on tin alloyed with small amounts of silver, copper or bismuth, with melting points in the range 206-221°C. Solder peak temperatures are higher, 217°C to 235°C.  The window for lead-free processing is smaller than with leaded solder materials.  The time above reflow is often reduced from 60-90 seconds for eutectic tin-lead solder down to 15-30 seconds for lead-free. Rework systems must be capable of ramping up and down very fast to achieve these temperature requirements. Using convection heating permits the development of a precise, repeatable thermal profile that will not overheat or over stress the package.  Establishing the ideal profile is only part of the process.  Profile development requires knowledge of materials and usage of equipment designed to meet process requirements.

The higher temperatures needed for lead-free, coupled with the thermal sensitivity of BGAs and CSPs, demands precise t0emperature control and the addition of a ramp stage where temperatures rise at a rate that will not harm packages. Today’s more sophisticated rework systems employ four heating zones and one cooling zone. Successful lead-free rework is difficult to achieve without this extra step.  Higher temperature requirements and thermal sensitivity of area arrays can be problematic without the ability to ramp temperatures at a rate that will not harm components. Having a controllable pre-heater allows for efficient pre-heating that avoids the thermal damage risk when working with expensive, but sensitive, packages unsuitable for heating above 235°C with quick reflow times.

A typical lead-free profile would be to pre-heat to 140°C in 100 seconds, followed by a soak zone below 170°C for 90 seconds , then a ramp up to 225°C in 100 seconds, reflow up to 235°C for 20 seconds and then a cool down for 60 seconds. The differences between this and a tin-lead profile are substantial, and the key is system control with the ability to ramp up and cool down quicker.  Another factor to consider with lead-free solder is the temperature difference, or delta T, across the soldering area. A ∆T of 10°C is considered acceptable to produce a good tin-lead joint, but this is halved to 5°C for lead-free joints.  The wetting process and temperature profiles must be controlled to ensure the resulting joints are strong and not brittle. Improved heating regulation and a faster ramp-up are also needed with lead-free solder – particularly in the under-board heater, which means that hot plates should not be used. Temperatures must be high enough to melt and form intermetallics, activate flux and optimise wetting, yet low enough to avoid damaging the PCB and component.

While the basic rework steps remain the same, the substantial temperature differences between eutectic and lead-free solders require tighter processes, better temperature profiles, and the use of precise rework systems with closed-loop process control to ensure high quality, low cost rework.  For additional information on OK Industries rework-repair equipment or to schedule a demonstration of the OK Industries rework-repair equipment located at the EMPF, please contact Robert N. Berta; telephone at 610-362-1200 ext 253 or via e-mail at rberta@aciusa.org.