- Packaging and footprints of leaded and lead-free components can be different. Those differences can be the basis of costly manufacturing errors.
- A mixed technology PCB with leaded and lead-free components can introduce new and costly mistakes. These components require different thermal profiles, hence correct component placement is critical.
- Quality control measures embedded in assembly processes must be increased with lead-free PCBs. With lead-free, the probability of defects such as BGA voids, tombstoning effects, and tin whisker formation increases.
- Effective lead-free component procurement is critical at an EMS provider. Lead-free components replacing leaded ones must be carefully cross-referenced by experienced procurement specialists.
There are other lead-free areas that need to be resolved and the industry is coming together to provide solid answers. Take for example the tin whisker problem, which is not new, but is aggravated in lead-free applications. Tin whiskers are electrically conductive crystalline structures of tin. Sometimes they grow from surfaces where tin, especially electroplated tin, is used as a final finish.
Lead-free alloys are composed of tin, silver, and copper. The lack of adequate lead content and greater amount of tin in this alloy triggers tin whisker growth. Tin whiskers can grow to lengths of several millimeters (mm) and in some cases, to lengths up to 10 mm. Electronic system failures are often attributed to short circuits caused by tin whiskers that bridge closely-spaced circuit elements.
Intel Corporation has made a major contribution in this particular area. It is tin-plating the copper in its lead-free products at 150ºC for 24 hours to reduce the potential of creating tin whiskers. Companies like this are increasingly engaging in consortiums and forums dedicated to discussing and resolving lead-free issues and its associated problems.
PCB surface finishes and the correct solder paste represent another problematic area. Lead-free assembly requires finishes such as electro-less nickel immersion gold (ENIG), immersion silver, organic solderability protectants (OSP) and lead free hot air solder leveling (HASL).
Lead Free surface finishes are critical because their conductivity is considerably higher compared to the tin-lead variety used for eutectic PCBs. These finishes withstand higher reflow temperature cycles. There is also less probability of the pads peeling away from the board surface when exposed multiple times to higher temperature cycles.
The question is which one to use since there are trade-offs and limitations. HASL has a shelf life of about 18 months, while OSP has only six months. Immersion silver has a shelf life ranging from 12 to 16 moths, while ENIG is the most durable at 24 months. OSP cannot undergo more than two or three reflow cycles. If more rework is required, then the SMT pads on the OSP finish begin peeling off. OSP, therefore is not the best finish when multiple rework cycles are involved. But immersion silver or gold can undergo six to eight reflow cycles. Also, they have a flatter surface finish, which is more conducive to a perfect assembly than a HASL finish, specially for fine pitch devices and BGAs and CSPs.
As for solder pastes, alloy selection has been a major consideration for assuring solder joint quality, reliability and production yields. Most EMS providers have chosen tin-silver-copper alloys (SAC) for leaded solder replacement. Two popular ones are SAC305 and SAC307 which are slightly different in alloy compositions. However, SAC305 or Sn96.5 Ag3.0 Cu0.5 with a melting range of 217-220°C has been the alloy most EMS providers use due to a higher degree of solderability and flux wetting.
Other problematic areas currently being resolved by solder paste suppliers and EMS providers are shrink holes and black pad. A shrink hole is an anomaly caused by a crack in a solder joint that uses either SAC305 or 307 alloy. At times, alloy chemistry reacts in such a way that it creates this crack or opening. This occurs occasionally, but not all the time. When it does, costly rework is required.
Black Pad is a defect in ENIG boards. It is a separation of solder joints formed on the surface of the electro-less nickel underplate. This is commonly attributed to excessive phosphorous in the electro-less nickel. However, any kind of nickel contamination, phosphorous or not, resulting in defects are often reported as "black pad" failures. In most instances, black pad emerges due to inexperienced PCB ENIG fabrication and not assembly.
Therefore, it is always prudent to rely on experienced EMS providers with extensive knowledge of lead-free PCB design, fabrication, and assembly. Trained technicians and engineers can assist OEMs to make a successful transition into lead-free.