Leland Teschler
Executive Editor

Chipmaker <B />Texas Instruments Inc. </b>studied how lead-free solders would work with its surface-mounted components. One study concerned a leadframe finish comprised of four layers of nickel and palladium. TI had previously used a thin layer of lead to protect the underlying pure nickel layer from oxidation. Use of a lead-free finish changed the contact angle of solder. At top is an artist's depiction of an SOIC lead cross section finished with tin-lead. Below is an SOIC finished with nickel-palladium. Both cases use Castin lead-free solder paste. TI says the higher contact angle proved to be only a cosmetic issue and didn't change the mechanical strength or reliability of the joints.

Chipmaker Texas Instruments Inc. studied how lead-free solders would work with its surface-mounted components. One study concerned a leadframe finish comprised of four layers of nickel and palladium. TI had previously used a thin layer of lead to protect the underlying pure nickel layer from oxidation. Use of a lead-free finish changed the contact angle of solder. At top is an artist's depiction of an SOIC lead cross section finished with tin-lead. Below is an SOIC finished with nickel-palladium. Both cases use Castin lead-free solder paste. TI says the higher contact angle proved to be only a cosmetic issue and didn't change the mechanical strength or reliability of the joints.


An example developed by the National Electronics Manufacturing Center of Excellence depicts the diference in wetting properties of leaded and lead-free solder as defined by contact angle for solder on a copper surface. SnPb typically has contact angles of between 4 and 6 on a copper surface. The lead-free solder alloy SnAgCu generally has contact angles ranging from 6 to 12.

An example developed by the National Electronics Manufacturing Center of Excellence depicts the diference in wetting properties of leaded and lead-free solder as defined by contact angle for solder on a copper surface. SnPb typically has contact angles of between 4 and 6° on a copper surface. The lead-free solder alloy SnAgCu generally has contact angles ranging from 6 to 12°.


Electronics manufacturers are becoming familiar with a European regulation called Restriction of Hazardous Substances (RoHS). RoHS, together with another EU directive called WEEE (for Waste of Electrical and Electronic Equipment) are designed to reduce the amount of heavy metals and old electronics dumped in European landfills. RoHS is scheduled to take affect next year and will, among other things, ban lead-alloyed solder from all but a handful of applications.

There is some irony to these new dictates; experts say electronic solder comprises less than 1% of all the lead going into municipal solid waste. (Among the biggest culprits are CRTs, each contributing between 2 and 5 lb of lead.) Nevertheless, manufacturers must eliminate lead from solder by next year for many product categories sold in Europe. Domestically, numerous states are also considering legislation that installs RoHSlike limits on lead. Countries that include Japan and China are in the process of doing likewise.

The pending legislation and trends toward lead-free electronics have raised several issues among manufacturers. One of the most important is the need to increase the thermal tolerances of electronic components. Lead-free solder alloys such as tin-silver-copper (SnAgCu) have a melting point of 217°C and require higher processing temperatures than traditional tinlead (SnPb) alloys. Thus soldering operations must take place at higher temperatures. Moreover, components are exposed to soldering temperatures for a longer time than with leaded solder. This is because nonleaded solders have poorer wetting qualities than traditional formulations. Thus it takes longer for the necessary capillary action to take place.

Tin-silver-copper solder alloys along with tin-copper (SnCu) seem to be generating the most interest as replacements for leaded solder. IPC, the organization that develops standards for electronics assembly and printed circuit boards, says it is conducting performance testing of several SnAgCu alloys. NIST, the National Institute of Standars and Technology, also maintains an extensive database of metallurgical properties that may be helpful in finding tin-lead solder replacements.

All in all, the higher soldering temperatures can potentially stress electronic components built for temperatures encountered in leaded soldering.

Lead-free soldering processes will be more energy intensive as well because of the need to generate higher temperatures over longer dwell times. Electronics assemblers say these factors are pressuring them to maximize the efficiency of their thermal processes. The task seems to be especially difficult for soldering through-hole (TH) components in complex mixed-technology printed circuit boards (PCBs). Manufacturers say there is a potential for damaging TH components because their maximum tolerable internal temperature can potentially be exceeded by either rapid heating or excessive heating of the PCB assembly during lead-free soldering processes.

Lead-free soldering will also dictate use of special fluxes able to withstand exposure to the higher process temperatures. Assemblers say fluxes used for lead-free TH flow soldering must be able to withstand topside PCB preheat temperatures as high as 130°C and solder temperatures as high as 280°C for at least 3 sec of contact time.

Similarly, lead-free alloys require higher temperatures for the preheating that takes place in automated processes prior to soldering. The preheating is to limit the thermal shock as the PCB touches the lead-free molten solder. The preferred approach is to heat up the PCB as quickly as possible and then continue heating with forced hot-air convection. The fast heating limits the time components get exposed to high temperatures and also helps evaporate water from the water-based fluxes commonly used for lead-free processes.

Other issues that can arise because of the higher heat used in lead-free assembly concern circuit-board material. Thinner boards, for example, may be more prone to delamination, blistering, and peeling soldermasks. Plastic on components and connectors must also be spec'd for lead-free processing temperatures. In processing using wave soldering, boards may be much hotter when they exit the soldering station. This could make them too hot to handle when they get to human inspectors.

There are also accommodations for leadfreealloys in hand soldering operations. The American Competitiveness Institute (www.aciusa.org) found that solder tip temperature had to be set at 343°C for leadfree solders, compared to 315°C for SnPb. Researchers there also found that soldering irons had to be held on solder joints longer to promote adequate heat transfer. It was their experience that soldering irons had to be removed more quickly from lead-free joints to prevent the creation of icicles. ACI says the size and frequency of solder icicles depends on the purity of the alloy used and the soldering-iron temperature setting.

Lead-free solders are more sensitive to the effects of a dirty soldering iron, says ACI. The higher soldering temperatures can oxidize the tip if it isn't cleaned and coated.

Finally, solder joints made with lead-free alloys will have a grainy dull finish. This contrasts with the bright, shiny finish associated with good joints made of lead solder. The difference in appearance may make it more difficult for inspectors and inspection systems to distinguish good joints from bad ones.

MAKE CONTACT:

American Competitiveness Institute,
(610) 362-1200
www.aciusa.org
IPC
(847) 615-7100
leadfree.ipc.org
Texas Instruments
(972) 644-5580
www.ti.com