VERY HIGH TEMPERATURES AFFECT EVEN "STANDARD" POLYIMIDE LABEL MATERIALS.

POLYONICS'XF 581 AND 582 SOLVE THAT PROBLEM

The Push for "Lead-Free" Soldering

The European perspective on waste management and recycling as described in the European Union's Waste of Electrical and Electronic Equipment (WEEE) and Restriction of Hazardous Substances (ROHS) strongly suggests that lead-free electronic assemblies will be mandatory in Europe by 2008. Japan is moving toward voluntary compliance by 2003 with a lead-free initiative that focuses on environmental marketing of new products such as mobile phones, consumer electronics and automotive electronics (with the exception of recycled lead-acid storage batteries). The viewpoint concerning lead-free electronic assemblies in the U.S. is somewhat different since lead usage in electronic solder comprises less than 1.0% of total lead consumption. Despite this fact, technological obsolescence of end-of-life (EOL) electronic products resulted in 21 million used computers being dumped into the solid-waste recycling process in 1998.

An additional concern is that although the electronic solder contained in electronic products represents less than 1.0% of total lead consumption, 28.5% of all lead bearing materials going into municipal solid waste (MSW) in the U.S. is from associated cathode ray tubes (CRT's) contained in television sets and computer monitors. If lead-acid storage batteries could be 100% recycled and thereby removed from MSW, electronic solder and CRT's would represent more than 83.7% of the remaining lead bearing source material.

Implications for PCB Manufacturers

The pending legislation and market trends leading toward the implementation of lead-free electronic assemblies have raised several issues including the need to increase the thermal tolerances of electronic components. Lead-free solder alloys such as tin-silver-copper (SnAgCu) with a melting point of 217° C, require higher processing temperatures than traditional tin-lead (SnPb) alloys thereby reducing the process window and focusing on the need for rigorous control of the thermal process during soldering. This paradox between higher melting points of lead-free alloys and the thermal thresholds of electronic components forms the basis of the lead-free challenge. Raising component thermal tolerances will place a significant economic burden on electronics manufacturers since they will be faced with higher overall thermal processing costs. This increase in costs associated with the thermal process will drive electronics assemblers to maximize the efficiency of their thermal processes. This is especially true when soldering through-hole (TH) components in complex mixed technology printed circuit boards (PCB's), where greater thermal demands are placed on the process capabilities of existing soldering methods such as wave soldering.

Implications for Manufacturing Processes

Lead-free soldering of surface mount technology (SMT) has been studied extensively over the past several years while lead-free flow soldering of TH components has received little attention. However, in order for an electronic assembly to be considered as truly lead-free, every step of the assembly process must utilize lead-free materials including SMT reflow, TH soldering, rework, field repair and EOL recycling infrastructure.

Since the wetting characteristics of lead-free alloys are less than that typically exhibited by SnPb alloys at lower process temperatures, the selection of a good flux is mandatory to assure good solderability. The flux selected must be able to withstand exposure to the higher process temperatures required for lead-free alloys. In general, 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 a minimum of 3 seconds of contact time or longer. The major difference between SbPb and lead-free flow soldering is the higher melting point required by the lead-free alloy. Because lead-free alloys require higher preheat temperatures, thermal shocking of components as they enter the molten liquid wave should not exceed 100° C. For most applications, VOC-free, water-based fluxes, applied with a spray fluxer or inkjet drop fluxer, are recommended since they can withstand these higher processing temperatures.

Implications for Process Thermal Profiles

 In order to form quality solder joints, the flow soldering process irregardless whether wave soldering or site-specific selective soldering is used, must:

1) raise the temperature of the base metals high enough to allow sufficient wetting,

2) provide adequate contact time for capillary action to take place, and

3) provide adequate thermal energy to create an intermetallic layer, and a good electrical connection.

While lead-free solder alloys require higher processing temperatures due to the nature of their wetting properties, TH components can be damaged if their internal threshold temperature is exceeded by either rapid heating or excessive heating of the PCB assembly during the soldering process.

This is especially true in lead-free flow soldering since the process temperatures are typically 30-40° C higher than traditional SnPb alloy process temperatures for which most TH components were designed. A minimum dwell time is required to promote wetting of the through-hole component lead while a maximum dwell time should not be exceeded that could result in damage to the component. Likewise, a minimum temperature is required for the solder to melt and flow, while exceeding a maximum temperature could cause internal damage to the component.

Some Specific Temperatures Will Be Affected

There are two areas of thermal increases:

PREHEAT CYCLE: For most lead-free flow soldering applications, the topside PCB temperature will increase by as much as 125% to approximately 110- 130° C to limit the thermal shock as the PCB contacts the higher temperature of the lead-free molten solder. The ideal method is to heat up the PCB as quickly as possible to 100° C and continue to heat the PCB with forced hot-air convection preheating for optimal evaporation of the water medium from the water-based flux.

SOLDER POT:Due to the higher melting point of lead-free alloys, the temperature of the solder pot will also increase to improve solderability and shorten contact times. For AgSnCu with a melting point of 217° C, the solder pot temperature will be between 250-270° C or as high as 260-280° C for SnCu.

Impact of These Changes on the Circuit Board Labels Used

Each label material used must be re-evaluated against these new thermal profiles. If polyester was used in a location which now experiences the higher temperatures (and renders it useless), then the material must be changed to a different material, such as PEI or Polyimide Label materials. The higher temperatures may also require that standard polyimide labels be changed to  newer XF 582 Polyimide Labels,or one of the other NEW Lead Free Labels offered by Polyonics.