Assuring Reliability in Electronic Products Utilizing Lead-Free Non-Hazardous Materials in Soldering
Tin/Lead - "solder" - has served as the primary bonding and conducting agent for electrical interconnection of wires and components in electronic and electrical equipment for decades. However, lead has been determined to be a hazardous substance and the use of tin/lead solders are being restricted by government laws and regulations such as the European Union's RoHS Directive (Restriction of Hazardous Substances).
Alternate solder alloys such as "Tin/Copper/Nickel" – "Tin/Copper" – "Tin/Copper/Silver", and "Tin/Silver" are available as potential replacements for Tin/Lead. Although the Tin/Lead solder technology has been well documented for it’s reliability, durability, and maintainability, the Lead-Free solder alloys are not equivalent "drop in" replacements.
Several Lead-Free solder alloys are available as shown in Table 1. Each has advantages and potential problems. Their effectiveness under all potential operating conditions has yet to be established. Their process characteristics are more likely to result in solder deficiencies, component overstress and tin-whisker growth. When these compromising conditions are introduced into a severe operating environment the result can be premature product failure.
Accordingly, the RoHS Directive provides some exemptions to permit Tin/Lead in solder joints where reliability and long life are of critical importance (i.e telecommunications infrastructure, servers, security and data storage). Products, which utilize these exemptions are referred to as RoHS 5 compliant, whereas Lead-Free solder assemblies are referred to as RoHS 6 compliant.
As new Lead-Free solders and processes are implemented, due caution should be taken to assure that the process and procedures are meticulously correct.
The detailed procedure to create reliable Lead-Free solder assemblies is influenced by a significant number of variables including:
- Process time and processing temperature of the assembly
- The mass and size of the assembly including the printed circuit board (PCB) and mounted components
- Solder pot contaminants. Pots must be analyzed regularly for proper alloy composition and the absence of contaminants
- PCB type as defined by the Association Connecting Electronic Industry Standard IPC-610 (see footnote 1)
- The compatibility of alloys used on PCB, the electrical and electronic components, and the manufacturing processes
- Length of time allowed for the solder to flow (wetting time)
- Continuous maintenance of the equipment used in the soldering process is an absolute requirement.
Tin/Lead and Lead-Free automated soldering processes are both influenced by these variables. However, Lead-Free processing tolerances are significantly less forgiving and challenging to control. The problem becomes more evident in “Low Volume” production of multiple products.
Figure 1 illustrates the allowable margins for error between Tin/Lead and typical Lead-Free solders and their relationship to component thresholds. As shown, a smaller margin of safety results when using higher temperature Lead-Free solders.
When selecting a Lead-Free solder, the following factors should be considered:
- Reasonable melting temperature not exceeding 227°C
- Low surface tension for better wettability/solderability
- Limit peak process temperature to maintain reasonable margins of safety
- Self-centering characteristics of SMT devices to the PCB land area
- Cosmetic appearance of the final assembly