The lead-free (Pb) assembly process is used to assemble the DS2502 flip chip with high lead content
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Introduction The European Union recently issued a decree restricting the use of metallic lead (Pb) in electrical and electronic components. The decree is officially known as the 2002/95 / EC directive, but it is often referred to as the Restriction of Hazardous Substances Directive (RoHS). The RoHS appendix (page 5) contains the exemption conditions for this directive. Among them, Article 7 is: "Lead in solder melted at high temperature (eg, the lead content in tin-lead solder alloy exceeds 85%)." Maxim's solder joint process, commonly referred to as "flip chip", due to the use of lead ) 95% of the high melting point solder joint structure, which meets the exemption conditions of Article 7, is exempted from RoHS. Please refer to the DS2502 hazardous substances listed in Appendix A.
The environmental impact of lead is in the United States. In 1998 alone, about 10,900 tons of metallic lead were used for soldering electronic components. These metallic lead will eventually flow into the dump with waste electronic components. In 2006, more than 4.6 million tons of waste electronic components were stored in the US dump. With the widespread implementation of the RoHS directive, the amount of lead used in the soldering of electronic components and the total amount of lead in waste electronic components will be greatly reduced.
To meet the requirements of the RoHS directive, many manufacturers have begun to use lead-free assembly processes. These assembly processes can reduce the total amount of metal lead flowing into the dump, which in turn contributes to environmental protection.
According to the requirements of the RoHS directive, in the lead-free reflow soldering process, many assembly processes must use lead-free solder. Although flip chips are exempt from the RoHS directive, Maxim still uses a reflow soldering process with a peak temperature of 250 ° C to assemble more than 396 flip chips. After assembly, the flip chip is subject to industry-standard reliability testing. The critical failure number of the DS2502 flip chip is zero, and it has successfully passed the reliability test. This article describes the reflow soldering process used and related content of reliability testing.
The circuit board lead-free reflow soldering assembly process DS2502 flip chip is assembled with high temperature, FR4 PCB board with Tg> 170 ° C. See Appendix B for the CAD drawing of the circuit board. The flip chip is soldered to the circuit board using Indium5.1 lead-free solder paste from Indium CorporaTIon of America® (see Indium5.1LS product data sheet). The DS2502 circuit board is assembled in a 5-temperature BTU SSA 70 reflow oven. The welding furnace conveyor is set to move 25 inches per minute, and the temperatures in the 1 to 5 temperature zone are set to 185 ° C, 200 ° C, 215 ° C, 270 ° C, and 300 ° C, respectively. The peak reflow temperature is 250 ° C. The reflow soldering temperature curve is shown in Figure 1, and also refer to Appendix C. 
Figure 1. DS2502 reflow soldering temperature curve
The DS2502 reliability test performed industry standard reliability tests on 396 circuit boards assembled through a lead-free reflow process. Including working life, storage life, temperature cycle, temperature and humidity bias and unbiased moisture resistance. After all tests are completed, the critical failure number of the DS2502 flip chip is zero. Reliability reports containing all test results are available online.
Summary According to Article 7 of the RoHS Directive Appendix, Maxim ’s DS2502 flip chip is exempted from the RoHS Directive. However, in order to be more environmentally friendly, the flip chip can still be assembled using a lead-free reflow soldering process. Maxim has assembled more than 396 flip-chip circuit boards using a process with a peak reflow temperature of 250 ° C. After assembly, the industry-standard reliability test was performed. The critical failure number of the DS2502 flip chip was zero, and the reliability evaluation was passed. From this it can be concluded that the DS2502 flip chip can use the parameters provided in this document for lead-free reflow soldering assembly.
Appendix A. Composition of DS2502 hazardous substances 
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Appendix B. High temperature FR4 material drawing 
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Appendix C. Temperature curve of lead-free reflow soldering 
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