Lead-Free Soldering Methods

The introduction of the lead-free soldering method has led to an increase in the temperature requirements of between 20-30 °C and thus created a demand for products with improved properties. 

---

The introduction of the lead-free soldering method has led to an increase in the temperature requirements of between 20-30 °C and thus created a demand for products with improved properties. At the same time, materials that are resistant to high temperatures such as ceramics, metals and thermosets, may be driven out of their existing application areas and replaced by the more cost-efficient injection moulding process with LCP. 

B. Liu. Lead free solder friendly thermoplastic blends and methods of manufacture thereof. US Patent 7 037 986, assigned to General Electric Company (Pittsfield, MA), May 2, 2006. 

Solder requirements are the same as for any other process. There are no ahoy composition requirements specific to laser soldering, as this soldering method is compatible with leaded or lead-free solder alloys. Even high-temperature alloys can be soldered by this technique. When single-point laser reflow is apphed, the board quality and integrity are not compromised if parameters are chosen and adequately controlled. 

Rework for lead-free solders has been found to be more difficult, because the lead-free solder alloys typically do not wet or wick as easily as the Sn-Pb solder due to their difference in wettability. This can be easily seen with QFP packages. In spite of these differences, successful rework methods (both manual and semi-automatic) have been developed (Ref 74-75) with lead-free solders (Sn-Ag-Cu, or Sn-Ag), for many different types of components. Most of the rework equipment for tin-lead can still be used for lead-free solder. For area array packages, it is helpful to use a rework system with split vision and temperature profiling features. The soldering parameters must be adjusted to accommodate the higher melting temperature and reduced wettability of the lead-free solder. The other precautions for tin-lead rework (such as board baking) still apply to lead-free rework. 

J. Sjoberg, D.A. Geiger, D. Shangguan, and T. Castello, Alternative Assembly Methods for Lead-Free Solder Rip Chips on FR-4 Substrates, Proceedings of IM-APS Nordic, 2004... 

This chapter has focused on the numerical tools used in the design for reliability of lead-free solder joints. These tools include the finite element method, in general, and, in particular, the use of 3D strip models, specific material properties, and the use of a creep strain energy... 

J.P. Clech, An Extension of the Omega Method to Primary and Tertiary Creep of Lead-Free Solders, Proceedings of ECTC 05, Orlando, FL, May 31-June 2, 2005... 

MUCH HAS BEEN WRITTEN REGARDING THE EFFECTS OF LEAD-FREE SOLDER ON THE ASSEMBLY PROCESS. HOWEVER, IT IS STILL UNCLEAR HOW ALTERNATIVE SOLDER MATERIALS WILL AFFECT TEST AND INSPECTION METHODS. THIS ARTICLE EXAMINES THE ISSUES AND TRENDS ASSOCIATED WITH LEAD-FREE SOLDER QUALITY AND PCB INSPECTION TECHNIQUES. 

This article examines the issues and trends of lead-free solder quality and PCB inspection methods. It introduces a computationally based 3-D AXI technique designed to achieve false, failure-free inspection to detect subtle voids and other lead-free solder defects accurately, even as board complexities grow. 

For these reasons, more attention is focused on AXI methods. The use of lead-free solder has little effect on AXI systems (Figure 3). While lead-free solder is about 15 to 20% less dense than leaded solder, AXI has no problem imaging lead-free materials. AXI can see the full shape of the joint, make volume measurements, provide full heel-and-toe measurements and measure angles based on slope of solder density. In particular, AXI has the ability to easily detect voids, a capability that may become more important in the future. The loss of electrical and visual access, a side effect of recent manufacturing trends, does not affect AXI. 

The manufacturing processes for soldering electronic assemblies were developed over decades as electronics technology advanced. The substitution of lead-free solders for the standard tin-lead alloys has a considerable deleterious effect on manufacturing methods, the entire supply chain of materials and parts, and the environment. The main contributing factor is the higher temperature required to melt and use the lead-free solder alloys. Development and performance testing of... 

Incorporation of dispersoids has been pursued to improve the mechanical and, particularly, thermomechanical behavior of solders and their service-temjjerature capability without significantly altering the processing parameters [16-19]. This chapter addresses the role of dispersoids, their requirements, methods to incorporate them in a solder, and their influence on the microstructure and properties. Prior studies have dealt with several aspects of dispersoids in leadbearing solders, which will be briefly reviewed as they provide the basis for developments in lead-free solder systems. 

The NCMS project developed a large data set of the mechanical properties of lead-free solder alloys. For three Pb-free solder alloys and eutectic Sn Pb, these include creep data, constitutive equations, and microstructural data, as well as detailed descriptions of test methods, sample geometries, and material data for the components and boards used in the experimental testing, as listed in Table 15. Partial datasets for 75 additional solder alloys were generated and are included in the NCMS CDROM [2]. 

JWES is investigating the standardization of lead-free solder evaluation methods. It is necessary to standardize test methods related to the evaluation of material properties such as melting, solidification, mechanical properties, and wetting properties for comparison of Pb-free alloys. New standard test methods are expected to be established by 2002. JEITA plans to establish comparable standards for the evaluation of components and soldered parts as well. 

---