Pb-free solders

Panasonic achieved the full adoption of Pb-free solder using Sn-Cu in 2001. 

The classical electronics solder alloy (Sn + 37 wt% Pb) contains the inhalation-toxic element lead and is being replaced in electronics for health reasons. A Pb-free solder is Sn-3.5wt% Ag indium and bismuth alloys are other eutectic-forming alloying additives to Sn. The classical brazing alloy is at the Cu-Ag eutectic (Ag-F 28.1 w% Cu) and is well suited for mechanically strong contacts with Cu and its alloys. 

Generally, ICs, passive devices, and connectors are not rated to survive temperature excursions much above that of eutectic Sn-Pb solder and are rated only to survive a brief thermal cycle. IC makers and component manufacturers have worked to understand the impact of the higher process temperatures required for Pb-free soldering. 

The most widely used workmanship standard, IPC-A-610, has been rewritten to accommodate the inspection of Pb-free soldered assemblies. The changes were introduced with version D of that document. Inspection personnel require retraining to give proper attention to the... 

In the United States and other countries, some legislative initiatives aimed at restricting the use of Pb have failed. This failure spurred research for solder replacement alloys. Several corporations and universities devised Pb-free solders, some of which were patented. As is the case with any patented item or process, the rights to invention reside with the inventor or sponsoring corporation in the short term, and several of these alloys may not be used freely. In some cases, only slight modifications in alloy composition separate corrunonly available solder alloys from patented solder compositions. The cost of patent licensing, whether by the end user or solder manufacturer, will be borne largely by the end user. 

In almost every case, the cost of Pb-free solder is at minimum 20 percent higher than for Sn-Pb solder. PWB laminates are more costly, as are many of the components. Energy costs go up with the need for higher reflow soldering temperatures. There is significant cost in training of persormel to deal with the change to a Pb-free environment. Separate lines for Pb d and Pb-free solder may be required. 

China, Japan, and Korea have legislative initiatives taking shape that are similar to the EU s, WEEE, and RoHS directives. This is the case for several U.S. states also. There is no escaping the fact that Pb-free soldering will be the mainstay of all future electronics interconnections. 

Good Pb-free soldering with no need for nitrogen... 

The need for higher processing temperatures limits the assembly process window of Pb-free solders. A higher nominal temperature is needed to accommodate the temperature variation at components across a circuit board to ensure melting of the solder and adequate wetting and spreading at each interconnection. On the other hand, the maximum temperature must be limited to prevent thermal damage to heat-sensitive devices and the circuit board. 

The intrinsic solderability performance of Pb-free solders is being improved by two means. First, new flux formulations are available that more effectively reduce the surface tension of the solder. Second, alternative surface finishes can be specified for the component I/Os and/or circuit boards that improve wetting and spreading activity exhibited by the Pb-free alloys. 

Strictly from an assembly process point of view, the mixing of Pb-free and traditional Sn-Pb solder can be beneficial. The Sn-Pb solder can improve the wetting and spreading performance of the Pb-free solder by two phenomena. First, Pb contamination lowers the molten solder surface tension of the solder. Second, the Pb contamination reduces the melting temperature of the Pb-free alloy. However, concerns are raised by the mixing of Sn-Pb and Pb-free solders and its effect on the long-term reliability of interconnections under thermal-mechanical fatigue environments. 

Lastly, the use of Pb-free solders impacts the post-assembly cleaning step (step 4) and the inspection step (step 5). The higher process temperatures can produce more tenacious flux residues that require more aggressive cleaning steps to ensure their removal. Also, the more tenacious residues affect the ability of test probes to contact test site pads on the circuit board. Poor contact can be responsible for detecting false opens on the assembly. 

Impact of Pb-Free Soldering on Through-Hole Technology... 

Referring to the solder paste printing process with a stencil, Pb-free solder pastes behave very similar to the Sn-Pb pastes for leaded and area-array pitches of greater than 0.5 mm. At the smaller pitches that are characterized by smaller apertures, it has been observed that the Pb-free solders have a slightly reduced transfer coefficient. The likely... 

The change to Pb-free solders does not have an explicit impact on component placement machine technology. Indirectly, however, the need for alternative surface finishes on both the components and circuit board fiducials, which have different reflectance characteristics, can affect the performance of the vision systems used to locate accurately both the circuit board and the tooling that delivers the component to the board. 

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