Lead-free wetting

A semiconductor mechanism was proposed for the electrocatalytic action of tin on the oxidation of PbO to PbO and Pb02. Tin has substituted antimony as an additive to the alloys for lead—acid battery grids. It is added in considerably lower concentrations than Sb and in combination with calcium which improves the mechanical properties of the alloys. Thus the interface problem of lead—calcium grids was resolved and the way was open for the manufacture of maintenance-free wet-charged batteries. 

The process window for lead-free soldering requires higher peak reflow temperatures and longer times above liquidus. Soldering in a nitrogen environment improves wetting and reduces oxidation of the flux residues and the solder. Due to the higher reflow temperature. 

Solderability Lead-free solder does not wet as well to solderable surfaces as leaded solder does, so pad size, solder quantity, and flux type and quantity must be taken into account. 

Surface Finishes. Tin-lead solder alloys (e.g., 63/37) are the most popular alloy used for surface finishes on PCBs. Other surface finishes are rapidly finding their way onto the PCBs. IPC-6012 lists more than 20 different surface finishes that are now in use for PCBs. Lead-free alloys are also appearing on PCBs. It is important to understand that the composition and type of the surface finish influence solderability. The procurement documentation must state specifications for surface finish. Methods available for analyzing the alloy composition on the plated PCB include wet analysis, atomic absorption, and x-ray fluorescence (XRF). XRF is popular because of the ease of obtaining the alloy composition and thickness nondestructively. 

In the presence of organic matter, after destruction by wet oxidation evaporate the solution until only 2 ml of acid is left. Dilute with 10 ml of water, add 2 g of lead-free ammonium acetate, make ammoniacal and then boil the solution until acid to litmus. Add concentrated nitric acid until the iron is redissolved, dilute with water to 30 ml and proceed as above. 

Surface Finishes. The search for alternatives to hot air solder leveling (HASL) has been ongoing for several years, primarily because of the inherent inconsistency in the quality of the HASL finish. For example, the thickness (and therefore, solderability) of HASL is difficult to control. In areas with a very thin layer of HASL, consumption of Sn by the formation of tin-copper intermetallics will render the areas non-wet-table. The HASL finish is typically non-flat (with a dome shape), making it difficult to deposit a consistent amount of solder paste during solder paste printing and difficult to place fine pitch (<25 mil) devices. The HASL process itself is not as clean and easy to control as some plating processes. The current move towards lead-free solder has provided the additional impetus towards alternative surface finishes. 

Reflow Process. The key parameter for the reflow profile is the peak temperature. Adequate reflow temperature is needed for the solder to melt, flow and wet, interact with the copper on the pad and the component termination, and form sound intermetallic bond when cooled and solidified. Typically, 30 °C (55 °F) superheat (above the melting temperature) is desired. Eor lead-free soldering, because of concerns about the thermal stability of the components, efforts are needed to minimize the soldering temperature. For SAC alloy with the eutectic temperature at 217 °C (422 °F), the minimum reflow... 

Lead-free reflow may be done in air or N2. Typically N2 is not required, and the use of N2 may even increase certain defects (such as tomb-stoiflng) especially for small passive components. In certain situations, N2 may help improve wetting (which in turn may help reduce the amount of voids in the solder joints). For flip chip applications, where flux is used instead of solder paste, N2 becomes necessary to form reliable solder interconnects. An N2 atmosphere with O2 level below 1000 ppm has been found to be effective (Ref 65, 67). 

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. 

M. Arra, D. Shangguan, and D. Xie, Wetting of Fresh and Aged Lead-Free PCB Surface Finishes by Sn-Ag-Cu Solder, Proceedings of APEX 2003, March 2003, p S12-2-1/7. Also, Journal of the Hong Kong Printed Circuit Association, 2004, p 22-29... 

It may be noted that the tin-lead solder is shiny with excellent wetting. The only lead-free solder joint that comes close to this feature is Sn96.5 Ag3.5, a eutectic solder. All lead-free solders show appearance of a graininess and lack of good wettability. 

Fig, 10 Optical photographs of J-lead packages assembled with Sn-Pb solder and four lead-free solders. Note differences in solder wetting angle and appearance. The lead-free eutectic solder (Sn96.2Ag3.5) has the closest appearance to tin-lead solder. 

Another issue with printing lead-free solder paste is stencil aperture design. Traditionally, stencil aperture size is reduced in relation to PCB pad size. This ensures the stencil aperture seals, or gaskets, to the PCB pad. Gasketing reduces solder paste that can get under the stencil and eventually cause wet solder bridges if not cleaned properly. Lead-free solder paste does not spread as well, so... 

With lead-free alloys, wetting generally is less than that of Sn/Pb systems. This may leave exposed comers or edges on the pads. If full coverage is required, then a change in stencil design is needed so that the stencil aperture covers 100% of the pad. 

Printability of lead-free solder paste will not change, but its spread during reflow will, which may require tightening of the stendl-printing process. One possible issue is print accuracy, or the alignment of the printed solder paste onto the PCB pad. Because lead-free alloys do not spread or wet as well as tin/lead, any solder paste that is not accurately printed onto the PCB will stay close to where it was printed after the reflow soldering process. Figures 3 and 4 depict the same deposits before and after reflow for QFPs and passives. 

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