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Choice of lead-free PCB manufacturing pad coating

In pcb manufacturing, lead-free soldering requires that the plating material on the PCB pad surface is also lead-free. Currently, Pb-Sn hot air leveling (HASL), electroless nickel/gold (ENIC), electroless nickel/palladium/gold (ENEPIG), Cu surface coating OSP, dip are mainly replaced by non-lead metal or lead-free solder alloys. Silver (I-Ag) and immersion tin (I-Sn).

Choice of lead-free PCB manufacturing pad coating

Choice of lead-free PCB manufacturing pad coating

 

The choice of lead-free PCB manufacturing pad coating must take into account the compatibility of the solder, smt process and PCB pad coating.

 

PCB  manufacturing pad coating and solder compatibility

Different metal platings are different from the solder alloys in the formation of the alloy at the interface, so the joint strength between them is also different. For example, the interface alloy Ni3SI14 of Sn and Ni has the strongest connection strength, so the general high reliability product selection (Ni/Au) or electroless nickel/palladium/gold (ENEPIG); and the same alloy hot air Leveling (HASL) as the lead-free solder is used, which has the best compatibility in pcb manufacturing.

 

PCB manufacturing pad coating and process compatibility

(1) Lead-free solder alloy hot air leveling (HASL)

At present, the solder for hot-air leveling of lead-free solder alloys mainly includes Sn-Cu-Ni+Ge  or Sn-Cu-Ni+Co (cobalt). The composition of Sn-Cu-Ni+Ge is Sn, 0.7% Cu, 0.05% Ni, and Ge having a nominal content of 65×10-6. Not only can it prevent the growth of oxides, but it can also prevent the solder joints from yellowing and tarnishing during the HASL process and subsequent reflow and wave soldering processes. In addition, niobium can also inhibit the formation of slag in lead-free wave soldering .

 

Lead-free solder alloy hot air leveling and Pb-Sn solder HASL-like, because the thickness of the plating layer and the flatness of the pad (dome shape) are difficult to control, it cannot be used in narrow pitch, high density assembly, and can be applied in Double-sided reflow soldering of general-purpose lead-free products, and wave soldering process for consumer electronics in pcb manufacturing.

 

The biggest trouble in the lead-free HASL process is the copper blockage of the tin bath during the using of the equipment.

 

The typical operating temperature range of the HASL process is 265 to 275 ° C, which can be used in almost all practically produced laminates in pcb manufacturing. At this temperature, even CEM1 has no problem of delamination degradation. However, the actual process temperature increases as the copper content in the tin bath increases. When the copper composition is 0.3% higher than the optimum value of 1.2%, then the soldering temperature must be increased to 285 ° C, which is unacceptable to the laminate. Although a copper-free solder alloy can be added to reduce the copper content in the tin bath, it is difficult to control the ratio.

 

It is also possible to use a so-called “lyophilization” method. When the temperature of the tin-lead eutectic solder (63Sn-37Pb) drops to about 190 ° C (about 7 ° C higher than the melting point of 183 ° C), the tin-copper intermetallic compound ( Cri6Sn5 ) in the melt is “lyophilized”. In high-density lead-containing solders pcb manufacturing, Cu6Sn5 floats on the surface of the molten solder and can be removed using a colander. However, in lead-free solders, Ct16SIl5 is denser than lead-free solder, and CLJ6Sn5 sinks to the bottom of the tin bath. Some organizations have lowered the temperature to about 235 ° C (about 8 ° C higher than the melting point temperature), and the tin bath is shut down for at least two hours, preferably overnight. At this time, most of the alloy is still in a molten state, and special tools can be designed. The precipitated Cr16Sn5 is removed from the bottom of the tin bath, but the difficulty is still great pcb manufacturing.

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