Lead-free HASL

Material Surface Finish Conventional 140°C Tg FR-4 Lead Free HASL (HASL not Typical) Treat as an additional reflow cycle... 

Lead-free HASL, using lead-free alloys (such as tin-copper) in place of tin-lead, is commer-... 

HASL (Pb-free) The majority of Pb-free HASL deposits use Cu.This is constantly added to as a function of the leveling process. In the tin-lead world, this copper is a source of contamination and needs to be controlled tightly. For Pb-free HASL it is a more important control point for the deposit. Additional copper in the bulk may mean a shorter shelf life as well, and may have an impact on the Liquidus of the surface finish. Be cautious of using this finish unless the supplier has excessive amounts of data and can show control of the alloy con josition... 

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. 

Reflowed Conventional Leadless SMT Assemblies with Pb Contaminant. In Fig. 8, we have plotted cycles-to-1% failures for SAC paste assemblies with various Pb contaminant, versus cycles-to-1% failures for 100% lead-free SAC assemblies under thermal cycling conditions -40 to 125 °C (-40 to 257 °F), or -55 to 125 °C (-67 to 257 °F). When data points appear above the main diagonal, life for assemblies with Pb contamination is of a longer duration than that of 100% lead-free assemblies. The data (Ref 7, 8, 13) is for conventional leadless SMT components 20 Input/Output LCCCs and resistors ( R ) of sizes 0603, 1206, and 2512. The source of Pb contamination is the Sn-Pb HASL board finish, or the Sn-Pb component termination, in the 1206 resistors. In the 20 I/O LCCC SAC boards with Sn-Pb HASL finish, Woodrow (Ref 13) measured a 0.5% Pb contamination level. The cycles to 1% failure were calculated from Weibull parameters (characteristic life and slope of failure distributions) provided... 

Area Array Assemblies with Sn-Pb Balls and SAC or Sn-Pb Paste. Another scenario of interest during the transition to lead-free technology is that of conventional area-array components using Sn-Pb balls assembled with SAC paste. This scenario is often described as a forward compatibility situation. In Fig. 11, we show cycles-to-1% failure for Sn-Pb ball area array components assembled with SAC paste versus cycles-to-1% failure for similar components assembled with Sn-Pb paste. The data was gathered from relevant test cells in several independent studies (Ref 3,11,12, 22, 29, 32, 33). Figure 11(a) shows the data for assemblies that were cycled between — 40 and 125 °C (— 40 and 257 °F) (6 data points). Figure 11(b) shows similar test data for thermal cycling under milder conditions 0 to 100 °C (32 to 212 °F) (7 data points) and 15 to 95 °C (59 to 203 °F) (2 data points for 144 Input/Output PEG As assemblies with Ni-Au or Sn-Cu HASL board finish). 

The vapor phase lead-free solder process operates at a maximum temperature of 230°C, whereas many convection or IR reflow processes range from 250°C to over 300°C in some zones of the reflow process, depending on the type of PCBAs being processed. Vapor phase is compatible with typical PCB finishes, including HASL, bare copper with OSP, copper/nickel/gold, immersion tin, and immersion silver. 

The surface finishes selected were Immersion Silver, Immersion Tin, two HASL (lead-free) and four Organic Surface Protections (OSPs). Four solder alloys were planned for testing, but two were eliminated due to cost, leaving SACX and Sn/Cu/Ni. Five fluxes were evaluated, two of which were chosen because they were commonly used in other experiments. 

TABLE 6 Several Candidate Lead-Free Alloys for HASL Deposition of Printed Circuit Board Finishes... 

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