HASL

/ˈhæsəl/

n. "Hot air solder leveling PCB finish dipping copper in molten SnPb/SAC305 then shearing excess via air knives unlike flat ENIG."

HASL, short for Hot Air Solder Leveling, immerses bare copper pads/traces in 250°C Sn63Pb37 or SAC305 bath followed by dual hot air knives (400°C, 50psi) shearing excess leaving 25-250μin uneven solder—protects Cu oxidation while ensuring reflow solderability for through-hole/SMT. Contrasts ENIG's atomically flat Au cap by creating solder protrusions (1-2mils high) problematic for 0.3mm BGA but cost-effective ($0.01/ft²); lead-free variant uses higher-melting SAC305 requiring 30°C hotter reflow profiles.

## Key Characteristics - Solder Thickness 25-250μin uneven topography from air knife shear angle. - Leaded 63/37 SnPb 183°C liquidus; Lead-free SAC305 217°C demands hotter profiles. - Air Knives dual opposed 400°C/50psi remove 95% excess solder in 50ms pass. - Shelf Life 12+ months; unlimited rework cycles as solder renews itself. - Cost Leader $0.01-0.02/ft² vs ENIG 5x premium.

-- HASL process monitor for PCB fab SPC
-- Tracks solder thickness uniformity post air knife

library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;

entity hasl_monitor is
    port (
        clk         : in  std_logic;
        rst_n       : in  std_logic;
        thickness_um : in  integer range 0 to 500;  -- XRF measurement μin
        pass_fail   : out std_logic
    );
end entity;

architecture behavioral of hasl_monitor is
    constant HASL_MIN : integer := 25;   -- μin IPC-2221
    constant HASL_MAX : integer := 250;
    
    signal thickness_valid : std_logic := '0';
begin
    process(clk, rst_n)
    begin
        if rst_n = '0' then
            thickness_valid <= '0';
        elsif rising_edge(clk) then
            if thickness_um >= HASL_MIN and thickness_um <= HASL_MAX then
                thickness_valid <= '1';
            else
                thickness_valid <= '0';
            end if;
        end if;
    end process;
    
    pass_fail <= thickness_valid;
end architecture;

Conceptually, HASL sacrifices ENIG planarity for infinite reworkability—solder volume renews itself across 10x thermal cycles while SerDes QSFP28 escapes HASL unevenness via land grid; lead-free SAC305 trades 5% higher dinoise for RoHS while wave-soldering THT survives HASL bridges that plague PAM4 BGAs backhauling Bluetooth piconets on budget fab runs.