Hot air solder levelling—is it a HASL?
Bob Willis
Hot air solder levelling—
is it a HASL?
I have said it many times in articles and ures 2-4 were taken from a test board with lead plated finish but this is not generally
seminars: Nothing Solders Like Solder. But different component formats. The assembly true today.
design and manufacturing engineers need and soldering performance on boards like Introduction of the solder levelling pro-
to understand all the different process these will provide good yields. cess also eliminated significant mismatch
options available. There are advantage and The original introduction of HASL
disadvantages to any process, and process eliminated solder coating under the solder
and material improvements have seen wise resist and reduced the possibility of the
companies revisit HASL with lead-free to resist lifting during the assembly and sol-
overcome some of the other problems with dering operations. Using the original etch
lead-free assembly. resist, tin/lead coating was still a popular
HASL hot air solder levelling became process in the 80-90’s. Introduction of hot
popular in the early 80’s and was still the air solder levelling provided a guaranteed
most commonly specified finish for surface solderable surface from the PCB manufac-
mount boards in the late 90’s and early ture. It also provided a further hidden ben-
2000. With other independent inspectors, efit to the assembler in stressing the board.
this author conducted production trials If the solder resist coating was poor or
on different finishes to demonstrate PPM the lamination of a multilayer circuit was
yield variations, comparing standard solder questionable, then it would generally show
Figure 2. Lead-free HASL on 0201 pads.
levelled boards with other finishes like up during exposure to the molten solder
gold and OSP. Trials were conducted on bath prior to shipment to the customer.
printing, placement, adhesive application, This would still be the case today with lead-
reflow and wave soldering. The results (Fig- free and some of the poor quality laminate
ure 1) clearly showed the defects associated circulating in industry. As the coating
with these finishes and is in line with many process was conducted after solder mask/
other evaluations conducted over the years. resist application part of the process it was
If you have large variations in flatness, it also referred to as SMOBC solder mask
can impact yields. over bare copper. Originally the coating
The example termination points in Fig- was more expensive than traditional tin/
Figure 3. LGA QFN pad surface.
Figure 1. Independent trials conducted by the author in 1992 to monitor the impact of levelling on printed board Figure 4. BGA pad surfaces.
assembly yields down to 0.025” pitch.
4 – Global SMT & Packaging – September 2008
www.globalsmt.net
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