Title
Bob Willis
Vaprour phase or convenction for lead-free: a question to be debated during one of the author’s
workshops at APEX Expo, Las Vegas in March. If you are a medium volume producer, want to
solder tin/lead and lead-free with one process and like guaranteed reflow the answer is simple.
Vapour phase or
convection for lead-free?
Vapour phase soldering (VPS) has been board and turns to liquid, encasing all the will often be at elevated temperatures, just
around in the industry for many years and assembly, component movement can occur. below reflow, for long periods. This can
was one of the only two serious options In a vacuum-assisted unit, poor control of cause flux exhaustion prior to reflow, leav-
during the early introduction of surface the rate of pressure change can also make ing incomplete coalescence of all the solder
mount technology. In the early days, a nice mess of assembled parts. Is this a balls as in Figure 3. With modern VPS this
engineers had the option of brown belt for reason to dismiss VPS? No, it’s often just does not occur, but it could happen with
single-sided products, infrared and VPS. an excuse to stick with the poor design. It’s old VPS processes when using a secondary
With the increasing availability of batch simple and quick to change footprints by a vapour layer.
and inline VPS equipmen,t the market few mills, so update design when you order There is no argument that VPS solder-
is likely to grow to 10% compared to the a new batch of boards. A recent lead-free ing provides better wetting performance
dominant position held by convection sys- trial comparing VPS with convection for a with tin/lead and lead-free alloys and on
tems. The first reflow system used by this client showed an increased number of lift- different solder finishes when compared
author was VPS, preferring the simplicity ing defects with VPS. If you only looked at to air convection. However, the results
of the process and process set-up over the the total PPM levels, you may say that VPS can become blurred when compared with
problems of accurate loading, board/belt was the reason, but the defects were all nitrogen convection. It is often stated that
positioning, profiling and over heating of on one component size (0402), hence the VPS soldering is an inert soldering process,
paste causing flux residue charring. need for good documentation during NPI and that is true when the board assembly
A well designed board works very well design reviews. enters the vapour layer. Prior to that, or
and gives high soldering yields. A poor One lead-free process defect which has when the board in some designs is not in
design, particularly on passive compo- been apparent with a number of lead-free a vapour during cooling, this may not be
nents, amplifies the number of lifted trials on 0201 chip resistors/capacitors the case. Wetting balance measurements
and tombstoned components. All vapour and small chip scale packages is not seen of boards subjected to reflow cycles in VPS
phase systems can show a difference in with VPS. When soldering boards with a do show changes in wetting after the first
component lift due to the fundamental wide range of other components, including reflow cycle but not to the same degree as
nature of the process, but with the degree QFP, BGA and through-hole connectors, with convection reflow in air.
of control/flexability offered today, even there can be a large ΔT. To optimise the Vapour phase as a process has always
marginal design can produce good yields. process and decrease ΔT, the surface of the been around as an option in the industry,
As vapour condenses on the surface of the board, including very small paste deposits, and there will be new engineers that select
Figure 1. Three microsections showing gull wing, BGA, 0603 chip joints produced in a batch VPS using 230˚C boiling fluid. The solder alloy used on these joints was
a tin/silver/copper alloy.
4 – Global SMT & Packaging – December 2008
www.globalsmt.net
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