Title Double-sided reflow in one reflow cycle
Bob Willis
Reduce time, energy, cost, floor space—this and more will be discussed during one of the author’s
reflow workshops being at APEX Expo, Las Vegas in March.
Double-sided reflow in
one reflow cycle
In any process there are always opportuni-
stage side
ties to improve, reduce the number of pro-
cess stages or reduce the cost of manufac-
Stencil print solder paste one
ture. Each may not improve yield initially,
may even increase manufacturing defects,
Dispense adhesive one
but it must in time benefit some compa-
Alternatively screen print adhesive
nies’ processes or products. Engineering is
(using recessed stencils)
always looking at alternative processes, and
Alternatively dispense paste and
Figure 1. Screen printing side one is conducted in
simultaneous double-sided reflow solder-
adhesive
the normal way for the SDSRS process unless solder
paste dispensing jetting is going to be used for both
ing (SDSRS) is a possible new candidate Place components one process stages.
for investigation. It has been used by two
(Possible UV curing stage for one
Japanese companies, one large computer
adhesive)
manufacturer in the USA, and is report-
edly being used by one other company in
Invert board
Europe. It was originally patented by one
Screen print solder paste two
large US producer.
The question has to be asked: Why
Place components two
consider SDSRS, as it will undoubtedly Possibly insert through-hole two Figure 2. After the application of paste, adhesive
cause problems with loss of components in components
is placed on the surface of the board by dispensing
the early stages of process development? If
prior to component placement and curing.
Reflow both sides simultaneously both
we examine the time taken for the reflow
cycle of a product, typically this is three to Table 1. The process sequence of SDSRS
four minutes. By reflowing both sides of
the assembly, the time equates to a total of
advantages of sDsrs
eight minutes. By conducting simultane-
Eliminates one reflow process stage
ous reflow, the total process cycle time is
With simultaneous reflow soldering, both
reduced.
sides of a surface-mount board can be
It used to be the case that placement
processed in one operation. If through-hole
Figure 3. An alternative to dispensing adhesives is
was the slowest part of the process, but
parts are also designed to be reflowed, this
stencil printing, which may be conducted using a
with increasing placement speeds, this is
too can be done in a single operation.
recessed stencil.
no longer the case. Printing was the last
part of the process to be attacked by the
Only one heating process for
speed merchants, focusing on the solder
double-sided boards
paste specification, which was the limiting
With only one heating process, there is a
factor in high-speed printing. Paste formu-
potential benefit to the reliability of the
lations, allowing printing cycle speeds of
joints, with a thinner intermetallic forma-
less than 10 seconds per board, have pro-
tion being possible. The issues related to
vided the speed to make other parts of the
reduction in solderability of surface-mount
process the weak links in the chain. SDSRS
pads are also eliminated.
Figure 4. After the second side has been printed
tackles reflow process speed.
With only one heating process during
and components placed, the whole assembly may be
reflowed in one operation reflow cycle.
reflow soldering, in which the board may
be supported with a central wire, it is pos-
sible to reduce warpage of boards suscep-
4 – Global SMT & Packaging – January 2009 www.globalsmt.net
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