Practical Considerations For
Attaching Surface-mount
Components
The photos included illustrate some of these components soldered to circuit boards.
What automated soldering methods can be considered for Mini-Circuits surface-mount
components? There are two basic methods: reflow and wave soldering. Generally, reflow
soldering can be done when (l) there are only surface mount components, or (2) these are
present together with through-hole components and the latter will be soldered in a separate
(wave soldering) step. The surface-mount components can be located on both the upper and
lower sides of the board, when reflow is used. Wave soldering is suitable for through-hole
components mounted on top of the board as well as for surface-mount components on the
bottom of the board. Both ceramic and polymer-based boards can be accommodated.
reflow
Reflow requires applying a controlled amount of solder and flux to the areas where
connections are to be made. A common technique is to print a pattern of solder paste on the
board, and then put the components onto their places. They tend to stay in position because of
the stickiness of the paste. Optionally, components can be held with chip-bonding epoxy.
When heat is applied, it must produce a time-temperature profile suitable for accomplishing
several process steps as illustrated in Figure l.
Fig.1 Solder Reflow Phases
First, solvent evaporation is done at temperatures up to about l00oC. Second, flux reduces
metal oxides as temperature rises to the solder melting point, typically l83oC. Third, as
temperature continues to rise, the solder particles in the paste melt and wetting and wicking
in the joint area begin. In the next step when temperature reaches a peak around 215o, surface
tension shapes the fillet of fully molten solder. It takes only a few seconds for proper solder
wetting at that temperature. The length of time that the work is actually above 200o is usually
limited to one or two minutes to avoid damage. Many plastic encapsulated components can
withstand several minutes of solder-melt temperature when re-flowed. Even at the higher
temperature experienced in wave soldering (see below) such components tend to withstand
several seconds immersion in molten solder.
There are several techniques for applying the heat needed for re-flow. Two of the most
common are infrared and vapor phase. Infrared heat sources operate at very high temperature
and are placed at the inner walls of the chamber, not in contact with the work. The actual
temperature of the work is strongly affected by its mass, geometry and composition, as well
as bel speed. Organic materials such as epoxy-based boards tend to absorb the IR radiation
and conduct the heat to metal parts which, by themselves, would tend to reflect the IR away.
On the other hand, in the vapor-phase soldering process the vapor surrounding the work is
maintained at the optimum solder-wetting temperature. This is accomplished by boiling an
inert liquid in a tank. The boiling point is approximately 215oC. When the work is held in the
vapor just above the liquid, the heat at that temperature is transferred to all surfaces quite
uniformly.
Figures 2 and 3 illustrate profiles obtained with both re-flow processes. Separately controlled
heating zones in an IR oven provide an opportunity to shape the profile more optimally than
is typical of the vapor phase process. However, the infrared method may require more careful
setup to avoid undesired nonuniformity of heating.
Fig. 2 Typical Five-Zone Infrared Profile.
Fig. 3 Typical Vapor Phase Profile
wave soldering
This method applied molten solder to the bottom side of a circuit board after a preheating
sequence, with cool-down following the soldering. There are variations to the method,
mainly characterized by the shape of the solder wave; the choice among them is influenced
by the type of assembly and the components being mounted. Solder temperature, typically
260o C, is higher then the peak temperatures used in the re-flow method. The temperature
profile experienced by the board and components increases during preheat to about 150o at a
rate similar to re-flow, but then rises rapidly when the solder wave impinges.
Components housed in molded plastic cases may be better suited to re-flow soldering
because of the possibility of micro cracks developing at the leads if they are wave soldered.
Resistance to this defect is enhanced when moisture is baked out of the plastic material
before soldering by either method; 24 hours at 125o C is usually effective.
some pointers
Among the considerations for successful attachment of surface-mount components are:
● Lead coplanarity, and wettability by the solder.
● Compatibility of solder composition with finish on components and circuit boards.
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Amount of solder deposited.
Internal heating of components.
Component placement and orientation accuracy.
Leads on a component should all be in the same plane. More departure from coplanarity can
be tolerated in the reflow method than in wave soldering because in reflow the surface
tension in the molten solder tends to bridge small gaps and ensure an acceptable fillet. In
wave soldering, special techniques are used to counteract the tendency of surface tension to
deposit excess solder and cause undesired bridging between metallization areas. Even so,
better coplanarity may be required when wave soldering to ensure good joints.
Wetting, or the ability of solder to adhere to the entire joint surfaces uniformly, is enhanced
when clean surfaces on the component leads and circuit board are provided. If the quality of
initially suitable surface finishes is preserved by proper handling and storage, the flux applied
in the soldering operation should suffice to remove any metallic oxides. Compatibility of the
solder includes closeness of its melting point with that of the metallization finishes, and the
minimization of materials such as gold which promote the formation of brittle inter-metallic
compounds with tin-lead solder. The amount of solder to be deposited depends upon joint
geometry, but once determined the deposition can be controlled to a few thousandths of an
inch by using pre forms or printing the solder pattern.
Concern about internal heating of components during soldering becomes greater when the
size or complexity of individual components increases, or where assembly contain a mix of
large and small components. In the latter case the heating rates can be disparate, or a large
component may mask a small one from the source of heat. Careful study an experimentation
may be needed to optimize the process. Measurement of air temperature in the processing
chamber, even at several points, may not be sufficient with infrared heating. Monitoring with
a surface temperature profiler can ensure that small components are not being overheated.
While high accuracy is needed for component placement on a circuit board, this requirement
is eased somewhat the case of re-flow because surface tension in the molten solder has a
self-centering action. This assumes that a proper metallized footprint has been provided.
Correct orientation of components requires assurance that when they are supplied in
tape-and-reel, all have been inserted into the tape in a constant and known orientation. Also,
the tape cavity dimensions must ensure that the orientation is maintained. Especially when
component placement is done manually, a clear orientation index is necessary on each
component. The following considerations tend to keep the components, once placed, from
becoming dislodged before the solder solidifies: When all surface-mount components are on
top of the board, the stickiness of solder paste and then the surface tension of molten solder
usually suffice. Components on the bottom of the board may need pre-attachment with an
adhesive such as epoxy. Caution is then necessary to prevent low-viscosity constituents of the
adhesive from spreading onto the metallization areas ("creep"), which could inhibit solder
wetting.
Automated soldering of Mini-Circuits surface-mount components
Of course, the surface-mount models listed at the beginning of the article can also be
mounted by hand-soldering techniques. But, pertinent to the advantage afforded by
automated assembly is that most can be soldered in their standard configurations, or require
simple modification to their internal connection method or omission of a plastic enclosure,
depending upon the particular temperature profile specified for a customer's application.
Fig. 4 Temperature Profile, RMS-2 Mixer
Fig. 5 Temperature Profile, T4-1 Transformer
Fig. 6 Temperature Profile, Ceramic-Base Transformer
Fig. 7 Temperature Profile, YSW-2-50DR Switch
Mixer model RMS-2 meets the temperature profile shown in Figure 4 in its standard
configuration. Transformer model T4-1 meets the profile in Figure 5 when a solder having
260oC solidus is used for its internal connections. A special transformer model designated
TC, having a smaller version of case style TT100, has a ceramic base and no lid. It accepts
the temperature profile shown in Figure 6. All three of these profiles were set by customer's
specifications. As a final example, GaAs switches with drivers, in plastic case style KKK155
are designed to withstand a reflow heating and cooling cycle lasting 4 minutes, in which the
component itself exceeds 200oC for 2 minutes. Figure 7 shows actual temperatures measured
on a switch and on the circuit board to which it is being mounted, using an infrared reflow
machine having four zones and seven source panels in a 72-inch overall length. The same
GaAs switches in metal-ceramic case style AG191 will withstand a peak case temperature up
to 275oC.
Last Updated: 12/17/1999
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