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IMDES CREATIVE SOLUTIONS
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5.0 Double-sided printed circuit boards
There is no difference in the vapor-phase processing of assemblies mounted on both sides to that of the
conventional soldering processes, i.e. radiation or convection. Heavy components with unfavorable mass-solder-
surface ratios have to be glued - if they are on the bottom side of the assembly. This glue-process may be
avoided, as long as sufficient attention is paid during the layout phase and all the heavy components are placed on
the top side. Generally, in the vapor-phase, significantly heavier components stay in place without additional
adhesive than in a convection process. Because in vapor-phase no oxidation occurs on the solder-joint and the
surface tension of the solder is highest so that even big components stay in place held only by the wetting force. If
heavy components must be glued, there are SMD adhesives, which cure at the process temperatures of the vapor-
phase. This results in enormous savings as it does away with an additional adhesive curing process when soldering
SMD‘s on both sides of an assembly.
Documents to these adhesives are available with IMDES creative solutions.
More information regarding
DUAL SIDED SMD SOLDERING you will find on the pages 28, 29, 30 and 31.
5.1 Curing of adhesives
Curing SMD adhesives in the vapor-phase is straight forward. Due to its careful and steady energy supply, as well
as its totally oxygen-free environment, the adhesive agents may be cured under the same conditions as solder
pastes may be reflowed.
This leads to the following advantages:
• Shorter dwell-times in the hot zones for curing, resulting in positive effects relative to the longevity of the
components.
• Curing the adhesive and soldering is one process step. There is no other in-series curing process, thus saving
costs.
• As the adhesives are cured without oxygen no oxides will form on the printed circuit boards - which results
in improved solder quality.
• Advantage of having a single profile to cure adhesive and to solder is an added advantage.
6.0 Cleaning and cleanliness of the printed circuit board
Using the mildest activated or no-clean pastes, the cleaning of the printed circuit boards is, in most cases no longer
necessary. Certain products, e.g. those according to MIL or ESA specifications, electrically tested assemblies, etc.,
must be cleaned. However, printed circuit boards soldered in a vapor-phase system may easily be cleaned off their
flux residues with normal cleaning processes, as the flux residues are not burnt and their chemical structure is not
altered. In future, the cleanliness of the printed circuit boards after soldering will become more important.
Particularly with edge connectors, LED´s, switches, potentiometers, any flux contaminations or outgassing from
the printed circuit board will become more problematic and undesired. Especially in these cases it is the vapor-
phase process, which offers enormous advantages. In difference to the convection machines - in which the whole
surface of the printed circuit board is always exposed to some contamination by the circulating air, the vapor-
phase processes the boards in a very clean environment. The vapor-phase is actually a distillation column and thus
always extremely clean - due its physical properties. Consequently and independently of the machine‘s
contaminated condition, no foreign particles are deposited on the printed circuit boards during the condensation of
the vapor,. Outgassing of the paste or of the printed circuit boards escape immediately from the vapor-zone, due
to their lower specific gravity. Any solid matter which might accrue during the soldering process, e.g. solder balls,
flux residues, will be transported into the sump of the machine where they will be filtered. As no oxygen is present
during the heating process, pastes with lowest and mild fluxing contents may be used. Consequently, the
contamination of the solder pads by flux residues, after the solder-joints solidified, is minimal.
