Handbook of Solvents - George Wypych - ChemTech - Ventech!

14.8 Electronic industry 895
that these residues might cause failures in production steps far beyond the initial printing
process. Therefore it is important to assure clean surfaces in each production step.
The cleaning applications (2-4) outlined in the SMT process line, shown in Figure
14.8.1, refer to the soldering process. Within this production step the components are placed
on the bare board using solder paste that is printed on the solder pads through a stencil. The
stencil can be cleaned continuously during the printing process by using a stencil printer underside
wiping system. However, it has definitely to be cleaned after removing the stencil
from the printer. The solder paste is removed from the apertures of the stencils in order to assure
an accurate printing image (3). 2,3 If the assemblies are to be mounted on both sides, the
components are placed on one side by using an epoxy adhesive (SMT adhesive), in order to
prevent them from being accidentally removed during the reflow process (3). 3 The soldering
process is carried out in a specially designed reflow oven. The cooling coils which contain
condensed flux residue of such reflow ovens also have to be cleaned (5). In the case of
wave soldering processes, the flux contaminated solder frames in addition must be cleaned.
The contamination that must be removed after the soldering process is predominantly
from flux residues. The removal of flux residues from the soldered assemblies (6) is generally
the most critical application. Thus it is the important that the assemblies are cleaned. A
subsequent coating process demands a very clean and residue-free surface to assure long
term stability of the coating against environmental stresses such as humidity. 1
Asked for the reasons for cleaning most of the process engineers give the following
answers:
1) Stencils and other tools, such as squeegees, are cleaned in order to assure a
reproductive and qualitative satisfying printing process. 2
2) Contaminations from printed circuit assemblies are removed to achieve long-term
reliability.
3) Cleaning is an important manufacturing step within the SMT process line if
subsequent processes such as coating and bonding are required.
In recent times an additional process step, which cleaning definitely is, was regarded
to be time and cost inexpedient. Therefore a lot off optimization techniques have been applied
to the SMT production process to try to avoid cleaning. However, considering new
technologies in the electronic industry like fine pitch, flip chip and micro-BGA applications,
the above statement is no longer true. The question if the removal of flux residues
from printed circuit boards is necessary can only be answered by carrying out a detailed process
analysis including the costs for cleaning and the increased reliability of the products. A
large number of global players in the electronic industry prefer the advantages of better field
reliability of their products. This aspect is even more important taking into consideration the
outstanding competition in these industries.
Whether a product has to be cleaned, and, especially, when cleaning is necessary in the
SMT production depends on the following factors.
• Process costs: cleaning is an additional process step that demands additional
investment and resources. However, failures and downtime in other process steps
can be diminished.
• Process reliability: due to a larger process window, the manufacturing process as a
whole is more stable.
• Product reliability: the functionality and long term reliability of the products has to
be assured.