
Wypych Handbook of Solvents
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•External heaters often may not be needed when using aqueous medium, because of the bath heating that the ultrasonic energy accomplishes.
However, because of this heating, these units are not recommended for use with solvent cleaners. Also, precise control of the ultrasonic energy is required to prevent possible damage to the stencil and apertures. Comparatively, also, ultrasonic machines are more expensive than either type of spray equipment. The size, and thus the cost, is directly proportional to the size of the stencil to be cleaned.
14.8.3.6 Stencil cleaning in screen printing machines
Today, many screen printers incorporate an automatic cleaning system for the undersides of stencils. During the process a wiper is moistened with a cleaner that wipes the stencil. On high-end printers this unit is programmable to permit subsequent dry wiping and vacuum drying. Cleaning the underside of stencils may not replace machine stencil cleaning, but it does extend the time, in some cases significantly, until machine cleaning becomes necessary.
A disadvantage of these automatic-cleaning units is the fact that only solder paste removal is possible. The main problem associated with cleaning the stencil underside of adhesive stencils is that it does not remove the adhesive from the stencil holes. This means that only the surface is cleaned, while the holes are untouched. Cleaning agents suitable for this application already exist.
14.8.3.7 Summary
The cleaning process, which a user finally selects, depends to a major extent on the operating conditions. Each process has its own specific advantages and disadvantages, so users are supported by advice and testing. These can be accomplished under tightly simulated process conditions by a technical center.
14.8.4CLEANING AGENTS AND PROCESS TECHNOLOGY AVAILABLE FOR CLEANING PCBs
14.8.4.1 Flux remove and aqueous process
With the banning of CFCs - enforced in Germany since January 1993 - users have been confronted with the problem of ensuring cost effective elimination of ozone depleting chemicals from the production process while having to satisfy constantly rising quality requirements. In the electronic industry it is customary to set certain quality standards in the manufacturing process of electronic assemblies. Nevertheless, it is often not enough to check the quality of a part immediately after it has been manufactured as long-term quality must also be taken into account. However, technically proficient and efficient processes are available that can exclude long-term risks and even reduce overall costs.
A large number of companies have decided in favor of no-clean processes. Though, with the expansion of packing density and increased quality demands, more and more companies are now returning to cleaning processes.
14.8.4.1.1 The limits of a no-clean process
The no-clean concept (i.e., no cleaning after soldering) proceeds from the following assumptions:
(1) The used soldering fluxes have a low solid content of approx. 2 to 3% and contain strong activators. These activators, however, are critical as they can easily form highly conductive electrolytes under humid climatic conditions.




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14.8.4.2.1Water-free cleaning processes using HFE (hydrofluoroethers) in combination with a cosolvent
The cleaning process:
A cosolvent cleaning process is one, in which the cleaning and rinsing solvents are of significantly different composition. For example, in semi-aqueous systems (see Cleaning Technologies on the PCB Assembly Shop Floor (Part II) in EPP Europe, October 1999) cleaning is done with organic solvents and water is used for rinsing. In the cosolvent cleaning process, cleaning is accomplished primarily by the organic cosolvent, and rinsing is effected by a fluorochemical. Typically, in cosolvent systems, the boil (wash) sump contains a mixture of cosolvent and fluorochemical rinsing agent. The rinse sump normally contains essentially 100% fluorochemical rinsing agent.
Referring to Figure 14.8.16, a cosolvent process operation can be summarized as follows. Printed circuit boards containing flux residues are immersed into the boil sump, which contains a mixture of cosolvent and fluorochemical rinsing agent, typically about equal volumes of each. After being cleaned in the boil sump, the substitutes are immersed in the rinse sump, which contains nearly 100% fluorochemical.
The primary function of the rinse sump is to remove the fluorochemical/cosolvent mixture and the soil dissolved in it. Following immersion in the
rinse liquid, the parts are moved into the vapor phase for a final rinse with pure fluorochemical vapor. Finally, the parts are lifted into the freeboard zone, where any remaining fluorochemical rinsing agent evaporates from the parts and returns to the sump by condensation from cooling coils. At this point, the cleaning cycle is complete and the parts are clean and dry.
Furthermore, pure NovecTM HFEs can be used for the removal of light soils, halogenated compounds like fluorinated greases or oils, and other particles.
The increasing popularity of the cosolvent process is mainly due to its flexibility in allowing independent selection of both the solvating and rinsing agents that best meet the needs of a particular cleaning application. The fluorochemical cosolvent process is even capable of defluxing components with very complex geometry due to the advanced physical properties of the cosolvent.
The cleaning chemistry:
NovecTM hydrofluoroethers (NovecTM HFEs) are rapidly becoming recognized as the solvents of choice for superior cleaning in the electronics and precision engineering industries. The combined properties of these new materials have proven to be very effective alternatives to ozone depleting substances (ODSs) like the previously mentioned CFCs and HCFCs. The new developed chemical compounds methoxyfluorobuthylether (HFE 7100) and ethoxyfluorobuthylether (HFE 7200). HFE rinsing agents can be used in combination
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with respective cosolvents. The most effective cosolvents for defluxing applications in the market are based on alcoxypropanols which are specially designed for applications in combination with HFEs.
Since the boiling points of these two HFEs lie at the upper range covered by the traditional ODSs such as HCFC-141b, CFC-113 and 1,1,1-trichloroethane, a better cleaning efficiency is provided due to the increase in solubility of most soils with temperature.
Like ODS solvents, the HFEs are non-flammable, which leads to very simple and thus inexpensive equipment designs.
The low surface tension of HFEs helps these fluids to easily penetrate tight spaces, especially between and under components of populated printed circuit boards. In addition, this is supported by the low viscosity of the HFEs that also allow it to quickly drain from the parts that have been cleaned.
One useful parameter for assessing the potential performance of a cleaning agent is the wetting index, which is defined as the ratio of the solvent’s density to its viscosity and surface tension. The wetting index indicates how well a solvent will wet a surface and penetrate into tight spaces of complex cleaning substrates. The higher the index, the better the surface penetration.
Table 14.8.3. Properties of HFEs compared to common CFCs and HCFCs
Property |
HFE-7100 |
HFE-7200 |
HCFC-141b |
CFC-113 |
|
|
|
|
|
Boiling point, °C |
60 |
73 |
32 |
48 |
|
|
|
|
|
Flash point, °C |
none |
none |
none |
none |
|
|
|
|
|
Surface tension at 25°C |
13.6 |
13.6 |
19.3 |
17.3 |
|
|
|
|
|
Viscosity, Pa s |
0.61 |
0.61 |
0.43 |
0.7 |
|
|
|
|
|
Wetting index |
181 |
172 |
149 |
133 |
|
|
|
|
|
Since HFEs contain no chlorine nor bromine, they do not have an ozone depletion potential. In addition, their atmospheric lifetime is short compared to CFCs. Furthermore, the Global Warming Potential of HFE fluids is significantly lower than other proposed fluorinated solvent replacements. Practical studies as well as laboratory tests have found that HFE solvent emissions are 5-10 times lower than the emissions of CFCs or HCFCs. Although, HFEs are volatile organic compounds (VOC), they are not controlled by related US directive regulations. Finally, toxicological tests have shown, that these products have extremely low toxicity which allows for higher worker exposure guidelines, unlike many of the chlorinated and brominated solvents offered as ODS alternatives.
14.8.4.2.2Water-free cleaning processes in closed, one-chamber vapor defluxing systems
Liquid vapor degreasing using solvents has been an accepted method of precision cleaning for over 50 years. This cleaning process incorporates washing, rinsing, drying and solvent reclamation in a compact, cost-effective unit. Consequently, this makes it a very attractive process for many production cleaning applications. Environmental concerns raised in the last decade have changed both the chemistry and equipment technology for degreasing. Solvents have been modified or replaced to eliminate ozone depletion potential and other haz-

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tive tests may be required to furnish a more in-depth statement of regarding the condition of the cleaning bath.
Refractive index:
An optical method by which a change of the refractive index of liquids, depending upon the content of liquid or solid contaminants, is used to measure the level of bath contamination.
Solid residue from evaporation:
This method establishes the non-volatile residues in cleaning and rinsing baths. This produces a very accurate statement of the bath contamination level.
Electrochemical potential:
The electrochemical potential is determined by the pH value, which furnishes an indirect statement of the level of contamination.
Conductivity measurement:
This is used to check the quality of the rinsing bath in assembly cleaning processes. The conductivity measurement indicates the amount of process-related ionic, and therefore conductive, contaminants carried over into the rinse medium.
Ionic contamination measurement:
Ionic contamination-expressed in sodium chloride equivalents is primarily used to assess the climatic resistance of electronic assemblies. Ionic measurement is suitable for statistical process control, but it does not furnish an absolute statement concerning the climatic resistance of an assembly.
Ink test:
Most organic residues on assemblies are not detected by ionic contamination meters. However, such residues do change the surface tension of assemblies, thereby influencing the adhesion of subsequent coatings. This method determines the surface tension by iterative use of corresponding testing inks.
Furthermore, the remaining protective ability of organic solder (OSP-coatings) on copper can be estimated. This test also provides an indication of the adhesion of lacquers and conformal coatings to the assembly.
Water-immersion test:
This economical test exposes the climatically unstable points of electronic components. Due to the nature of the test, the entire board is evaluated. This test accelerates the mechanisms of electrochemical migration. Consequently, faults that previously would appear after months or even years can be detected during the development process. To identify potential weak points, the assembly is operated in standby mode and immersed in deionized water. Testing while the assembly is in full operation is even more effective. The sensitivity of the circuit to moisture exposure is assessed on the basis of the recorded test current, combined with a subsequent examination of the assembly. Through weak point analysis, a Yes/No decision can be determined concerning the expected service life, of the assembly.
The growing diversity of cleaning media, cleaning equipment, and cleaning processes, together with more stringent legal requirements and quality expectations, are making it increasingly difficult for companies to develop their own cost-effective and result-optimized cleaning processes. Consequently, a lot of well known manufacturing companies opted for turnkey solutions for the entire development and installation of complete cleaning processes.

