March, 2015
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The Benefits of Soldering with Vacuum Profiles
Helmut Öttl, Head of Application and Product Management, Rehm Thermal Systems F
orming solder joints is a critical part of print- ed-circuit-board (PCB) manufacturing and is impacted by a large number of parameters. A
workgroup on solder voids prepared an overview of these factors to provide a greater understanding of the problem. However, the overview only included two factors that can be addressed prior to produc- tion of PCBs to reduce or eliminate the formation of voids. The two factors concern the stencil and the layout of the aperture and the use of vacuum technology during the soldering process. Vacuum soldering can even be used during the production processes as a sort of "fire brigade" when a short- term increase in the number of voids occurs. As a result, nothing stands in the way of using standard processes and profiles for series PCB produc- tion. With standard approaches, it's possible to react flexibly to any fluc- tuation in the quality of purchased components, PCB surfaces, and sol- der paste batches. In addition to series production,
PCBs can be repaired in cutting-edge vacuum soldering systems such as the CondensoX system from Rehm Thermal Systems. In this way, PCBs can be upgraded when excessively large voids have occurred during the initial soldering process in a conven- tional soldering system. Otherwise, these PCBs would have to be scrapped because they do not fulfill the criteria of the applicable IEC standards or IPC directives. Vacuum is defined in DIN
28400 (in agreement with ISO 3529- 1) as: "Vacuum is the state of a gas when the pressure of the gas, and thus the particle-number density, is lower within than it is outside of its container, or when the pressure of the gas is less than 300 mbar, i.e. lower than the smallest atmospheric pressure which occurs on the surface of the Earth." Today's vacuum systems are no
longer simply concerned with evacu- ating atmosphere, but of adjusting the gradient at which vacuum is gen- erated and adjusting dwell time at specified pressures. This level of con- trol enables sensitive components and volatile flux ingredients to adapt to prevailing ambient conditions. Otherwise, damage to components may take place or solder splashing may occur. This type of vacuum pro- filing must not be considered as a separate step in the soldering process, but available for the full duration of the process. Pressure can be adjusted along with temperature, making it possible to fulfill various requirements. Moisture absorbed by the solder
paste can be removed before solder- ing. For example, all of the process gas might be exchanged at 160°C to remove residues which have evapo- rated out of either the PCB material or the solder paste. This ensures that they won't condense onto sensitive components during the cooling process. The CondensoX system pro- vides a great deal of flexibility in this regard, allowing operators to impact the PCB assembly manufacturing process in a targeted and repro- ducible fashion. In the tests, SAC305 solder paste
was selected and applied by a stan- dard stencil printer. The stencils vary with regard to thickness and manufac-
turing process. One stencil was used without any additional "finishing steps" with a thickness of 120µm. In contrast, the other one-half of the circuit
Making void-free solder joints is
critical in today's complex electron- ics manufacturing. It's important to
reduce or eliminate cavities between connectors and connector pads.
boards was printed with a plasma-coated, electro- polished variant and stencil thickness of 110µm.
The difference between these soldering pro-
files becomes apparent and results from the use of the controlled vacuum profile. A so-called pre-vac- uum approach was used in the preheating zone. This stabilizes the test procedure because the sol- dering tests were distributed over a full day and, for example, absorption of moisture into the solder paste flux chemistry might lead to distorted results during shorter-duration testing. Stable conditions can be assured by reducing pressure, which lowers the boiling point of liquids, and therefore promotes easy evaporation of volatile flu-
Continued on next page
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