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May, 2018
Zero-Defect Soldering: Quality Management Based on Quality Assurance
By Heike Schlessmann, Marketing Manager, SEHO Systems GmbH T
he ultimate goal of manufac- turing is to reach zero defects. Manual rework is time-con-
suming and expensive, personnel must be trained and assemblies often go through production a second time. These entire boards are exposed to the thermal load again, not only the faulty solder connections, which can negatively affect their reliability. An automated zero-defect pro-
duction process can provide a cost- effective solution. Automated process control and integrated automated rework can enable a soldering process free from defects, with com- plete traceability. Compared with other automat-
ed processes, selective soldering is particularly demanding. Structures with small pitches require a small process window and variable param- eters, such as flux quantity, temper- ature or wetting time, impact solder joint quality and reliability. Material-related issues also need to be considered.
Soldering Defects Zero-defect production requires
a completely controlled and reliable process. SEHO’s zero-defect produc- tion concept includes selective mini- wave processes with monitoring and control functions for all steps. The process includes integrated AOI, as well as defined and automated rework soldering. Only defective solder joints go
through the process again, not the entire board. All workstations are linked with bidirectional data trans- fer and all process steps are trace- able and reproducible. Analysis of trend and series faults allows process optimization at an early stage. This is particularly useful for component placement and the soldering process and design problems can also be identified quickly. One of the most common solder
defects is the formation of solder bridges, which have many possible causes. In lead-free applications, the reduced wetting-force of the solder alloy contributes to a change in flow behavior, which can create bridges. Bridges might also be caused by an inadequate quantity of flux applied to the solder joints or insufficient
preheating of the assembly. Insufficient solder through-hole
penetration is another soldering defect, which is often related to poor heat balance or too little flux. The formation of solder icicles is another issue. Usually, icicles are caused by an insufficient preheat process or inadequate energy transfer during
flux quantity during deposition is much more reliable. The system monitors the flux quantity that is jetted to the PCB during fluxing. The data is compared with the nom- inal values set in the soldering pro- gram and sends an error message if there is any deviation. The cause of the error, as well
Software can preset the required
preheat temperatures for a PCB exactly, allowing a gradient-con- trolled preheat temperature profile.
Controlling Soldering There are many variables that
can influence soldering. The temper- ature of the solder alloy and the sol- der level in the bath need to be mon- itored continuously and controlled. Due to the very small solder nozzle geometries used in selective solder- ing, it is critical to ensure a constant and stable wave height for sufficient energy transfer and reproducible soldering. Multi-wave systems typically
SEHO integrates AOI into selective soldering.
the soldering process. Again, too lit- tle flux might be the cause.
Monitoring Fluxing Flux removes any existing oxide
layer and avoids re-oxidation to allow proper wetting. Enough flux must be brought to the solder joints to sufficiently fill through-holes. When using a no-clean flux, and if additional costs for a cleaning process are to be avoided, residues on the PCB should be minimized. Flux residues can result in a significant loss of quality, in extreme cases caus- ing total product failure. Therefore, fluxer monitoring systems are essen- tial. There are different methods to monitor flux deposition. The easiest, but not the most
reliable way, is to monitor the func- tion of the fluxer drop jet nozzle. This is usually performed in a test cycle using an appropriate sensor, which is wetted prior to the actual flux appli- cation in production. This monitors the function of the fluxer nozzle at the time of the test, however, it does not offer any information about the process overall or the quantity of flux being applied. Real-time monitoring of the
as the name of the faulty geometry, can be displayed. This real-time sys- tem ensures the highest reliability and stable process conditions with- out influencing cycle times.
Preheat Control It is essential to monitor the
preheat temperature to ensure repro- ducible profiles and activate the flux. Infrared themometry is one of the most common and reliable ways to monitor and control preheat temper- ature. Depending on its temperature, each object emits a certain amount of infrared radiation. Any change in temperature affects the intensity of the radiation. The wavelength range used for
infrared thermometry ranges between 1 and 20 µm. The intensity of the emitted radiation depends on the material. Most materials have a known emissivity that describes the intensity of their radiation. Infrared thermometers are
optoelectronic sensors. Their most important feature is that the meas- urement is contactless. They can measure the temperature of inacces- sible or moving objects without diffi- culty.
Cross sensor for solder wave height control and automatic tool measurement.
The easiest way to measure the
wave height in single-nozzle, mini- wave processes is to use a measuring needle made of titanium or other resistant material and perform a contact measurement at the surface
Continued on next page
use an eddy current sensor to control the solder wave height and simulta- neously monitor the solder level in the pot. A measurement funnel, which has the same level as the sol- der nozzles, is connected with the pump channel. Connected by tubes, the same solder wave height arises at the solder nozzles and at the measurement funnel. The signal of the eddy current sensor is constantly compared with a nominal value, keeping the wave height stable.
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