Cleanliness of a finished printed circuit board ( PCB) is subjective thing. There can be different contaminants that can affect a PCB such as dust particles, moisture or flux residue. All of these factors can leave adverse impacts on a PCB as they don’t only make the board look disorderly, but also influence the long-term reliability of the circuit.
Figure 1 Dust accumulated on a PCB
Dust particles consists of organic and inorganic materials, salts which are soluble in water and traces of water. With the advancements in electronics, PCBs are exposed to dust and other ambient conditions more than ever. For example, the desire to use natural air cooling for PCBs is a big factor in this regard. Additionally, the reduction in component footprints, smaller trace width and increase in the number of I/Os has also made PCBs more susceptible to the dust particles.
Dust particles result in several issues in the functionality of a PCB. First problem is the affinity of dust particles towards water molecules (also known as hydrophile). So, dust can make a conductive layer resulting in insulation failure during long term operation.
Flux residue can have even worse impacts on a PCB and as a result of that ‘no clean flux’ was introduced. No-clean flux was introduced to enhance the overall reliability and functionality of the product. However, due to smaller footprints in modern PCB designs the no clean flux has been confronted with additional challenges. The main cause for that is partial flux in the no clean process is unable to go thorough high enough temperature which results in its polymerization. Once the residue flux is polymerized, it acts as synthetic resin and starts interacting with the components and solder due to its low-PH acidic properties. Therefore, even with no clean flux process, it becomes crucial to perform cleaning to achieve best end-results.
If no clean flux results in the aforementioned adverse impacts on the PCBs, then question is how much efficient were the prior methods such as solvent based flux or water soluble? Major issue linked with the water-soluble flux was the management of enormous amounts of wastewater. With the growing concerns about the environmental protection, treatment of wastewater has become extremely crucial which results in additional cost. Furthermore, with the ever decreasing footprint of PCBs, trace width and smaller components, the traditional cleaning processes are unable to meet the codes and standards.
As previously described, no clean method has proven to be insufficient in the presence of high density and small footprint of components. As the number of components increase in a circuit ( spacing is reduced), probability of electrical failure under no clean methods increases. Hence, cleaning is the only viable solution in order to meet the design requirement in the modern era of electronics. Moreover, cleaning process itself has complicated in the presence of smaller components which has caused the need for innovation in not only the cleaning process but also the cleaning agents.
Advancements in the cleaning technology:
With the growing changes in the PCB designs and the components, the cleaning techniques have also evolved over time. Here is a list of different processes available. Selection of the process is dependent on the substance which needs to be removed and the quality of end-result ( cleanliness) expected.
1. Aqueous cleaning process
2. Semi-aqueous process
3. Vacuum cleaning process
4. Vapor phase degreaser
5. Mixed co-solvent process
6. Separated co-solvent process
Aqueous cleaning process can either include the water-based solution or Alkaline Saponified Water. If water-based solution is utilized then it must be clear from any salts or particles. Cleaning agent and cleaning equipment both are selected based on different criterion.
Must-haves for a cleaning process:
1. The cleaning process should be able to clear any contaminates with momentum.
2. Should provide appropriate cleaning time
3. Temperature during the process should be maintained within reasonable limits
4. The cleaning process should be able to regulate the amount of cleaning agent being deployed
5. It should also include drying mechanism
6. The use of cleaning equipment should not be hazardous
7. It should be easy to operate the equipment
8. The impacts on the environments should be minimal
Must-haves for a cleaning agent:
1. The cleaning agent should be well-matched with the type of flux residue and it should be able to remove flux residue
2. The cleaning agent should be matched appropriately with the base under consideration
3. It should not have a high surface tension
4. The cleaning agent should abide by all the environmental regulation set by the international bodies
5. It should be non-reactive and stable under normal circumstances
6. It should have anti-oxidation properties
7. The cleaning agent should not exceed the allowed temperature limits
Selection of flux with appropriate compatibility with the cleaning agent is also crucial often times. Due to the complexities involved in the cleaning process, a few flux types hinder the achievement of proper results at the end of the cleaning process. For flux, first requirement is still the compatibility with the solder. However, if this requirement fulfilled by multiple options, then selection of flux according to the cleaning agent is the most suitable solution. Factors which influence the selection of flux can be arranged in a priority list:
1) Solder compatibility 2) Effectiveness of cleaning 3) Cost optimization
In nutshell, the cleaning process is extremely crucial to achieve long term functionality and it can be achieved by optimized selection of cleaning agent, cleaning process and yet keeping everything within economic viability for a large scale production process.