Methods and specification of PCBA cleaning
The importance of the PCBA cleaning process is self-evident , including cleaning methods, cleaning agents, cleaning specifications, etc. Improper cleaning can lead to changes in device performance, such as reduced insulation performance of transformers, and reduced contact performance of key contacts . Therefore, PCBA cleaning is a crucial part.
Purpose of cleaning
After PCBA soldering is completed, cleaning can effectively remove organic and inorganic pollutants such as rosin flux, water-soluble flux, no-clean flux/soldering and other organic and inorganic pollutants remaining on the surface of SMT/THT PCBA after soldering.
Methods of cleaning
It is suitable for small batch sample PCBA cleaning. Through temperature control, chemical cleaning (such as alcohol) is completed in a constant temperature bath. When the board contains many devices that cannot be cleaned, it can be selectively cleaned by manual cleaning.
Batch solvent cleaning
The batch solvent cleaning is also called intermittent cleaning. The main process flow is: place the printed circuit components to be cleaned in the steam area of the cleaning machine. Since there are condensation pipes around the steam area, when the solvent located in the lower part of the steam area is heated, it becomes a vapor state and rises to a cooled state. When the surface of the component is condensed into a solvent, it interacts with the contaminants on the surface of the component, and then the contaminants are taken away with the droplets falling.
After the components to be cleaned stay in the vapor zone for 5-10 minutes, the components are sprayed with the clean liquid recovered by condensation of the solvent vapor to flush out the pollutants. When the surface temperature of the component that has been staying in the vapor zone reaches the vapor temperature, no condensate will be produced on the surface, and the component is clean and dry and can be taken out.
The components cleaned by this cleaning method have high cleanliness, and are suitable for occasions where small batch production and printed circuit components are not seriously polluted and require high cleanliness. Its operation is semi-automatic, and there will be a small leakage of solvent vapor, which has an impact on the environment.
The keypoints of the batch solvent cleaning include the following:
①A sufficient amount of solvent should be contained in the boiling tank to promote uniform and rapid evaporation, and care should be taken to remove the cleaning residue from the boiling tank.
②A cleaning table is arranged in the boiling tank to support the cleaning load; the contaminated solvent should always be kept at a safe height under the horizontal frame of the table, so that when the basket with the cleaning load is raised and lowered, the contaminated solvent will not be removed into another solvent tank.
③The solvent tank should be filled with solvent so that the solvent can always flow into the boiling tank.
④When the equipment is started, there should be sufficient time to form a saturated vapor area, and check to make sure that the condensing coil reaches the cooling temperature specified in the operation manual, and then start the cleaning operation.
⑤According to the usage, periodically replace the solvent in the boiling tank with fresh solvent.
Continuous solvent cleaning
The continuous cleaning is suitable for mass production and assembly line production, and the cleaning quality is relatively stable. Since the operation is fully automatic, it is not affected by human factors. In addition, in the continuous cleaning process, the mechanical decontamination methods of high-pressure inclined jet and fan jet can be added, which are especially suitable for the cleaning of surface-mounted circuit boards.
Continuous cleaning machines generally consist of a long vapor chamber, which is divided into several smaller vapor chambers to accommodate solvent cascading, solvent boiling, spraying and solvent storage, and sometimes immersion of components in boiling solvent. Typically, components are placed on a continuous conveyor belt, running at different speeds depending on the type of SMA, horizontally through the vapor chamber.
Solvent distillation and coagulation cycles are carried out in the machine. The cleaning procedure and principle are similar to batch cleaning, but it is clear that the procedure is carried out in a continuous structure.
The key to cleaning SMA with continuous technology is to choose a satisfactory solvent and an optimal cleaning cycle.
②Types of continuous solvent cleaning system
Continuous cleaning machines can be divided into the following three types according to the cleaning cycle:
- Steam → spray → steam cycle. This is the most common cleaning cycle used in continuous solvent cleaners. The components first enter the vapor zone, then enter the spray zone, and finally send out through the vapor zone. Spray up and down from the bottom and top in the spray area. This type of cleaning machine uses a combination of flat, narrow fan and wide fan nozzles, supplemented by high pressure, spray angle control and other measures for spraying.
- Spray → immersion boiling → spray cycle. Continuous solvent cleaners using this type of cleaning cycle are mainly used for difficult-to-clean SMAs. The components to be cleaned are first sprayed at an inclined angle, then immersed in the boiling solvent, sprayed at an inclined angle, and finally the solvent is removed.
- Spray → immersion boiling with spray → spray cycle. Washing machines using this type of cleaning cycle are similar to Type 2 washers with the addition of a solvent spray on top of the boiling solvent. Some also set nozzles in the immersion boiling solvent to create solvent turbulence. These are all to further strengthen the cleaning effect.
Water based cleaning
The water-based cleaning can be divided into saponified water cleaning and clean water cleaning.
Saponified water cleaning
For printed circuit assemblies soldered with rosin flux, a saponification cleaning should be used. This is because rosin acid, the main component of rosin, is insoluble in water, and water must be used as a solvent. Under the action of the saponification agent, the rosin is turned into a water-soluble rosin fatty acid salt, and then the rosin fatty acid can be removed under the high-pressure water spray, and finally it can be washed with clean water to achieve the purpose of cleaning. Saponified water cleaning process:
Wave soldering or reflow soldering → rosin type flux residue → 60% saponified water cleaning → deionized water rinsing → drying
The saponified water cleaning can remove a wide range of contaminants, and the appropriate saponification agent can be selected according to the flux used for cleaning, which affects the performance and quality of printed circuit components. In addition, the cleaning effect of the saponified water cleaning for non-rosin flux residues is not stable.
Clean water cleaning
Clean water cleaning is to use clean water or pure water for washing and rinsing. Compared with the saponified water cleaning, clean water cleaning is mainly suitable for printed circuit components soldered with water-soluble flux. This cleaning method is very simple, and its process flow is as follows:
Wave Soldering or Reflow Soldering → Water-Soluble Flux Residue → Deionized Water Rinse → Deionized Water Rinse → Drying
The clean water cleaning is simple in operation and low in cost, but the quality of the water-soluble flux is not stable enough, the process is not easy to control, and it is rarely used in actual production.
Boiling ultrasonic cleaning
The ultrasonic cleaning effect is comprehensive and the cleanliness is high; the cleaning speed is fast, which improves the productivity; it does not damage the surface of the components to be cleaned; it reduces the chance of hand contact with the solvent, and improves the work safety; it can clean the parts that cannot be reached by other methods.
But at the same time, because ultrasonic waves have a certain penetrating ability, they often enter the inside of the device through the package of the device and destroy the solder joints of transistors and integrated circuits. Therefore, many countries in the world have clearly stipulated that the use of boiling ultrasonic cleaning is not allowed for military electronic products.
(1) Principle: Semi-aqueous cleaning is a cleaning method between solvent cleaning and water cleaning, that is, first cleaning components with solvent, then rinsing with water, and finally drying the cleaned components. Semi-aqueous cleaning agent is the key, which can dissolve rosin and be dissolved in water.
When cleaning, it first quickly dissolves the rosin residue on the component, and then washes the component with water, and the solvent is miscible with water. At this time, the rosin residue will be separated from the component and float in the water to achieve the purpose of removing pollutants.
(2) Characteristics: Semi-aqueous cleaning The welded SMA is first cleaned with terpenes or other semi-aqueous cleaning solvents, and then rinsed with deionized water. The semi-aqueous cleaning process using terpene semi-aqueous cleaning solvent is as follows:
Wave Soldering or Reflow Soldering → Solvent Cleaning → DI Water Rinse → DI Water Rinse → Drying
Since terpene-based hemi-aqueous cleaning solvents have minor side effects on circuit components, rinsing with deionized water must be performed after solvent cleaning. It can be rinsed with flowing deionized water or a steam spray rinsing. In practical applications, different semi-water cleaning should be selected according to the needs.
One of the important part is to make the emissions meet the requirements of environmental protection regulations. In addition, since the solvent in semi-aqueous cleaning is relatively expensive, the solvent and water separation technology is used to extract the solvent after the step of “rinsing with deionized water” to realize the recycling of the solvent.
Devices that cannot be cleaned
- Sensitive componentswith special regulations cannot be cleaned, such as Integrated circuit blocks, triodes, diodes, transistors, transformers, key switches, network sockets, buzzers, batteries, super capacitors, LCD displays, plastic components, DIP switches, lenses,etc.
- Components that are easily deformed, deteriorated and discolored, components that will be damaged by contact with water, and components that are deformed by baking with plastic parts (conventional 60°C) cannot be cleaned.
- Components with mechanical lubricating oil and components that are not welded cannot be cleaned.
- Others such as super capacitors, speakers, power modules, display screens, etc.
Inspection standard for cleaning effect
When the PCBA board is cleaned, it needs to be evaluated for cleanliness to ensure the reliability of the product and meet customer requirements. According to the relevant provisions of the standard SJ20896-2003 and the reliability and performance requirements of electronic products, the cleanliness of electronic products is divided into three levels, as follows:
|Cleanliness||Product Type||Ionic Contaminant Content||Extraction Solution Resistivity||Flux Residue|
|1||High reliability PCBA products||<1.5mg/ml||>2*10^Ω.cm|
|2||Durable PCBA Products||1.5~3.0mg/ml||<100 ug/ml|
|3||General PCBA Products||3.0~5.0 mg/ml||<200 ug/ml|
In practice, eradication of contamination is practically impossible, and a compromise is to determine what level of contamination on the board is acceptable and unacceptable. According to the IPC-J-STD-001 standard flux residue level 3 standard regulations <40ug/cm2, the third level standard regulations for ionic contaminants content ≤ 1.5 (NaCl) ug/cm2, extraction resistivity > 2 × 106Ω.cm.
There should be no dust, fibers, solder splashes, tin slag, white residues, fingerprints, etc. on the cleaned PCBA, and the surface and solder joints should not be whitish or gray.
PCBA cleaning requirements
- During the cleaning process, do not use tweezers and other metals to directly contact the PCBA, so as to avoid damage and scratches on the PCBA surface;
- After the PCBA board is soldered with components, since the flux residue will produce a physical corrosion reaction over time, it should be cleaned as soon as possible.
After cleaning, the baking process must be entered within 4H; boards that have not been baked beyond 4H need to be cleaned again. Baking parameters: 60±2℃, 3.5±0.5H, or 50±2℃, 4.5±0.5H.
Also, you can click here to read another post about PCB cleaning.