In recent years, the application of inkjet printing technology to the printing of characters and logos on PCB boards has continued to expand, and at the same time it has raised higher challenges to the completion and durability of inkjet printing. Because of its ultra-low viscosity, the inkjet printing ink usually only has a dozen centipoises. Compared with the tens of thousands of centipoises of traditional screen printing inks, the inkjet printing ink is relatively sensitive to the surface state of the substrate. If the process is controlled Not good, it is prone to problems such as ink shrinkage and character falling off.
Combining the professional accumulation in inkjet printing technology, Hanyin has been cooperating with customers on process optimization and adjustment with ink manufacturers for a long time at the customer site, and has accumulated some practical experience in solving the problem of inkjet printing characters.
1
The influence of the surface tension of the solder mask
The surface tension of the solder mask directly affects the adhesion of the printed characters. You can check and confirm whether the character falling off is related to the surface tension through the following comparison table.
You can usually use a dyne pen to check the surface tension of the solder mask before character printing. Generally speaking, if the surface tension reaches 36dyn/cm or more. It means that the pre-baked solder mask is more suitable for the character printing process.
If the test finds that the surface tension of the solder mask is too low, it is the best way to notify the solder mask manufacturer to assist in the adjustment.
2
The influence of solder mask film protective film
In the solder mask exposure stage, if the film protective film used contains silicone oil components, it will be transferred to the solder mask surface during exposure. At this time, it will hinder the reaction between the character ink and the solder mask and affect the bonding force, especially The place where there are film marks on the board is often the place where the characters are most likely to fall off. In this case, it is recommended to replace the protective film without any silicone oil, or even not use the film protective film for comparison test. When the film protective film is not used, some customers will use some protective liquid to apply to the film to protect the film, increase the release ability, and also affect the surface state of the solder mask.
In addition, the influence of the film protective film may also vary according to the degree of anti-sticking of the film. The dyne pen may not be able to accurately measure it, but it may show ink shrinkage, resulting in unevenness or pinhole problems, which will affect the adhesion. Make an impact.
3
Influence of developing defoamer
Since the residue of the developing defoamer will also affect the adhesion of the character ink, it is recommended that no defoamer is added to the middle of the developer for a comparison test when finding the cause.
4
The influence of solder mask solvent residue
If the pre-bake temperature of the solder mask is low, more residual solvents in the solder mask will also affect the bond with the character ink. At this time, it is recommended to appropriately increase the pre-bake temperature and time of the solder mask for a comparison test.
5
Process requirements for printing character ink
The characters should be printed on the solder mask that has not been baked at high temperature:
Note that characters should be printed on the solder mask production board that has not been baked at high temperature after development. If you print characters on an aging solder mask, you cannot get good adhesion. Pay attention to the necessary changes in the production process. You need to use the developed board to print the characters first, and then the solder mask and the characters are baked at a high temperature.
Set the heat curing parameters correctly:
Jet printing character ink is a dual-curing ink. The entire curing is divided into two steps. The first step is UV pre-curing, and the second step is thermal curing, which determines the final performance of the ink. Therefore, the thermal curing parameters must be set in accordance with the parameters required in the technical manual provided by the ink manufacturer. If there are changes in actual production, you should first consult the ink manufacturer whether it is feasible.
Before heat curing, the boards should not be stacked:
The inkjet printing ink is only pre-cured before thermal curing, and the adhesion is poor, and the laminated plates bring mechanical friction, which can easily cause character defects. In actual production, reasonable measures should be taken to reduce direct friction and scratching between the plates.
Operators should standardize operations:
Operators should wear gloves during work to prevent oil pollution from polluting the production board.
If the board is found to be stained, the printing should be abandoned.
6
Adjustment of ink curing thickness
In actual production, many characters fall off due to the friction, scratching or impact of the stack, so appropriately reducing the curing thickness of the ink may help the characters fall off. You can usually try to adjust this when the characters are falling off and see if there is any improvement.
Changing the curing thickness is the only adjustment that the equipment manufacturer can make to the printing equipment.
7
Influence of stacking and processing after printing characters
In the subsequent process of completing the character process, the board will also have processes such as hot pressing, flattening, gongs, and V-cut. These behaviors such as stacking extrusion, friction and mechanical processing stress have an important effect on the character dropout, which often occurs The ultimate cause of character falling off.
In actual investigations, the character drop phenomenon we usually see is on the thin solder mask surface with copper on the bottom of the PCB, because this part of the solder mask is thinner and the heat transfers faster. This part will be heated relatively faster, and this part is more likely to form stress concentration. At the same time, this part is the highest convexity on the entire PCB board. When the subsequent boards are stacked together for hot pressing or cutting, It is easier to cause some characters to break and fall.
During hot pressing, flattening and forming, the middle pad spacer can reduce the character drop caused by squeeze friction, but this method is difficult to promote in the actual process, and is generally used for comparison tests when finding problems.
If it is finally determined that the main reason is the character falling off caused by hard friction, scratching and stress in the forming stage, and the brand and process of the solder mask ink cannot be changed, the ink manufacturer can only solve it completely by replacing or improving the character ink. The problem of missing characters.
All in all, from the results and experience of our equipment manufacturers and ink manufacturers in the past investigation and analysis, the dropped characters are often related to the production process before and after the text process, and they are relatively sensitive to some character inks. Once the problem of character falling off occurs in production, the cause of the abnormality should be found step by step according to the flow of the production process. Judging from the industry’s application data for many years, if appropriate character inks and proper control of the relevant production processes before and after are used, the character loss problem can be very well controlled and fully meet the industry’s yield and quality requirements.