Removing ablation residues from laser direct structuring of 3-D printed circuit boards with an added effect
Laser Direct Structuring (LDS) enables the circuit layouts of Molded Interconnect Devices (MIDs) to be produced on complex three-dimensional substrate structures. In the process, the laser beam inscribes the layout directly onto the injection-moulded plastic element. When removing ablation residues, CO2 snow jet cleaning not only ensures optimum results but also has advantages for mounting and assembly technologies. CO2 snow jet cleaning is a dry, residue-free technique which can be used to remove particles after laser direct structuring. Its ease of automation enables the cleaning process to be integrated into the laser system or carried out immediately afterwards.
Founded in Garbsen near Hannover in 1976, LPKF Laser & Electronics AG soon made a name for itself using unconventional methods to manufacture its printed circuit board prototypes. In the early 1990’s, the company extended its product range to develop, produce and sell laser systems for PCB manufacture and microelectronics. To manufacture its molded interconnect devices (MIDs), LPKF developed a technique using LDS technology which enables circuit layouts to be inscribed directly onto complex, three-dimensional substrate structures. The advantages of direct laser structuring include increased design possibilities, significantly reduced component weights and dimensions, faster alteration of electronic subassemblies and thus makes development and serial production considerably more cost-effective. The laser also generates much more precise structures than conventional techniques.
Removing residues in the μ-range
“When manufacturing the LDS-MIDs, a special additive is mixed with the application-specific thermoplastic synthetic material used. The laser beam activates it by inducing a physical-chemical reaction. This releases the additive in the polymer matrix which then functions as a catalyst in
the subsequent reductive copper coating process”, explained Dr. Wolfgang John, Senior Consultant of the LPKF LDS technology. During laser structuring, active ablation residues remain on the surface which also become metalized and can cause problems. To avoid this, such particles are usually removed in a cleaning process after structuring. “That’s what attracted my attention when I heard about acp’s CO2 snow jet cleaning technology about three years ago. Tests carried out in the system manufacturer’s technical center achieved very positive results”, Dr. Wolfgang John reported. LPKF was able to carry out fine adjustments to the cleaning parameters using a test system supplied by the Ditzingen-based company acp – advanced clean production GmbH.
Dry, gentle cleaning of fine structures
The cleaning process uses liquid carbon dioxide as a blasting agent. Through a combination of mechanical, chemical and thermal actions, the non-toxic, non-combustible CO2 snow removes solid and filmy contamination in a dry, residue-free process. On impacting on the surface to be cleaned, the snow crystals liquefy and then sublimate. The sublimation impulse detaches and removes the tiny particles of
contamination adhering to the surface. Due to the low degree of hardness of the tiny snow crystals, the technology developed by acp ensures that the surfaces of filigree interconnect devices do not become damaged. The main reason behind the effectiveness of the snow crystals is the patented acp cleaning head which is equipped with a supersonic, two-component ring nozzle. Liquid carbon dioxide expands on exiting the nozzle to create a mixture of snow and gas which forms the core jet. Compressed air is also fed to the nozzle as a jacketed jet, accelerating the CO2 snow crystals to supersonic speed. Acceleration with compressed air results in a significantly higher degree of cleaning efficiency than that attained with single-component nozzle systems – it also considerably reduces carbon dioxide consumption. “In contrast with conventional processes such as cleaning with ultrasound or pressure washing, the CO2 system is a dry-cleaning process which leaves no residues. It is also very simple to automate. It can therefore be integrated into the laser structuring system or be connected up downstream as a separate system”, said the senior consultant.
Research project confirms added effect
LPKF and acp participated as partners in a project sponsored by the Federation of Industrial Research Associations AiF “Otto von Guericke” e.V. (AiF). The project was concerned with finding ways to improve LDS-MID contacting through planarization and showed up another advantage of CO2 snow jet cleaning. The fine snow crystals also smooth the roughened LDS structures. “This simplifies mounting and assembly processes for LDS-MIDs, such as wire-bonding, the application of non-housed chips and flip-chip technology”, remarked Dr. Wolfgang John.
Future technology LDS-MID
Moulded Interconnect Devices combine electrical and mechanical functions in one component. The PCB tracks are integrated into the housing, thus replacing conventional circuit boards. This creates the need for innovative techniques such as the CO2 snow jet, both for manufacturing and cleaning processes. The reduction in weight and die space already made possible by MIDs can be further decreased through LDS technology. Furthermore, the process also simplifies prototype manufacture and minimizes time to market. Among others, LDS MIDs are used in the automotive industry, medical technology, telecommunications and mechanical engineering.