Material Effects of Laser Energy When Processing Circuit Board Substrates During Depaneling

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Figure 4: Reaction of various circuit board constituents with different wavelengths.

Shorter wavelength and excellent optics allow for a very small beam size, often around 15 to 25 µm. This allows cutting a very narrow kerf in the panel resulting in minimal waste between boards, especially as the mechanics of the system allow very precise beam location. The example in Figure 5 shows part of a panel with very small boards. When the routing process was used the number of boards per panel was approximately 125, and after re-layout of the panel to use laser cutting the number of boards increased almost three-fold. This resulted in a very significant economic advantage.

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Figure 5: Example for a panel with very small circuits.

In the laser system used in this example, a panel is placed on a perforated surface with downdraft, or mechanically mounted on the high precision X-Y movable table to prevent the panel from moving during the cutting operation. For boards with components on both sides a special support pallet is required.

To cut all the paths on a panel the area is divided in blocks of 50 x 50 mm in which the laser beam is moving using precision computer controlled mirrors mounted on galvanometers. The beam movement speed within this area is well controlled and can be as high as 1000 mm/s. While cutting, airflow passes across the panel as shown in Figure 6 to remove debris and minimizing any deposits on top of the panel. When the cutting inside the 50 x 50 mm area is finished, the table is moved to the next square until the project is finished.

Location precision

From the original design data (e.g., Gerber file) the laser system can use panel fiducials to locate where the cut is intended to go. The table movement, in conjunction with the galvo movements, is computer controlled and allows the beam to be located within 25 µm of where it is supposed to be. However, when singulating boards or flex circuits, the precision of the panel image is typically less and therefore it often becomes necessary to use additional fiducials for smaller portions of the panel. It is even possible to use recognizable sections of the board pattern for more precise board edge location requirements.

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Figure 6: Airflow and exhaust.

Residue on Board Surface

Even though an airflow passes across the area being cut (Figure 6), not all of the material expelled from the kerf is caught. Some remaining particles are powdered epoxy and glass particles. None of these are measured to be larger than 20 µm and they averaged around 10 µm. (For reference see the circled area in Figure 7.) Their size and quantity should not raise any concerns.

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Figure 7: Surface after laser cutting.

But to determine if the redeposited material can cause any problems, a test board was designed made of FR4 material, 800 µm thick (Figure 7). The test board had four patterns with sets of two groups of interdigitized fingers. Each pair of these fingers was connected to the edge of the board for easy measurement of the surface insulation resistance (SIR). As part of the test, a slot was cut in close proximity to the fingers. After cutting the slot, these test boards were subjected to a climate test (40°C, RH=93%, no condensation) for 170 hours and the SIR was measured. In all measurements, the values exceeded 10E11 ohm—indicating that the SIR is not negatively impacted (Figure 8).


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