JP2002144060A - Method for laser beam machining - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a method for laser beam machining by which a remaining quantity of resin in a burst mode machining is decreased to a level in a cycle mode. SOLUTION: In the method for laser beam machining, the time interval of the last two pulses is made longer than that of the preceding two pulses which are consecutively irradiated when every position to be machined is irradiated with the pulsed laser beam in the burst pulse mode.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser processing method for irradiating a laser beam to form holes such as via holes and through holes in a printed circuit board or the like.

[0002]

2. Description of the Related Art In the following description, an object to be processed is a printed circuit board, and a processing position indicates a position at which a hole is processed and formed. Generally, in order to form a plurality of holes on a printed circuit board (hereinafter, appropriately referred to as “substrate”), it is necessary to irradiate a plurality of laser pulses to form each hole. As a method of irradiating a pulse to a plurality of holes a plurality of times, a galvanomirror is stopped until processing of one hole is completed, and pulse irradiation is continuously performed a plurality of times at a position where the hole is to be formed. The case where the galvanomirror moves to form the next hole after the first hole is formed is referred to as a burst pulse mode (hereinafter, appropriately referred to as "burst mode"). Then, irradiate the laser beam once to the position where the next hole is to be formed. Irradiation in the same cycle is the cycle pulse mode (hereinafter, appropriately referred to as “cycle mode”). FIG. 7 shows the pulse irradiation modes of the above two types of laser beams. (A) is a cycle mode, and (b) is a burst mode. FIG.
The figure shows a case where pulse irradiation per hole is performed three times and three holes are processed. In each of the figures (a) and (b), pulse irradiation is performed in the order indicated by the thin-line arrow from pulse irradiation with the reference sign S (indicated by a thick arrow) to pulse irradiation with the reference sign E. Go. In the case of the cycle mode, the galvanomirror is moved each time pulse irradiation is performed, so that the hole processing portion is hardly affected by the heat of the previous pulse irradiation, and the finished quality of each formed hole is relatively good. However, the time interval between successive pulse irradiations becomes longer,
There is a disadvantage that the processing time as a whole also becomes longer.

On the other hand, in the burst mode, the galvanomirror is stopped until pulse irradiation is completed as many times as necessary to complete one hole. For this reason, in the time interval between the preceding and succeeding pulse irradiation, the pulse irradiation is continuously performed on the same portion without moving the galvanomirror, so that the processing time can be shortened. However, since the time interval between successive pulse irradiations is short, there is a disadvantage that the formed holes are likely to be affected by heat. Further, compared to the cycle mode, the remaining amount of the resin inside the hole tends to increase. Even if the number of pulse irradiations is increased to reduce the remaining amount of the resin, the effect of reducing the thermal effect is small, and conversely, the shape of the hole is distorted due to the thermal effect. Due to such a difference in properties, conventionally, it is common to selectively use the cycle mode when giving priority to the finish quality of the hole and to perform the burst mode when giving priority to the machining time. Intermediate processing methods between the burst mode and the cycle mode, such as moving the galvanomirror and repeating the cycle when irradiating several pulses so that one hole is not completely opened, have been tried for some time. The effect was not clear. Also, in this case, the processing time is longer than in the case of the normal burst mode, so that it is hard to say that this method is very practical.

[0004]

In the conventional laser processing method, as described above, the cycle mode and the burst mode have advantages and disadvantages, respectively. However, if the hole processing quality can be improved in the burst mode. It is possible to perform high-speed and high-quality drilling. The most important of the processing quality issues is the amount of resin remaining. There was a problem that it was desired to reduce the amount of resin remaining in the burst mode processing to the level of the cycle mode.

[0005] In addition, due to the problem of thermal influence during the burst mode processing, differences in the ease of heat release occur due to the size of the area of the inner land, resulting in a difference in the processing state of the inner land surface. For example, the surface may be damaged on a small area land, but the remaining resin may be seen on a large area land. There has been a problem that it is desired to eliminate such a difference in the processing state depending on the size of the area of the inner layer land.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problem, and an object of the present invention is to provide a laser processing method capable of reducing the amount of resin remaining in burst mode processing to the level of a cycle mode.

[0007]

According to the laser processing method of the present invention, at the time of irradiating a laser beam with a pulse to each processing position in a burst pulse mode, the time interval between two consecutively radiated pulses is reduced to the nth time. The time interval between the (n + 1) th pulse and the (n + 2) th pulse is made longer than the time interval between the (n is a natural number) pulse and the (n + 1) th pulse.

Further, at the time of irradiating a laser beam pulse to each processing position in the burst pulse mode,
The time interval between two pulses is longer than the time interval between two previously irradiated pulses.

Further, the beam intensity of the last one pulse is made higher than the beam intensity of the previous pulse.

In addition, the beam intensity of the last one pulse is increased by reducing the pulse width.

Further, the beam intensity of the last one pulse is increased by increasing the energy.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1 Conventionally, quantitative measurement of the residual amount of resin, which has been dependent on visual inspection with a microscope or the like, has become possible with a resin residual amount inspection device (hereinafter, appropriately referred to as an inspection device) using fluorescence amount measurement. By using this inspection device, the amount of resin remaining can be grasped as a numerical value called the output voltage value, and by comparing and examining this measurement data, it is possible to investigate a processing method that is really effective in reducing the amount of resin remaining. became. As a result of studying various irradiation methods, it has been found that it is effective to change the frequency, that is, the time interval between successive pulses, as shown in FIG. 1, for example. Until then, 70 in burst mode machining
It has become possible to reduce the value of the output voltage of the inspection apparatus from about 0 mV to about 400 mV, which is the level of the cycle mode processing. This is considered to be due to the following reasons. When the substrate is pulsed with a laser beam,
First, laser beam absorption occurs in the resin part on the surface,
This raises the temperature of the resin portion, causing instantaneous melt evaporation. The evaporant at this time becomes plasma, and the processing of the plasma further proceeds by heating the plasma. However, this plasma has a high absorptivity to the laser beam and also has the property of preventing the laser energy of the later pulse from reaching the resin portion. Therefore, when the time interval between successive pulses is short as in the burst mode, it is conceivable that the energy of the laser cannot be sufficiently transmitted. However, since the plasma itself is rapidly expanding, it is considered that this pulse can be effectively used for processing if the next pulse irradiation is performed after a certain period of time. In other words, the last pulse irradiation of a plurality of pulse irradiations may be performed at a timing slightly longer than the time interval between preceding and succeeding pulse irradiations.

In general, it is said that a pulse having a higher beam intensity (W / cm ² ) of a laser beam is more effective in removing the resin, and it has been confirmed by a test apparatus. Therefore, it is better that the last pulse has a high beam intensity. However, it is known that the higher the pulse intensity of the beam, the larger the amount of absorption by the plasma, and even if the next pulse irradiation with the increased beam intensity is performed from the time when the plasma is growing, the effect is small. The last pulse irradiation may be performed by increasing the time interval from the immediately preceding pulse irradiation and then increasing the beam intensity. This makes it possible to reduce the amount of resin remaining in the burst mode.

Until the depth of the hole being processed reaches the inner-layer land, there is no particular effect due to the difference in land area. When the hole reaches the inner layer land, a difference occurs in the temperature rise on the land surface due to a difference in the area of the inner layer land, that is, a difference in heat capacity, and a variation occurs in a finished state.
In particular, when the influence of the plasma still remains, the variation becomes more remarkable because the energy of the laser beam that can reach the land is small. So, like the above,
After a sufficient time for the influence of the plasma to be neglected, the next pulse irradiation is performed. Further shortening the pulse width of this pulse, before the diffusion of heat quantity by heat conduction occurs,
Heating by the laser beam ends, and the influence of the difference in heat capacity can be reduced. Further, by shortening the pulse width, the beam intensity increases, and the ability to remove the resin can be improved.

FIG. 2 shows a schematic configuration diagram of a laser processing apparatus for piercing a printed circuit board or an electronic material by irradiating a laser beam. In FIG. 2, reference numeral 1 denotes a laser oscillator, 2 denotes a laser beam, 3a and 3b denote galvanomirrors, 4a to 4d denote reflection mirrors, 5 denotes an fθ lens, and 6 denotes a workpiece. As described above, the laser beam 2 is scanned by the galvanometer mirrors 3a and 3b, and the position on the workpiece 6 where a hole is to be formed is irradiated with the laser beam 2.

FIG. 1 is a waveform diagram of a pulse according to an embodiment of the present invention, that is, a diagram showing the timing of pulse irradiation. In this case, description will be made on the assumption that the burst mode is basically used. In the case of the burst mode, the advantage is that the processing time is short, and although it depends on the material of the work, pulse irradiation is usually 5 times or less, and usually 3 to 4 times.
The processing of one hole is completed by one pulse irradiation. FIG. 1 shows the case of four times. As shown in FIG. 1, the first pulses P1 and P2
Between the pulse P2, the second pulse P2 and the third pulse P
3 is the same period ts, while the period between the third pulse P3 and the fourth pulse P4 is a period tl longer than ts (in this specification, the periods (ts, t
l) is appropriately referred to as "time interval"). That is, thereby, the plasma generated by the irradiation of the first three pulses P1, P2, and P3 expands and becomes thin, and the fourth pulse P4 has no influence on the irradiation. Specifically, it has been found that the long cycle (time interval) tl is 1 ms to 10 ms, and that the effect is obtained at a frequency of about 100 to 1000 Hz.

As described above, in principle, the higher the beam intensity, the higher the resin removal ability.
It is preferable to make the pulse width shorter than that before, as shown in FIG. 3A, or to increase the peak value, that is, the energy of the pulse, as shown in FIG. The beam intensity is W /
cm ² = J / s / cm ² . Here, J indicates energy (joules), s indicates pulse width (seconds), and cm ² indicates the diameter of the laser beam. In the case of FIG. 3A, the energy of the fourth pulse (indicated by the area of the pulse in the graph) is the same as that of the first to third pulses, but the pulse width is short, so that the beam intensity increases. In the case of FIG.
In the fourth pulse, the pulse width is not changed and the energy is increased, so that the beam intensity is higher than that in the first to third pulses.

FIG. 4 shows the relationship between the amount of resin remaining in the hole at the time of burst mode processing and the time interval of the last pulse irradiation, as examined by the inspection apparatus. In FIG. 4, a low voltage value of the output of the inspection device shown on the vertical axis corresponds to a small resin remaining amount. The horizontal axis represents frequency, and the smaller the frequency value, the longer the period, that is, the time interval between successive pulse irradiations. The data shown in FIG. 4 shows that the total number of pulse irradiations is four times, and the frequency is 2000 Hz between the first and second pulse irradiations and between the second and third pulse irradiations.
It has become. Therefore, if the interval between the third and fourth pulse irradiation is set to 2000 Hz, all four irradiations are performed at the same time interval. It is apparent from FIG. 4 that the resin remaining amount decreases as the time interval between the third and fourth pulse irradiations is increased (frequency is reduced). FIG. 4 shows that when the frequency is set to 1000 Hz or less, the resin remaining amount is reduced to about 400 mV, which is the level of the cycle mode processing.

FIG. 5 shows the relationship between the amount of resin remaining in the hole at the time of burst pulse processing and the beam intensity of the last pulse, as examined by an inspection apparatus. In this experiment, of the total number of pulse irradiations, the first three pulse irradiations were performed at 2000 Hz, and the last pulse irradiation was performed at 1000 Hz. Also, only for the last pulse, the beam intensity is changed by shortening the pulse width or increasing the energy. Beam intensity 1 indicates that the beam intensity is the same as the previous pulse. It is apparent from this figure that the resin remaining amount decreases as the beam intensity of the last pulse is increased.

FIG. 6 shows a substrate provided with two types of inner layer lands, a small land having a small area and a large land having an area approximately four times as large as the small land. This is a result of processing under the two conditions of the pulse mode and the processing method according to the present invention, and measuring the resin remaining amount by an inspection device. It is apparent from FIG. 6 that the processing method according to the present invention reduces the influence difference between inner layer lands having different areas.

[0021]

As described above, according to the present invention, there is an effect that the amount of resin remaining in the burst mode processing can be reduced.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing a pulse waveform according to an embodiment of the present invention.

FIG. 2 is a schematic configuration diagram of a laser processing apparatus.

FIG. 3 is an explanatory diagram showing an increase in beam intensity of a last pulse.

FIG. 4 is an explanatory diagram showing the relationship between the pulse frequency of the last pulse and the remaining amount of resin.

FIG. 5 is an explanatory diagram showing the relationship between the beam intensity of the last pulse and the remaining amount of resin.

FIG. 6 is an explanatory diagram showing a change in resin remaining amount variation due to a difference in processing method when a land area is different.

FIG. 7 is an explanatory diagram of two types of pulse irradiation modes.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Laser oscillator 2 Laser beam 3a, 3b Galvano mirror part 4a, 4b, 4c, 4d Reflection mirror 5 fθ lens 6 Object to be processed

Claims (5)

[Claims] At the time of irradiating a pulse of a laser beam to each processing position in a burst pulse mode, an n-th (n is a natural number) pulse and an (n + 1) -th pulse are set for a time interval between two pulses irradiated before and after. A laser processing method, wherein the time interval between the (n + 1) th pulse and the (n + 2) th pulse is longer than the time interval between the pulses. 2. When irradiating a pulse of a laser beam to each processing position in a burst pulse mode, the time interval between the last two pulses is set to be shorter than the time interval between two pulses irradiated before and after that. Laser processing method characterized in that the length is also increased. 3. The laser processing method according to claim 2, wherein the beam intensity of the last one pulse is set higher than the beam intensity of the previous pulse. 4. The laser processing method according to claim 3, wherein the beam intensity of the last one pulse is increased by reducing the pulse width. 5. The laser processing method according to claim 3, wherein the beam intensity of the last one pulse is increased by increasing the energy.