CN109950162B - Laser surface treatment method for improving ultrasonic bonding quality of bonding pad - Google Patents
Laser surface treatment method for improving ultrasonic bonding quality of bonding pad Download PDFInfo
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- CN109950162B CN109950162B CN201910222941.2A CN201910222941A CN109950162B CN 109950162 B CN109950162 B CN 109950162B CN 201910222941 A CN201910222941 A CN 201910222941A CN 109950162 B CN109950162 B CN 109950162B
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Abstract
The invention discloses a laser surface treatment method for improving ultrasonic bonding quality of a bonding pad, which comprises the following steps: placing the surface of the bonding pad to be bonded at a laser processing position; carrying out surface treatment on the welding disk by adopting pulse laser; and bonding the processed bonding pad. The invention has the beneficial effects that: the laser surface treatment efficiency is high, no mechanical damage exists, and the consistency is good.
Description
Technical Field
The invention relates to the technical field of laser processing, in particular to a laser processing method for microelectronic assembly, and specifically relates to a laser surface treatment method for improving ultrasonic bonding quality of a bonding pad.
Background
In the field of microelectronic packaging, ultrasonic bonding methods such as lead wires, ball bonding, flip chip and the like are important ways for realizing internal electrical interconnection of a packaging body. The bonding process mainly utilizes mutual diffusion of metal atoms of a bonding interface, quality parameters such as bonding strength, consistency and the like are related to process parameters such as temperature, pressure, time, ultrasonic power and the like in the bonding process, the state of a surface to be bonded also has great influence on the bonding quality, and the uniformity of an electroplated layer and the surplus materials such as soldering flux and the like directly influence the ultrasonic bonding consistency and strength. If the bonding is carried out directly without processing, problems of cold joint, desoldering and the like are caused, so that the long-term reliability of the product is not ensured.
In order to improve the ultrasonic bonding quality, the surface treatment of the bonding pad can be carried out by adopting a mechanical friction method, and a relatively 'fresh' bonding surface is manufactured by adopting a mechanical friction mode of a bonding chopper. The method can improve the bonding quality to a certain extent, but has poor operation consistency, easy damage to the welding disc and low production efficiency. In addition, the surface of the bonding pad can be cleaned and activated in a cleaning mode. When the cleaning agent is used for cleaning by using a water-based cleaning agent or a gas-phase cleaning agent such as bromopropane and the like, various substances such as solder resist residues, oil stains and the like can be removed, but the used solution is corrosive, pollutes the environment, consumes more materials and has high cost. The bonding pad is cleaned and activated by adopting the plasma, and the surface energy of the bonding pad can be improved, the wettability is increased and the bonding effect is improved to a certain extent by utilizing the high-energy activation effect of the plasma. However, the plasma cleaning has a limited capability of cleaning the excess, the removal efficiency is low, and the bare chip may be damaged when the plasma energy is increased.
The information disclosed in this background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.
Disclosure of Invention
The technical problem to be solved by the invention is as follows: how to solve the problems of low bonding strength and large bonding stress difference caused by the surface state of a bonding pad in the assembly process of the existing microcircuit.
The invention solves the technical problems through the following technical scheme: the invention relates to a laser surface treatment method for improving ultrasonic bonding quality of a bonding pad, which comprises the following steps:
step SO1, placing the surface of the bonding pad at the position to be processed; in particular to a processing platform on a laser;
step S02, performing surface treatment on the welding disk by adopting pulse laser; wherein, the laser emission source is a laser;
step S03, bonding the processed pads.
Through high-speed galvanometer scanning, ultraviolet pulse laser regularly acts on treating the bonding pad, makes the pollutant break away from bonding pad substrate material surface, and the bonding pad substrate absorbs laser energy, makes the surface coating melting, and the cleanliness factor is high after the bonding pad surface remelting, and the uniformity is good, promotes ultrasonic bonding quality.
Preferably, the bonding pad to be bonded in the step SO1 is one of a printed circuit board, a co-fired ceramic substrate, and a bonding pad on a chip.
Preferably, in the step SO1, the surface of the pad to be bonded is a focal plane of laser processing, the emission source of the pulse laser is a pulse laser, and the processing platform on the laser is a position to be processed.
Preferably, in the step S02, the laser wavelength is 355nm, the laser spot diameter is 0.015mm, the laser average energy is 1W-1.4W, the frequency is 35-40KHZ, and the laser marking speed is 280-350 mm/S.
Preferably, the surface treatment in step S02 further includes performing laser cleaning, that is: surface cleaning can be further carried out by adopting low-energy laser after the surface treatment of the high-energy laser.
Preferably, the laser energy of the laser cleaning is 0.5W-0.8W, the frequency is 40-45KHZ, and the laser marking speed is 280-300 mm/s.
Preferably, in the surface treatment process in step S02, the machining surface is rapidly cooled by a cooling air flow by blowing with a cold air, so that the cleanliness is high after the reflow of the pad surface, and impurities can be removed.
Preferably, in step S02, the laser marking path in the surface treatment is in one or more of a transverse filling scan, a longitudinal filling scan and a coaxial filling scan,
preferably, in step S02, the path pitch is 0.005 to 0.010mm, and the number of paths is at least one.
Preferably, the bonding mode of the pad subjected to the laser surface treatment in step S03 is one or more of wire bonding, ball mounting, and flip chip bonding.
The mechanism of the invention is as follows: through high-speed galvanometer scanning, ultraviolet pulse laser regularly acts on a bonding pad to be bonded, on one hand, under the complicated physical and chemical action of the laser, a series of complicated physical and chemical processes such as vibration, melting, combustion, even gasification and the like can be generated after pollutant materials attached to the bonding pad absorb the laser energy, and finally the pollutants are separated from the surface of a bonding pad substrate material; on the other hand, the bonding pad substrate also absorbs laser energy, the surface coating can be melted instantly under the action of laser, the ultraviolet laser has short wavelength, concentrated energy and small heat affected zone, and is rapidly cooled under the action of external cooling airflow, the cleanliness is high after the surface of the bonding pad is remelted, the consistency is good, and the ultrasonic bonding quality is improved.
Compared with the prior art, the invention has the following advantages:
(1) the invention solves the problems of low bonding strength and bonding caused by the surface state of the existing bonding padThe invention has high laser surface treatment efficiency, no mechanical damage, good consistency and surface treatment rate up to 1mm2/s；
(2) The ultrasonic bonding quality is obviously improved by laser surface treatment, taking a gold-plated pad of a printed circuit board as an example, the mean value of the ultrasonic gold wire pressure welding tension is improved from 7.1g to 9.2g, and the standard deviation is reduced from 1.5g to 0.4 g;
(3) the laser surface treatment is flexible processing, has good selectivity and good compatibility with the ultrasonic bonding process. Environment-friendly, and avoids environmental pollution caused by cleaning fluid and the like.
Drawings
FIG. 1 is a schematic diagram of a laser surface treatment method for improving ultrasonic bonding quality of a bonding pad in an embodiment of the invention;
FIG. 2 is a schematic diagram of a laser surface treatment lateral filling scanning mode for a bonding pad in an embodiment of the invention;
FIG. 3 is a schematic diagram of a pad laser surface treatment vertical fill scanning mode in an embodiment of the present invention;
FIG. 4 is a schematic diagram of a laser surface treatment coaxial filling scanning mode for a bonding pad in an embodiment of the invention.
Reference numbers in the figures: the bonding method comprises the steps of bonding pads 1 to be bonded, laser beams 2, laser spots 21, laser overlapping spots 22, laser marking paths 23 and laser marking path intervals 24.
Detailed Description
The following examples are given for the detailed implementation and specific operation of the present invention, but the scope of the present invention is not limited to the following examples.
The first embodiment is as follows:
the bonding pad 1 to be bonded in the embodiment is a gold-plated bonding pad of a printed circuit board, and the bonding mode is wire bonding.
Referring to fig. 1, 2 and 3, the laser surface treatment of the gold-plated pad of the printed circuit board in the embodiment includes the following steps:
step S01: placing the printed circuit board on a laser processing platform, and fixing the bonding pad 1 to be bonded by vacuum pumping; the height of the laser head is adjusted as required, so that a laser spot 21 is focused on the surface of the printed circuit board, and the upper surface of the printed circuit board is a focal plane for laser processing;
step S02: carrying out surface treatment on the printed circuit board by adopting a pulse laser; the processing size of the surface of the printed circuit board is set to 0.1mm x 0.1mm, the laser marking path 23 adopts a transverse filling scanning mode, and the distance 24 between the laser marking paths is 0.008 mm; the laser wavelength of the surface treatment is 355nm, the diameter of a laser spot 21 is 0.015mm, the average laser energy is set to be 1.4W, the frequency is 40KHZ, and the laser marking speed is 300 mm/s; processing the laser for 1 time according to a set path, and opening auxiliary compressed air to carry out blowing cooling on the surface of the bonding pad in the processing process; and then, carrying out laser cleaning, changing the average energy of the laser for cleaning to 0.5W, keeping the other parameters unchanged, and reprocessing for 1 time according to the set path.
Step S03, bonding the processed bonding pad; a25 micron gold wire was selected, the ultrasonic frequency was set to 20KHz, the time was set to 0.055s, the bonding pressure was set to 20g, and the temperature was set to 120 ℃. Carrying out gold wire pressure welding bonding on the bonding pad before and after the laser surface treatment, wherein the result of a gold wire pressure welding tension test shows that: the mean value of the wire bonding tension is increased from 7.1g to 9.2g, and the standard deviation is reduced from 1.5g to 0.4 g;
if the laser marking path 23 adopts the longitudinal filling scanning mode, the laser marking path distance 24 is still 0.008mm, and other parameters are also unchanged.
Example two:
the difference between the second embodiment and the first embodiment is: step S01 is the same as step S02, but step S03 is different, and the bonding method in the second embodiment is ball bonding.
Selecting 25 micron gold wire for ultrasonic ball planting, setting ultrasonic frequency at 15KHz, bonding pressure at 10g and temperature at 120 deg.C. The test result of gold wire ball-planting shearing force on the bonding pad before and after laser surface treatment shows that: the mean value of the shearing force was increased from 30.21gf to 68.44gf, and the standard deviation was decreased from 4.84gf to 4.54 gf.
Example three:
as shown in fig. 1 and 4, the difference between the third embodiment and the first embodiment is: the bonding pad 1 to be bonded in the embodiment is a gold-plated bonding pad of a thin-film alumina ceramic substrate, and the bonding mode is flip-chip bonding.
Step S01: placing the thin-film alumina ceramic substrate on a laser processing platform, and fixing a bonding pad 1 to be bonded by vacuumizing; by adjusting the height of the laser head, a laser spot 21 is focused on the surface of a gold-plated bonding pad of the thin-film alumina ceramic substrate, and the upper surface of the gold-plated bonding pad is a focal plane for laser processing;
step S02: carrying out surface treatment on the welding disk by adopting a pulse laser: the surface treatment size is set to 4mm x 3.5mm (slightly larger than the size of a chip), the surface treatment size comprises a pad array, a pad metal layer and a ceramic dielectric layer are treated together, a laser marking path 23 adopts a coaxial filling scanning mode, the laser marking path 23 is of a square spiral structure, and the distance 24 between the laser marking paths is 0.005 mm; the laser wavelength of the surface treatment is 355nm, the diameter of a laser spot 21 is 0.015mm, the average laser energy is set to be 1W, the frequency is 35KHZ, and the laser marking speed is 280 mm/s; processing the laser for 1 time according to a set path, and opening auxiliary compressed air to carry out blowing cooling on the surface of the bonding pad in the processing process;
step S03: bonding the processed bonding pad; the ultrasonic frequency was set to 28KHz, the time was set to 0.8s, the bonding pressure was set to 60N, the substrate heating temperature was 200 ℃, and the shear strength reached 45gf, which was 20% higher than that of the untreated surface.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents and improvements made within the spirit and principle of the present invention are intended to be included within the scope of the present invention.
Claims (7)
1. The laser surface treatment method for improving the ultrasonic bonding quality of the bonding pad is characterized by comprising the following steps of:
step SO1, placing the surface of the bonding pad at the position to be processed; the bonding pad to be bonded is one of a printed circuit board, a co-fired ceramic substrate and a bonding pad on a chip; the bonding pad to be bonded is a bonding pad with gold plated surface;
step S02, performing surface treatment on the welding disk by adopting pulse laser; the laser marking path in the surface treatment adopts one or more modes of transverse filling scanning, longitudinal filling scanning and coaxial filling scanning; in the surface treatment process, cold air is used for blowing; the laser wavelength is 355nm, the diameter of a laser spot is 0.015mm, the average laser energy is 1W-1.4W, the frequency is 35-40KHZ, and the laser marking speed is 280-350 mm/s;
step S03, bonding the processed pads.
2. The laser surface treatment method for improving the ultrasonic bonding quality of the bonding pad according to claim 1, wherein the surface of the bonding pad to be bonded in the step SO1 is a focal plane of laser processing, an emission source of the pulse laser is a pulse laser, and a processing platform on the laser is a position to be processed.
3. The laser surface treatment method for improving the ultrasonic bonding quality of the bonding pad according to claim 1, wherein the surface treatment in the step S02 further comprises performing laser cleaning.
4. The laser surface treatment method for improving ultrasonic bonding quality of a bonding pad according to claim 3, wherein the laser energy of the laser cleaning is 0.5W-0.8W, the frequency is 40-45KHZ, and the laser marking speed is 280-300 mm/s.
5. The laser surface treatment method for improving the ultrasonic bonding quality of the bonding pad according to claim 3, wherein in the step S02, the laser marking path in the laser cleaning process adopts one or more of a transverse filling scan, a longitudinal filling scan and a coaxial filling scan.
6. The laser surface treatment method for improving ultrasonic bonding quality of a bonding pad according to claim 1 or 4, wherein in the step S02, the path pitch is 0.005-0.010mm, and the number of paths is at least one.
7. The laser surface treatment method for improving the ultrasonic bonding quality of the bonding pad according to claim 1, wherein the bonding pad subjected to the laser surface treatment in the step S03 is subjected to ultrasonic bonding in one or more of wire bonding, ball mounting and flip chip bonding.
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| CN111112842A (en) * | 2019-12-24 | 2020-05-08 | 中国电子科技集团公司第三十八研究所 | Gold-removing tin-coating method and application |
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