Ceramics substrates, Al2O3 and AlN, are commonly used as electronic packaging substrate materials, have many advantages such as small dielectric coefficient, small thermal expansion coefficient, high thermal conductivity, good insulation performance, and good corrosion resistance. In short, ceramics substrates can meet all the performance requirements of microelectronic device packaging and are widely used in the fields of aerospace and military engineering. However, due to the hard and brittle characteristics of ceramics materials, the traditional machining method is easy to cause damage to this kind of materials. Laser processing is an advanced processing technology with no contact processing, no tool wear, high precision, and high flexibility. It is the preferred method for hard and brittle materials processing. At present, there are many researches on the effect of characteristic dimension of laser ablated holes, but the study on the characteristic morphology of laser drilled holes is often ignored. Moreover, when a high energy laser is focused on the material, the strong thermal effect cannot be avoided, which leads to the recast layers, micro-cracks, and heat affected zones on the surface of the ceramics substrate, influencing the morphology of the holes and subsequently influencing the performance of the ceramics substrate. It must be pointed out that the thermal effect has a high correlation with the laser pulse duration. Specifically, the thermal effect of the long pulse laser represented by the millisecond laser is the most serious, and the holes ablated by them have serious spatters, recast layers and micro-cracks. The thermal effect of a short pulse laser represented by the nanosecond laser is relatively small, and the morphological quality of holes is improved greatly. The ultrafast laser (generally referred to the pulse duration ≤12 ps) has the characteristics of "cold ablating" , which can limit the influence of the thermal effect to the maximum, and thus can be used to process the holes with a high morphological quality. In this paper, the morphological characteristics of holes in the electronic ceramics substrate ablated by a millisecond long pulse, a nanosecond short pulse, and an ultrafast laser pulse are reviewed.
As for the hole circularity, it becomes worse when the scanning speed increases during the millisecond laser processing. During the nanosecond laser processing, the higher the repetition rate, the better the hole circularity. During the ultrafast laser processing, the circularity and machining efficiency can be ensured by selecting appropriate interpolation errors. For the spatters on the holes surfaces, the higher the laser energy, the higher the repetition rate, the wider the spattering range. During the millisecond laser processing, the larger the pulse duration, the more the surface spatters, the wider the spatters range. During the ultrafast laser processing, the faster the scanning speed, the wider the spattering range. The surface cracks of holes induced by laser ablating on the ceramics substrate are mainly caused by the surface stress concentration coming from the thermal effect. The radial cracks are generally induced by the tangential stress, and the ring cracks are by the radial stress. The cracks propagate to form the propagation path of the group hole cracks, which maybe cause the sample to fracture finally. The laser compound machining assisted by water and other liquids, the spatters and cracks on hole surfaces can be reduced and the surface morphologies of holes can be improved.
The laser processed hole taper on ceramics substrate is related to laser energy, repetition rate, pulse duration, focus position, air pressure, and processing environment. The faster the scanning speed, the larger the hole taper. During the nanosecond laser processing, the wider the scanning filling circle’s interval, the larger the hole taper. For different hole diameters and depths, a smaller hole taper can be obtained by choosing the appropriate processing filling style. For the recast layer on the hole sidewall, even using an ultrafast laser with the "cold working" characteristics, it cannot be completely avoided. The hole recast layer processed by the millisecond laser is thicker. Under the optimized parameters, the thicknesses of the recast layer processed by the nanosecond and ultrafast lasers are almost the same. Furthermore, the thicknesses of the recast layer can be reduced effectively by the water jet assisted machining and the water environment assisted machining, and it can be completely removed by a post-treatment such as solution corrosion. It should also be pointed out that a large number of microcracks extending along the grain direction are generally distributed on the surface of the recast layer, especially for the millisecond laser processing.
High quality holes have been required on the surface of a hard-brittle electronic ceramic substrate processed by a laser in order to achieve high quality and high density interconnection of electronic devices. Morphological features of holes drilled by a long pulse duration millisecond laser, a short pulse duration nanosecond laser and an ultrafast laser are reviewed, mainly including the hole surface morphological characteristics such as hole circularity, spatters, micro-cracks, and heat affected zones on the hole surfaces, and the hole side-wall morphological characteristics such as hole taper, recast layers and micro-cracks on the side-wall surface.
