Laser Marking

Laser Marking is the process of placing marks on the package of a semiconductor package using a laser beam. Laser marking is the other major alternative for marking semiconductor packages aside from ink marking.

Laser, which stands for "light amplification through stimulated emission of radiation", is simply a high-intensity beam of light emitted by atoms that have been supplied with enough energy either by optical or electrical means. Optical stimulation is primarily applied to a solid and usually crystalline material, using special lamps or diode bars to bombard the material with wavelength-specific light. On the other hand, electrical stimulation employs either a DC or RF signal to excite gas atoms.

The optical or electrical excitation of the atoms of the laser material causes their electrons to absorb energy and move to a higher energy level. They subsequently return to a lower energy level, releasing photons in order to do so. These photon emissions build up into the laser beam as more and more photons line up in phase.

Among solid-state lasers, the Nd:YAG laser is the most widely used for laser marking. Nd:YAG stands for 'Neodymium:Yttrium Aluminum Garnet'. Among gas lasers, the CO2 laser is the most popular, wherein carbon dioxide is excited into emitting an infrared beam that can engrave a mark on the surface.

All laser beams, regardless of type or source, are: 1) monochromatic, or of single color; 2) highly collimated, or can travel over large distances without spreading too much; and 3) coherent, or possess light waves that are in phase with each other.

Figure 1. Example of a back-end handling equipment from Ismeca that's also capable of laser marking

Laser marking is a non-contact thermal process that alters the surface to be marked by using the heat generated by a laser beam. There are three major ways to achieve mark contrast by laser marking. These are: 1) surface annealing, which applies relatively low temperatures to metallic surfaces to produce sharp, contrasting lines with very shallow penetration (making it non-disruptive to the surface); 2) surface melting, which is commonly used to induce a color change on plastic surfaces by melting the marking areas; and 3) material vaporization, which marks a surface by removing material from it through vaporization. Among these three, material vaporization is the most common method used for industrial marking.

A laser marker must allow its operator to optimize some parameters to produce excellent markings on the surface. Two common parameters for this purpose are the lamp current and the laser pulse rate. The lamp current determines the power or energy being used to stimulate a material into photoemission, i.e., increasing the lamp power increases the laser amplification. The pulse rate, on the other hand, determines the amount of time between laser 'bursts' are allowed to strike a surface. A higher the pulse rate shortens the time for the laser system to charge up and therefore lowers the peak energy of the laser beam, reducing its ability to vaporize some material from the surface. Thus, vaporization capability is increased by using high-energy laser beams at lower pulsing frequencies.

The advantages of laser marking over ink marking include: 1) better marking capabilities; 2) permanence of the mark; 3) high speed of marking; 4) short set-up or changeover time; 5) elimination of ink costs; 6) elimination of mess caused by ink; and 7) easier integration into other processes or operations.

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