The laser transmits through the top material being bonded into the bottom material with laser light absorbing additives or fillers in the bottom material. Laser power, applied pressure between the two adjoining parts, and time are critical factors manufacturing components to have acceptable leaser welded joints. In some cases however, hot spots or cold spots on the joint may occur and affect the joint quality.  Cold spots occur if not enough power is applied or the pressure applied between the two components is too great.  Hot spots are caused by over separation between the top and bottom layers and can result in voids in the sealing area and in an unsealed assembly. This process can be controlled by temperature measurement of the weld and recorded with thermal map of the completed welded area.