One precise form of heighht sensing uses a non-contact laser beam to regulate the height of the dispensing tip from the receiving surface, as indicated in figure 6. The laser sensor automatically corrects for warp and thickness variations on a dot-by-dot basis. It emits a beam at the time of dispensing that reflects back from the receiving surface. This is checked in the systems computer against preprogrammed coordinates. If the height does not match the required coordinates, the sensor instructs the system to correct to correct the distance of the dispensing tip above the receiving surface. The required adjustment is made and the system continues dispensing. All in less that 10 µs. All calculations and adjustments are accomplished "on-the-fly" during the actual moment of dispense. With dots of solder paste in the volume range of 0.01 µl, even a minute height variance can be crucial to successful dispensing.
This type of sensor is not always needed, but this is a good option to keep in mind. If the hybrids are always flat the automated height correction is not needed. If there is a chance that uneven height may be present, it is a worthwhile option to have.
Hybrid assemblies come in a multitude of sizes and configurations and are relatively small when compaired to other types of printed circuit assemblies. For substrates on the larger size end of the scale and for those requiring less dense solder paste patterns and larger dots, screening can handle the application. When the size of the hybrid drops and the density of the dispense pattern increases, both size and placement of solder paste patterns become critical.
Most types of automated positioning systems for dispensing can give the speed and programmability needed. Figure 7 shows an example of a fully equiped automated dispensing system. A proper vision alignment system will make sure that the dispensing is repeatable in pattern positioning and if required, a non-contact height sensor will ensure the hybrid receiving surface is level in relation to the dispensing tip. However, due to the size of the substrates involved and the idiosyncrasies of the material itself, when dispensing solder paste only the most precise dispensing head can give the ability to accomplish repeatable, volumetric dispensing after all other parameters have been met.
Figure 7, ADM-1812/TS benchtop automated dispensing system with piston positive displacement pump, vision alignment system, and a laser height sensor.
Gary helmers, vice president, may be contacted at Creative Automation Company, 11641 Pendleton St., Sun Valley, CA 91352; (818) 767-6220; Fax: (818) 767-1243;