Automated Dispensing
The type of head used for hybrid solder paste dispensing applications is more critical than in other dispensing areas, due to the potential for solder paste to separate. To dispense solder paste in uniform size dots onto hybrids — whether trimmed or untrimmed, populated or unpopulated — reliable dispensing, usually at high speeds, requires high dispense pressure. However, keeping solder paste under very high pressure for any length of time in a substantial quantity will cause it to separate.

To counteract these problems the Piston Positive Displacement Pump* was developed, as shown in figure 3. This dispensing pump transfers solder paste out of the tip at a pressure of 10,000 to 30,000 psi. The dispensing system maintains a stand-off distance from the hybrid so there is no chance for contact that may cause the substrate to move, subsequently causing material misalignment. The pump literally "shoots" the solder paste onto the substrate. The solder paste is kept in a "staging" area of the pump under low pressure so there is no chance for material separation. Only the exact amount of solder paste is fed into the dispensing pump chamber is under high pressure. The diameter of the dot is directly related to the dispensed volume and the viscosity of the solder paste. Using the proper dispensing tip size in relation to the material viscosity is also imperative as indicated in figure 4.

In automated dispensing for this application, once proper dot volume and size are achieved, as shown in figure 5, the foremost concern is repeatability. Repeatability translates into success. If the dispensing process is not repeatable, accuracy and speed are of no value. It is that simple. If the dot of solder paste winds up in the wrong spot and shorts out two pads, chances are that all the value added for that assembly is lost and it must either be reworked or discarded. If the system fails to dispense the right amount of material and it is not discovered, the results could be even worse — the assembly may test well at the factory but fail in the field.

Typically, hybrids are small. Even uncut, multiple substrates are usually much smaller than the average PC board. There are many other aspects that make an automated dispensing system suitable for this particular application, aside from X-Y-Z programmability. Having the correct type of dispensing head and a vision alignment system are critical. Where there may be a problem with holding the hybrid on a level plane, a non-contact, laser based height sensor is also recommended.

Dispensing onto hybrids requires a highly accurate and capable dispensing system. In addition to a pump the system should automatically adjust for height variations and material inconsistencies as well as part position errors. These are requirements for truly effective solder paste dispensing in hybrid applications.

 

There are some physical problems encountered with the hybrid substrate that also gives credence to automated dispensing versus screening. As the size of the substrate increases, because of the ceramic material of which it is typically made, the substrate undergoes some shrinkage during the process of firing. Therefore, the stencils used for screen printing must typically be fabricated after, rather than before the firing process.