The dispensingprocess is mainly used in the mixed placement process where the through-holeinsertion (THT) and surface mount (SMT) coexist. In the whole productionprocess, we can see that one of the components of printed circuit board (PCB)can be soldered by wave soldering from the beginning of dispensing and curingto the end. During this period, the interval is long, and there are many otherprocesses, so the solidification of components is particularly important.
The dispensingprocess is mainly used in the mixed placement process where the through-holeinsertion (THT) and surface mount (SMT) coexist.
Process control indispensing process. The following process defects are easy to appear in theproduction: unqualified glue spot size, wire drawing, glue dip pad, poor curingstrength and easy chip dropping. Therefore, it is a solution to control thetechnical parameters of dispensing.
1. Size ofdispensing amount
According to workexperience, the size of glue spot diameter should be half of the pad spacing,and the glue spot diameter should be 1.5 times of the glue spot diameter afterthe patch. This will ensure that there is enough glue to bond the componentsand avoid excessive glue to contaminate the pad. The dispensing amount isdetermined by the dispensing time and the dispensing amount. In practice, thedispensing parameters should be selected according to the production conditions(room temperature, glue viscosity, etc.).
2. Dispensingpressure
At present, thecompany's dispensing machine applies a pressure to the dispensing needlecartridge to ensure enough glue to extrude the dispensing nozzle. If thepressure is too high, it will cause too much glue; if the pressure is toosmall, it will cause intermittent phenomenon of glue dispensing and leakage,which will cause defects. The pressure should be selected according to the samequality glue and working environment temperature. If the ambient temperature ishigh, the viscosity of glue will be reduced and the fluidity will be improved.At this time, the supply of glue can be ensured by lowering the pressure, andvice versa.
3. Size ofdispensing nozzle
In practice, theinner diameter of the dispensing nozzle should be 1 / 2 of the glue dispensingpoint diameter. During the dispensing process, the dispensing nozzle should beselected according to the pad size on the PCB: for example, the pad size of0805 and 1206 is not different, the same kind of needle can be selected, butdifferent dispensing nozzle should be selected for the pad with greatdifference, which can not only ensure the quality of glue point, but alsoimprove the production efficiency.
4. Distancebetween dispensing nozzle and PCB board
Differentdispensers use different needles, and the dispensing nozzle has a certaindegree of stop. At the beginning of each work, make sure that the stop rod ofthe dispensing nozzle contacts the PCB.
5. Gluetemperature
Generally, theepoxy resin glue should be stored in the refrigerator of 0-50c, and it shouldbe taken out 1 / 2 hours in advance to make the glue fully meet the workingtemperature. The use temperature of glue should be 230c-250c; the ambienttemperature has a great influence on the viscosity of glue. If the temperatureis too low, the glue point will become smaller and wire drawing will occur. Thedifference of ambient temperature of 50c will cause 50% change of dispensingamount. Therefore, the ambient temperature should be controlled. At the sametime, the environment temperature should also be guaranteed, the humidity issmall, the glue point is easy to dry, affecting the adhesion.
6. Viscosity ofglue
The viscosity ofglue directly affects the quality of dispensing. If the viscosity is high, theglue point will become smaller, even wire drawing; if the viscosity is small,the glue point will become larger, which may dye the pad. In the process ofdispensing, the glue with different viscosity should be selected withreasonable pressure and dispensing speed.
7. Curingtemperature curve
For the curing ofglue, the general manufacturer has given the temperature curve. In practice,higher temperature should be used as far as possible to solidify the glue sothat it has enough strength after curing.
8. Bubbles
There must be nobubbles in the glue. A small bubble will cause many pads to have no glue; everytime glue is filled, the air in the plastic bottle should be emptied to preventempty hitting.
The adjustment ofthe above parameters should be carried out from point to surface. The change ofany parameter will affect other aspects. At the same time, the defects may becaused by many aspects. The possible factors should be checked one by one andthen eliminated. In short, the parameters should be adjusted according to theactual situation in production, which not only ensures the production quality,but also improves the production efficiency.