SMT plastic patch often problems and solutions
1, empty points or glue adhesive dispensing excessive instability point uneven coating. Coating amount is too small, it will cause the bonding strength is not enough, when wave soldering components easy to fall off; rather excessive amount of glue, especially for small components, easily contaminated land, impede electrical connections.
Causes and Countermeasures:
a). Adhesive large particles, or adhesive mixed with air bubbles, resulting in an empty spot. Measures to remove oversize particles using a bubble plastic film.
b) adhesive warmed enough time, not when the viscosity stability began coating, causing glue instability. Prevention method: When used, placed in a sealed container to prevent condensation of the stand for about 1 hour, then loaded on each dispensing head, dispensing nozzle temperature to be stable before beginning dispensing. Use better if there thermostat.
c) placing the dispensing head is not used for a long time, resulting in micro blockage. Workaround: fully warmed recovery thixotropic adhesive patch. To start dispensing try several times, adjusting atmospheric pressure until the glue stable work in the beginning.
2, the drawing is dispensing patch glue dispensing head movement direction was filamentous connection this phenomenon. Drawing more patches of plastic will contaminate the land, causing poor welding. Patch of plastic drawing mainly by its principal component drawing resistance of the resin and setting dispensing parameters.
Workaround:
a) to increase the dispensing head stroke, reduce the speed.
b) low viscosity, high thixotropic adhesive patch is not easy drawing, so try to choose low-viscosity, high thixotropic adhesive patch.
c) the temperature of the thermostat set higher, forcing the adhesive patch to adjust to the low viscosity and high thixotropic adhesive patch. Then you must consider the storage of glue and dispensing of pressure.
3, there are two main reasons for the slump slump caused.
a) Due to poor thixotropic adhesive patch, making the adhesive patch and easy flow, causing collapse, possibly contaminated land, resulting in bad electrical connections, the solution: try to choose a thixotropic adhesive patch.
b) After the glue is too long because the storage time cause collapse. Solution: Try to be a patch shortly after coating. Cured.
4, components offset components is prone to offset adverse speed placement machine. Offset There are two: θ angle occurs when one is pressed into the components of glue offset; the second is offset PCB XY axis moving at high speed to produce a small patch of plastic components coated area On prone to this phenomenon, the reason is caused by insufficient adhesion. Appropriate measures, the selection of relatively high thixotropic, high wet strength adhesive patch. Test had proved that if the placement rate of 0.1 sec / chip, the acceleration component reaches 40m / S², therefore, of glue adhesive force must be sufficient to achieve this.
5, components sometimes fall into the wave trough QFP, SOP and other large components, the wave, due to stress its own weight in excess solder bath and solder adhesion of glue, falling in the solder pot, because that is inadequate adhesive patch red knot caused by force. There are several specific reasons:
a) When reflow, SMT adhesive is not fully cured Solution: extend the curing time or curing temperature increase;
Under b) wave high temperature for too long undermined the cohesion of glue; lower temperatures or reduce wave over wave of time;
c) patch of plastic glue inadequate, insufficient to provide adequate adhesion, strategy: to increase the amount of adhesive;
d) patch of plastic itself high temperature performance is poor, a significant decline in bond strength at elevated temperatures, countermeasures: use high temperature adhesive patch. Or due to high temperature caused by the adhesive force reduction. Therefore, the choice of adhesive patch, but also to pay attention to it in the adhesive strength at high temperatures.
