PCBA Tech - Common defects and solutions in SMT dispensing

Drawing/tailing

Drawing/tailing is a common defect in glue dispensing. The common causes are that the inner diameter of the glue nozzle is too small, the dispensing pressure is too high, the distance between the glue nozzle and the PCB board is too large, the patch glue is out of date or the quality is not good, and the reason is too small. The viscosity of the sheet glue is too good, the glue cannot be restored to room temperature after being taken out of the refrigerator, the dispensing volume is too large, etc. 

The solution: change the glue nozzle with a larger inner diameter; reduce the dispensing pressure; adjust the "stop" height; Change the glue and choose a glue with a suitable viscosity; after the patch glue is taken out of the refrigerator, it should be restored to room temperature (about 4h) before being put into production; adjust the amount of glue.

The glue nozzle is blocked

The fault phenomenon is that the amount of glue from the glue nozzle is too small or there is no glue point. The reason is generally that the pinhole is not completely cleaned; impurities are mixed in the patch glue, and there is a phenomenon of plugging; incompatible glue is mixed. 

Solution: change a clean needle; change a good quality patch glue; the brand of patch glue should not be mistaken.

Empty play

The phenomenon is that there is only dispensing action, but no amount of glue. The cause is that the patch glue is mixed with bubbles; the glue nozzle is blocked. 

The solution: the glue in the injection cylinder should be debubbled (especially the glue that is installed by yourself) ; Replace the glue nozzle.

Component shift

The phenomenon is that the components are displaced after the patch glue is cured, and the component pins are not on the pads in severe cases. The cause is that the amount of glue out of the patch glue is uneven, for example, the two-point glue of the chip component is more than one and the other is less; 

The component shifts during the chip or the initial adhesion of the patch adhesive is low; the PCB is placed for too long after the glue is placed. The glue is semi-cured. 

Solution: check whether the glue nozzle is blocked and eliminate the uneven glue output; adjust the work of the placement machine Status; change the glue; the PCB placement time after dispensing should not be too long (less than 4h).

The first piece of smt patch processing is tested and tested

The first piece inspection of smt patch is very important. As long as the component specification, model, and polarity direction of the first piece are correct, the machine will not place the wrong components during subsequent mass production: as long as the first piece placement position meets the placement deviation In general, the machine is able to ensure the repeatability during mass production. Therefore, the smt processing plant must carry out the first article inspection every shift, every day, and every batch, and an inspection (test) system must be established.

1. Program trial run

The program test run generally adopts the non-mounting component (dry run) method. If the test run is normal, it can be formally mounted?

2. The first trial sticker

1. Call out the program file.

2. Try to mount a PCB in accordance with the operating rules.

3. First article inspection

Inspection items

1. Whether the specifications, direction and polarity of the components on the tag numbers of each component are consistent with the process documents (or surface assembly samples).

2.  Whether the components are damaged, and whether the pins are deformed.

3. Whether the component mounting position deviates from the pad beyond the allowable range.

Inspection method

The inspection method should be determined according to the configuration of the inspection equipment of each unit.

Ordinary pitch components can be inspected visually, magnifying glass, microscope, online or offline optical inspection can be used for high density and narrow pitch

Check each (AOD).

Inspection standards

The patch processing plant is implemented in accordance with the corporate standards established by the unit or with reference to other standards (such as IPC standards or SUT10670-1995 general technical requirements for surface assembly processes).