Judging And Adjustment Method Of Defective Pcb Products

- Aug 13, 2019-

1. Defect name and meaning: Dislocation of Pcb board components

Judging criteria: the misalignment is greater than 25% of the pad area

Causes and solutions:

A. Board alignment is misaligned: re-alignment;

B. Printing machine printing accuracy is not enough



2. Defect name and meaning: Pcb printing is not complete, and some pads are not printed with solder (less tin)

Judging criteria: the unprinted portion should be less than 25% of the pad area

Causes and solutions:

A. leaking hole blocking: wipe the bottom of the stencil, use a fiber-free paper towel or a soft brush dipped with absolute ethyl alcohol.

B. Lacking of solder paste or uneven solder paste in the width direction of the blade: add solder paste to make it uniform.

C. Solder paste viscosity is not suitable, printability is not good: change solder paste.

D. Solder paste rolling performance is not good: slow down the printing speed, increase the blade delay, so that the solder paste on the scraper fully flows to the stencil.

E. Solder paste stick to the bottom of the stencil: slow down the leaving speed of the board.



3. Defect name and meaning: solder paste is too thin, solder paste thickness does not meet the specified requirements (less tin)

Judging criteria: The thickness of the solder paste is controlled between -10% and +15% of the thickness of the stencil.

Causes and solutions:

A. Slow down printing speed

B. Increase printing pressure

C. increase printing times



4. Defect name and meaning: the thickness of the solder paste is inconsistent, the solder paste on the pad is thin or thick in different portion, and there are breakpoints.

Judging criteria: The thickness of the solder paste is controlled between -10% and +15% of the thickness of the stencil.

Causes and solutions:

A. Stencil is not parallel with pcb: adjust the horizontal degree of pcb workbench

B. Uneven solder paste: stir evenly before printing.

C. Stencil window inner wall is poorly smooth, there is residual solder paste: cleaning stencil



5. Defect name and meaning: the figure collapses and the solder paste collapses to the four sides (collapse)

Judging criteria: 25% beyond the pad area or solder paste pattern adhesion

A. Cream viscosity is small, thixotropy is not good: change solder paste.

B.Room temperature is too high, resulting in a decrease in viscosity: control room temperature is 23 ° C ± 3 ° C



6. Defect name and meaning: solder paste pattern adhesion, solder paste pattern has depression

Judging criteria: the adjacent pad maps are connected together

Causes and solutions:

A.The bottom of the stencil is not clean: clean the bottom of the stencil

B. Too many printing times: correction parameters

C. Excessive pressure : correction parameters

D.Stencil window is too large, insufficient rubber scraper hardness : replace steel scraper




7. Defect name and meaning: solder projection, solder paste on pcb pad is hillock

Judging criteria: the upper surface of the solder paste is unfairness and more than 0.2mm

Causes and solutions:

A. Cream viscosity: change solder paste

B. High off-board speed: adjust parameters.



8.Defect name and meaning: PCB surface contamination

Judging criteria: PCB surface is contaminated by solder paste

Causes and solutions:

A.The bottom of the stencil is contaminated by solder paste: cleaning the bottom surface of the template

B. The pcb is contaminated during rework, the PCB should be cleaned.

C. The bottom of the template is polluted, and the frequency of cleaning the stencil should be increased in the program. 


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