Three important factors in the SMT quality

Three important factors in the SMT quality 

A.Correct components

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It is required that the type, model, nominal rating, polarity and other features of the components should be consistent with those on the assembly diagrams and the specifications of the products.

B. Accurate placement

The end and the pin of the components should aligh with the diagram on the bonding pad and be put on the middle part. The soldering end of the component should connect with the solder paste. The placement of the components should meet the processing requirement. The placement requirements for CHIP components that have two ends, the components with gull-wing lead, J-type lead and ball-type lead are as follows:

1.For chip components with two end: as the self-positioning effect is very large, more than half in the width direction of this kind of component is bonded on the bonding pad, and the two ends in the length direction need to be lapped on the corresponding bonding pad and connect with the solder paste, then it can achieve self positioning in the re-flow soldering process. But if onw of the end is not lapped on the bonding pad or connects with the solder paste, shift or suspension would occur in the re-flow process.

2.For components with gull-wing lead and J-type lead: as the self positioning effect components like SOP, SOJ, QFP(Quad Flat Package), PLCC(Plastic Leaded Chip Carrier) is relatively small, the shift of mounting can not be corrected in the re-flow process. If the placement position exceeds deviation range, manual correction is necessary before the re-flow process. Otherwise rework is necessary after the re-flow soldering process which would cause waste of time and materials even the product reliability. Timely correction of the mounting coordinate are necessary if there is deviation during the production process. Manual placement and correction should take the above requirements into consideration to avoid bridging caused by solder paste adhesion.

3.For components with ball-type lead: as components like BGA(Ball Grid Array) and CSP(Chip Scale Package) have comparatively larger bonding pads and better self positioning effect, so only two requirements should be met. Firstly the soldering ball of the BGA(Ball Grid Array) should correspond to the related bonding pad; secondly, the deviation between the center of the soldering ball and the center of the boning pad should shorter than half the soldering ball diameter .

C. Proper pressure( the height of the chip)

The patch pressure(the height of the Z-axis) should be controlled in proper range. Small patch pressure would lead to the failure to stick the components because the soldering end or the pin of the components float on the surface of the soldering paste. Shift and movement are easily caused in the passing and re-flow process. Besides, too high Z-axis would also cause shift of placement when the components are dropped from a high distance. Too large patch pressure would lead to the adhesion of solder paste and causes bridging in the re-flow process, and it would also caused damage to the components due to movement and shift caused by sliding of the solder paste.

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