Applied Statistics and Probability for Engineers

640 CHAPTER 16 STATISTICAL QUALITY CONTROL

observed defects. Furthermore, the first five defect categories on the Pareto chart are all solder-

related defects. This points to the flow solder process as a potential opportunity for improvement.

To improve the flow solder process, a team consisting of the flow solder operator, the

shop supervisor, the manufacturing engineer responsible for the process, and a quality engi-

neer meets to study potential causes of solder defects. They conduct a brainstorming session

and produce the cause-and-effect diagram shown in Fig. 16-23. The cause-and-effect diagram

is widely used to display the various potential causes of defects in products and their interre-

lationships. They are useful in summarizing knowledge about the process.

As a result of the brainstorming session, the team tentatively identifies the following vari-

ables as potentially influential in creating solder defects:

1. Flux specific gravity


2. Solder temperature

0

25

50

75

64

45

18

(^86)
(^5443)
(^21)
Number of defects
Insuffient solderSolder ballsDewettingPinholesBlowholesShortsMissing componentsMisaligned componentsComponent failureWrong componentUnsoldered
Figure 16-22 Pareto
diagram for printed
circuit board defects.
Machine Solder Flux
Wave turbulance
Conveyor speed
Conveyor angle
Exhaust
Maintenance
Temperature
Wave height
Contact time
Wave fluidity
Amount
Specific gravity
Type
Temperature
Orientation
Contaminated lead
Solderability
Pallet loading
Alignment of pallet
Operator Components Preheat
Solder
defects
Figure 16-23 Cause-
and-effect diagram for
the printed circuit
board flow solder
process.
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