PPM Monitoring in Assembly

All text Copyright Bob Willis, EPS

In today's manufacturing industry quality control of the Surface Mount process is all important. If the quality level and the control procedures are not correctly introduced or a misunderstanding exists over what is acceptable and rejectable any quality control programme will be ineffective.

The aim of this joint effort by the SMART Group and The Surface Mount Club is to provide a system by which engineering may effectively monitor a Surface Mount process. By providing a set of charts covering each part of the process and a procedure for collecting defect data it will be possible to compare the performance of different SM facilities. It is the intention to provide an industry average for different parts of the process so that any company can compare its own production performance.

It is the intention to provide a data recording chart for each process stage for a standard SM process, individual companies may then use all the charts to record data or only points that they feel are valuable.

It has long been realised that many engineers talk about the yield from a particular process and often compare this with other lines or companies. This can be very misleading unless the following points are considered:

Are all staff trained to a common quality standard?

Have all staff been trained to recognise/record defects correctly?

Do all staff understand the data collection forms?

How is corrective action monitored, the failure cause determined and documented and staff informed of the necessary process changes?

INTRODUCTION TO THE CHARTS  AND HOW TO COMPLETE THEM

The charts are divided into two levels:

	The first is for basic first level investigation and includes five or six general fault categories 

	The level two form includes the basic fault categories then extends the list and sub-divides the faults into both local and general faults. 

Full definitions of each of the fault categories are included with these instructions to make correct fault identification possible.

The number of fault categories may be increased as required by individual companies. For the purposes of process comparison the number of groups should be limited to those included on the two sheets. 

The charts in this manual cover the following inspection points:

	    Adhesive Dispensing
	    Paste Deposition
	    Component Placement
	    Wave Soldering
	    Reflow Soldering
	    Hand Soldering/Rework
	    Cleaning
	    In-Circuit Electrical Test

Blank inspection forms are also included in this manual to allow possible modification for data collection at other points in the process.

Each form requires the inspector/operator to fill in the following relevant information:

Process: This would be the name of the actual inspection point, e.g. component placement. An individual copy of the charts are already completed with the relevant process stages.

Sample Size: This would be determined by experience and depending on the complexity of the assembly. A further consideration would be the time available to perform the inspection task. It is suggested that a minimum of five samples should be used.

Machine: This would be the actual machine where the samples were taken recording the machine type or number.

Checking Frequency: The time at which the samples are to be taken, e.g. every 30 mins.

Operator: This would be the name of the inspector, or where the operator performs the inspection tasks, their name should be recorded.

Date: The date and time of the inspection record as the sheet allows for samples every 30 mins.

Board Type/No: The full board reference number should be recorded

Total Defects: The defects recorded against the different fault categories should be added together to give the total number of faults.

Baseline: This is a record of the possible total number of defects which could be recorded and will vary dependent on the process stage and the board reference number. The total number of possible defects are counted  and multiplied by the number of samples taken.

Parts Per Million: The defect level in Parts Per Million (PPM) is obtained by dividing the number of defects by the baseline figure and multiplying by 1,000,000 to give the defect level in PPM.

Process Defect Log: The PPM figures which have been calculated are recorded on the chart, a cross is placed against the number of PPM defects and a line is drawn from the last recorded reading to join the two sample points.

Action Line: A target line for the number of PPM defects is drawn on the form, this level is agreed with the quality and engineering department based on the performance of their process.

Reaction Log: Where the recorded PPM figures are over the action line the log must be completed with the relevant information.

	Number:

	Date:

	Time:

	Special Cause/Identified Defect:

	Initial:

	Corrective Action:

	Date:

	Time:

	Initial:

The process defect log number is entered, the number is the next in sequence on the existing form or from the previous copy of the form.

The date when the recorded defects crossed above the action line and the process defect was entered on to the log sheet.

The time when the recorded defects crossed above the action line and the process defect was entered on to the log sheet.

Record the the defect type and if possible the cause of the defect which has caused the action line to be crossed.

The initials of the person completing the form.

Indicate the corrective action to be taken to correct the process fault which has caused the action line to be crossed.

The date when the corrective action was completed and the PPM figure has dropped below the action line.

The time when the corrective action was completed and the PPM figure has dropped below the action line.

The initials of the person completing the corrective action for the current process defect log number.

All forms are to be retained by the production department. Where individual  process defect log actions have not been completed the individual sheets shall remain on the machine or at the inspection point. 

For further information on EPS services Email Bob Willis or visit his Internet Home Page.

Email: eps@bobwillis.co.uk
Home Page: http://www.bobwillis.co.uk