Solder Paste Inspection Criteria

The SMART Group is starting a new project on PPM Monitoring which will be providing average PPM yield data per month free to the industry. I would like to get feedback or exchange criteria for paste printing as non exists with in IPC or similar organisations. Below I have outline the project and some criteria I have produced. Any comments, criteria exchange or suggestions on this and criteria used for CSP and BGA packages would be useful as I want to make sure that it reflect industry.

There are no easy methods for companies to compare their manufacturing yields with other similar businesses. This procedure is a necessity to understanding their own world class manufacturing status. A common question asked by many companies is how does my process compare with other companies in terms of yield? The information does not exist or is not easily available to small and medium volume companies.

It is proposed to create a method by which companies can compare their performance with other companies in similar market places using similar equipment. This would allow each manufacturing process used to build printed board assemblies to be compared. This would initially include screen printing, component placement, reflow and wave soldering and final test.

The most commonly used method of illustrating yield is Part Per Million (PPM) Defective. This provides a measure of the defect level against a process stage or a specific product compared against a known number of opportunities for defects to occur. There are IPC and IEC documents and procedures which do exist to monitor a process, which would be considered during the development phase of this project.

Although this project would be co-ordinated by the SMART Group the information would be available to every company in the industry free.

Although the methods and procedures do exist in a variety of sources, no co-ordinated effort has been made with small and medium volume assembly companies to assist and provide a reference on performance. With support for this project, process data collection can be simplified, a benchmark for each process can be defined and an ongoing reference provided to industry.

CLASSIFICATION OF QUALITY STANDARDS

During the assessment the following classifications, which are pictorially illustrated, should apply:

SATISFACTORY

This is a satisfactory condition which should be achieved and used as the standard for manufacture.

ACCEPTABLE

This condition represents the maximum acceptable departure from the "Satisfactory" condition. Examples within this limit of deviation will not require reworking. Individual clarification accompanies each example illustration. Consideration should be given to modification to the process.

UNACCEPTABLE

This applies to an unacceptable condition which should not be reworked without the cause of the fault being established. Rework may be possible after assessment of the fault and corrective action taken on the process.

SOLDER PASTE INSPECTION CRITERIA

SATISFACTORY

The solder paste deposit should be flat with no peaks and conform to the stencil aperture dimensions. It should be positioned centrally on the pad surface.

ACCEPTABLE

The solder paste deposit may be mis-registered up to 10% from the pad surface provided 90% of the paste deposit is present on each pad.

UNACCEPTABLE

The deposit will not be acceptable if the print deposit is incomplete or the print is mis-registered by more than 10% from the pad.

SOLDER PASTE INSPECTION CRITERIA

SATISFACTORY

The solder paste deposit should be flat with no peaks and conform to the stencil aperture dimensions. It should be positioned centrally on the pad surface.

ACCEPTABLE

The solder paste deposit may be mis-registered up to 10% from the pad surface provided 90% of the paste deposit is present on each pad.

UNACCEPTABLE

The deposit will not be acceptable if the print deposit is incomplete or the print is mis-registered by more 10% from the pad.

SOLDER PASTE INSPECTION CRITERIA

SATISFACTORY

The solder paste deposit should be flat with no peaks and conform to the stencil aperture dimensions. It should be positioned centrally on the pad surface.

ACCEPTABLE

The solder paste deposit may be mis-registered up to 10% from the pad surface provided 90% of the paste deposit is present on each pad.

UNACCEPTABLE

The deposit will not be acceptable if the print deposit is incomplete or the print is mis-registered by more than 10% from the pad.

SOLDER PASTE PRINTING DEFECTS

MISALIGNMENT

Incorrect stencil position Poor board alignment Poor printer repeatability

PASTE SMEARING

Movement of PCB after printing Insufficient stencil separation Poor racking before placement

UNEVEN PRINT DEPOSIT

Soft rubber squeegee blade Excessive squeegee pressure Uneven squeegee force

SOLDER PASTE PRINTING DEFECTS

SMUDGED PRINT

Poor manual handling Stencil contact after print Insufficient stencil separation

BOARD MOVEMENT

PCB sticking to stencil PCB moving after printing Insufficient stencil separation

DOG EARS

Paste drying out High pressure and rubber blade Incorrect stencil apertures

SOLDER PASTE PRINTING DEFECTS

STENCIL MISALIGNMENT

Incorrect manual alignment Paste clogging aperture Missing stencil aperture

INCOMPLETE PRINT

Paste drying out Incorrect squeegee stroke Excessive print speed

SMUDGED PRINT

Poor manual handling Stencil contact after printing PCB movement after printing

SOLDER PASTE PRINTING DEFECTS

PASTE WASHOFF RESIDUES

Incorrect paste washoff Incompatible cleaning agent Poor manual or automatic process

PASTE IN VIAS

Incorrect manual washoff Poor automatic washoff Stencil with via apertures

STENCIL SEEPAGE

Stencil not contacting pad Excessive squeegee pressure Incorrect under stencil cleaning

SOLDER PASTE PRINTING DEFECTS

PASTE SLUMP

High factory temperature Poor quality solder paste Low paste metal loading

STENCIL CLOGGING

Paste dried out in stencil Incorrect stencil aperture Incorrect printer setting

PASTE SCOOPING

High pressure with rubber blades Uneven tin lead on pads Paste drying out

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