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4 Process Hardware 4.3 Stencil 18 Process Foundation DEK Micron-Series 12/2017 4.3 Stencil The stencil defines the volume and shape of the deposit, and the importance of a well-maintained stencil cannot be over-emphasise…

4 Process Hardware

4.2 Tooling

Process Foundation DEK Micron-Series 12/2017 17

Customised Tooling

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For one off applications

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Expensive

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Inflexible…

4.2.2 Correct Set Up

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Label tooling clearly to prevent mix-ups

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Tooling blocks should make use of the locating dowels on the rising table to ensure

repeatability of positioning

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To aid in the positioning of magnetic pins, create tooling maps or templates that can be laid on

the table surface with holes cut for the pins to go through

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Use tooling blocks designed with stencil support, or stencil support blocks if squeegee length

exceeds board length

Care and Maintenance

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Keep tooling and rising table scrupulously clean

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Inspect tooling regularly for wear or damage

4.2.3 Identifying Tooling Problems

With everything set correctly, the stencil surface should be clean at the end of the print stroke.

Therefore, any material residues left can provide clues to underlying problems.

These problems can be caused by:

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Damage or contamination of tooling or rising table

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Tooling pins positioned beneath underside components

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Tooling positioned directly under meath bar code labels, silkscreen mask or uneven pad finish

on the underside of the board

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Worn or damaged transport belts

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Underside components fouling on the surface of the tooling plate

Note: Adjusting the "Board Stop X" parameter slightly may allow the board to locate accurately

within the tooling block.

4 Process Hardware

4.3 Stencil

18 Process Foundation DEK Micron-Series 12/2017

4.3 Stencil

The stencil defines the volume and shape of the deposit, and the importance of a well-maintained

stencil cannot be over-emphasised. A good stencil will give high and predictable yields, will

minimise the need to clean and hence increase throughput, and reduce both downtime and

consumables use. A damaged or worn stencil may dramatically reduce yields and demand

constant cleaning and inspection, adding considerable costs to the manufacturing process. A poor

stencil, once identified, should be replaced immediately as the cost of replacement is negligible in

relation to lost production.

Good Stencil (Well designed/ good condition)

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Excellent Results

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Few Failures

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Predictable Yields

Bad Stencil (Poor design/ damaged)

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Lots of stencil wiping/ cleaning

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Lots of bridging and open circuits

Stencil Condition

Cleanliness – Screen Wash Machine

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Dirty apertures are one of the main causes of process defects

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Stencils should be cleaned thoroughly in an automatic washing

machine prior to storage and possibly even at shift change

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Remember that the under-screen cleaner supplied with the machine

is for regular, small cleaning only and cannot clean the top surface

Cleanliness – Manual

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For manual cleaning, use a lint-free cloth. Solvent-soaked cloth in

each hand, clean both sides simultaneously

Fiducial Condition

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The condition of the fiducials has a direct bearing on the alignment repeatability and accuracy

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Any misalignment can cause bridging and stencil smear and increase the frequency of under-

screen cleaning

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The stencil should be inspected regularly and damaged fiducials repaired

4 Process Hardware

4.3 Stencil

Process Foundation DEK Micron-Series 12/2017 19

Damage

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Most stencils are damaged before their po-

tential lifetime is reached, often as a result

of poor storage or handling

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A heavily used stencil may exhibit a "coin-

ing" effect where there is an outline of the

board pressed on the stencil surface

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Metal squeegees can cause surface dam-

age and wear

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The metal bars between QFP apertures

can be very thin and can eventually break,

leading to bridging

Best Working Practices

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Write or etch the stencil image number on the frame, or colour code it, to ease identification

and matching with production check sheets

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Store stencils vertically in a rack or cupboard so that there is minimal risk of damage from

feet, trolleys or objects placed against them

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In the absence of an automatic washing machine, a manual clean can be performed with

solvent and lint free clothes

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Maintain automatic washing machines regularly and remove any build up of waste material

Exercise

1. List as many ways that you can think of to extend the lifetime of your stencils

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2. List as many ways that you can think of to maintain the print quality from your stencils

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