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4 Process Hardware

4.3 Stencil

Process Foundation DEK Micron-Series 12/2017 19

Damage

●

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

●

The metal bars between QFP apertures

can be very thin and can eventually break,

leading to bridging

Best Working Practices

●

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

and matching with production check sheets

●

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

feet, trolleys or objects placed against them

●

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

solvent and lint free clothes

●

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

……………………………………………………………………………………………………………

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

……………………………………………………………………………………………………………

4 Process Hardware

4.4 Print Material

20 Process Foundation DEK Micron-Series 12/2017

4.4 Print Material

There are many types of material that can be screen-printed and many varieties and different

manufactures of each type. Materials include solder paste, fluxes, ink, adhesive, silver epoxy and

encapsulation. Basically, any materials that exhibit the correct viscosity can be screen-printed. This

course cannot go into specific details about each material type but can offer general guidelines and

good working practices based on the following factors:

Safe Use

●

Avoid any activities that may transfer the material to your mouth (wash your hands before

eating, drinking etc.)

●

Avoid direct contact with your skin

●

Wear gloves

Quality

Affected by:

●

Environment

●

Age

●

Storage conditions

●

Contamination (i.e. IPA)

●

Handling (i.e. stirring, kneading, gathering up tramlines)

Material Loading

●

Start with enough material to ensure a rolling motion during the print stroke

– Too little: material will not fall from squeegee

– To much: material characteristics will change over time, reducing performance

●

Approximately 15mm diameter for solder paste

●

Add small quantities of material regularly.

Maintaining Print Performance

●

Many print materials – such as solder paste – decrease in viscosity (become more liquid)

when they are stirred, and during printing

●

Some increase in viscosity (get thicker) when stationary

●

If this kind of material has been inactive for some time, use the ‘knead’ function

●

Keep within the optimum operating range as indicated in the chart below

Maintaining print performance:

1. Viscosity

2. Print strokes/ min

3. To go from A-B (i.e. to reach working

viscosity) may take several print strokes

4. To go from C-D (i.e. from working to non-

working) may only take a few minutes of

paste inactivity

4 Process Hardware

4.4 Print Material

Process Foundation DEK Micron-Series 12/2017 21

Environment

●

Most print materials decrease in viscosity (become more liquid) when they get hotter (though

solder paste may dry out and eventually become thicker)

●

Most increase in viscosity (get thicker) when they get colder

●

Minimise temperature fluctuations (keep covers closed, use an environment control module

(ECM), locate machine away from sources of heat, direct sunlight, etc.)

●

Some materials are also sensitive to changes in humidity. Water-soluble pastes, for instance,

tend to absorb water if humidity is high, decreasing their viscosity and degrading their later

performance in the reflow oven

Temperature:

●

Affects paste viscosity

●

Keep comfortably cool (22 - 24°C typical)

Humidity:

●

Too low may dry out paste

●

Too high may cause water absorption, particularly in aqueous pastes

●

Keep to mid-range (40-50% typical)…

In this example a change of room temperature from 20°C to 30°C reduced the materials viscosity

by some 10 – 15%, resulting in poorer print definition.