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7 Product File and Machine Optimisation

7.3 Design a Paste Knead Strategy

50 Process Foundation DEK Micron-Series 12/2017

7.3 Design a Paste Knead Strategy

Why Knead?

A characteristic of some print materials (solder paste, for example) is that when a force is applied

to it from a squeegee, spatula, etc. it becomes thinner and more liquid – its viscosity decreases.

Conversely, when the force is removed, i.e. when production stops, the material’s viscosity will

increase. This is why some materials that have been inactive for a certain time may require a

‘knead’ in order to return its viscosity to the desired working condition. To maintain a good process

the material must be kept within its optimum operating range on the viscosity chart as

demonstrated below.

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

When to Knead?

There are three occasions when a knead operation is required:

Start of Production When fresh material is loaded onto the stencil it may require kneading to

condition it for printing, i.e. to bring it onto its working viscosity range.

During Production When fresh material is added to the main mass, it may need to be mixed

thoroughly and conditioned to ensure process stability.

After Downtime If the line is prone to periods of downtime, an automatic knead strategy

may help to recover the material to working viscosity and hence reduce

defective printed boards.

Multiple print boards

The process of "kneading" across a board may result in bridging or high volume deposits on that

board. Some customers will choose to automatically reject the kneaded board. Another option is to

use a board covered with mylar or similar clear plastic which can be re-used every time a knead is

required or at the initial set up of a new print run.

Programming Options

There are several knead parameters that can be used to help ensure that the material is kept in

good working condition.

7 Product File and Machine Optimisation

7.3 Design a Paste Knead Strategy

Process Foundation DEK Micron-Series 12/2017 51

Exercise

Discuss the knead parameters with your instructor and complete the table below.

Parameter Description Comments

A careful inspection should be

made of the print quality on a

board that has been used for

kneading. Multiple print strokes

can increase the risk of

excessive print deposits or

bridging.

7 Product File and Machine Optimisation

7.4 Provide a Definition of all Advanced Product File and Machine Set up Parameters

52 Process Foundation DEK Micron-Series 12/2017

7.4 Provide a Definition of all Advanced Product File and

Machine Set up Parameters

The DEK software contains many programmable parameters. All of these serve a useful purpose,

but maybe not all at the same time or for the same application. For instance: some parameters are

only used for printing low viscosity materials such as ink; some are for adhesive printing; others for

solder paste. It is important that the person designing a new product file is aware of which

parameters are relevant to their particular application.

Exercise

Review all the parameters in the product file and machine set up menus then answer the following

questions.

Q1 Which parameter controls the speed that the squeegee moves upwards off the stencil at the

end of a print stroke?

………………………………………………………………………………………………

Q2 Under what circumstances would this parameter be adjusted?

………………………………………………………………………………………………

Q3 Which parameter enables the option to perform an automatic paste dispense whilst a screen

clean is being performed?

………………………………………………………………………………………………

Q4 List a minimum of four other product file or machine set up parameters that can be adjusted

to achieve the following:

1. Increase Machine Throughput

……………………………………………………………………………………………

2. Decrease Cost of Production

……………………………………………………………………………………………

3. Increase Percentage of Effective Printed Boards

……………………………………………………………………………………………

Notes:

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