1. The Screen Printing Process
The SMT screen printer printing process is a simple operation in theory, with solder paste squeegeed through a stencil onto the PCB pads.
However this process accounts for over 70% of all process faults on an SMT line.
To ensure quality results a number of steps must be completed accurately. Firstly the PCB must be supported properly and evenly either by tooling pins or dedicated tooling solutions, we must then ensure that the PCB is aligned excatly with the stencil, Once this is achieved the print stroke must be controlled for speed and pressure to get the best results, and finally it is important that you seperate the PCB from the stencil in a controlleed manner to get the best aperture release. For continued success the stencil must be kept clean either by manual cleaning or by the regular use of an under stencil wiper if fitted to your printer.
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2. Screen Printing Applications
Screen printing when applied to electronics manufacturing serves various applications. Primarily it is used for solder paste printing as the first stage in PCB assembly prior to component placement. However screen printers are used for a few other applications including, adhesive printing ( both conductive and SMT), special thick film deposition and even advanced semiconductor applications such as wafer bumping and ball placement.
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3. Screen Printer Machine Types
Screen Printers come in many different sizes offering various levels of cost and capability, from small manually operated desktop systems to large fully automatic inline printers with post print inspection and SPC.
The printer you require will depend on a number of factors such as volume requirements, component types (small fine pitch devices demand higher accuracy), whether you wish the printer to be inline or standalone and if you need post print inspection.
Small manual printers tend to have a smaller maximum PCB size, have a manual alignment system where you must align the stencil to the PCB by hand and have very little control over the pressures applied during print.
Semi-Automatic screen printers may have a larger PCB capability, probably maintaining a manual alignment system, but often have an automatic print sequence where speed and pressure are controlled,
Automatic screen printers are just that, they are inline systems which load, align and print your product to very high accuracy and repeatability. These types of screen printers are necessary for the consistent printing of products containing fine pitch devices.
Automatic printers often make use of enclosed head technology to replace the standard squeegees and 2d post print inspection to ensure quality.
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4. Troubleshooting Screen Printing Problems
As screen printing is responsible for the majority of yield problems in SMT production, it is imperative that you underdstand the principles of successful printing and that you can identify potential process faults quickly. Below are few examples of the issues you may face:
Insufficient/Excess Solder..
Insufficient or Excess solder paste on the PCB is a problem which is normally easily identified. From such obvious things as insufficient solder paste on the stencil to badly setup board tooling causing non contact areas on the stencil.
Adjustment of the squeegee pressures is often all that is required, with pressure just enough to clear the stencil of paste during the print stroke.
Bridging..
Bridging occurs for a number of reasons, not always associated with the screen printer so you must identify where the bridge is occuring in the process and that it is not as a result of poor placement or reflow issues.
Bridging resulting from the printer is usually the result of excessive squeegee pressures, inadequate under stencil wipe or as a byproduct of uneven tooling/board support ( usually with double sided PCB bottom components fouling on tooling nest).
Dog Ears/Peaking..
Dog Ears or Peaking is the term which refers to the edges of a paste deposit being higher than the body. This phenomenun is usually caused by incorrectly set PCB to stencil heights and poor seperation settings. Driving the PCB to high into the stencil results in a trampolining effect which causes paste to be ripped from the PCB and remain in the stencil aperture meaning ongoing and accumlative issues with print quality.
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5. Expert Advice..
Please visit individual supplier websites for further detailed information.Browse manufacturers screen printers here
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