1. The SMT Dispensing Process
SMT dispensers are used for depositing primarily SMT adhesive and conductive epoxy materials used in electronics assembly. The material being dispensed is formulated to enable it to be dispensed through a variety of needles etc connected to a dedicated pumping technology.
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2. Dispensing Applications
Dispensing equipment is used for a variety of applications. These applications include the dispensing of SMT adhesives, which are used to glue SMT components to the PCB prior to wave soldering etc. Dispensing of solder paste is also a popular application for prototype builds etc as it eliminates the need for a printer and stencil.
Advanced packaging also often requires the use of dispensers to dispense conductive epoxys, and is used in some die attach applications.
Other applications for dipensing include underfill, dam and fill, glob top and conformal coating.
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3. Dispenser Machine Types
Dispensers come in many different sizes offering various levels of capability, from small desktop systems to large fully automatic inline machines. Most of these systems also offer a choice of pump technology to suit the application.
The dispenser you need will likely be determined by the required dispense speed. As dispensing is by its very nature a relatively slow process the DPH ( dots per hour)) figure will be important.
Dispense units/pumps are split into a number of types; time/ pressure, archimedean screw, positive displacement and jet varieties.
Each pump technology has its place dependant on the application and you are well advised to seek speciific application advice from the suppliers as pump choice can have a significant effect on your dispensing results.
Other available machine options include height sensing, substrate and material heating and automatic needle cleaning amongst others.
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4. Troubleshooting Dispensing Problems
Dispensing is a process which requires a good machine setup and material conditioning to achieve good results. However a number of common problems are described below:
Stringing..
There are a number of causes for what is known as stringing or tailing. The primarly cause is incorrect needle diametr/dot ratio, the dispensed dot should be 1.5 times the inner diameter of the needle, this will ensure sufficient surface tension to minimise stringing. Another cause of stringing whilst dispensing in incorrect Z height between movements, if it is too low it may trail the needle in the dipensed deposit.
Components Falling Off..
When wave soldering surface mount components, it is usual to glue the component to the board. Howver occasionally these glued components can fall off in the wave. The reasons for this are mainly due to 3 potential causes. Dirty PCB's not allowing glue to adhere to surface, old adhesive going off and losing adhesive strength and insufficiently cured adhesive.
This failure can also be caused by insufficient glue deposit on components with a high standoff.
Missing/Variable Dots..
Missing dots and glue dots of variable size are a serious dipensing problem, normally associated with poor pump parameter setup, warped boards or bad material. Therefore you must ensure that all pump settings including speed and pressure/time settings are correct and that your PCB or substrate is level to the needle (a needle with a stand off can be a solution to this). Also you must ensure that your material has not exceeded its floor life etc, this is especially important with conductive epoxy material.
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5. Expert Advice..
Please visit individual supplier websites for further detailed information.Browse manufacturers dispensing equipment here
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