Central Midori's Technology Blog

Designing a Water-Proofing (IP67) Membrane Keypad

Front-450064-edited.jpg

(Top View)

Digital Dispensing: 7 key parameters to control

1.  Little history of component assembly
I am old enough to remember the time when components had long leads and PCB had holes other than these tiny via holes. It was a time when components were mounted on a bare PCB by inserting the component leads into holes that were drilled on the PCB and soldered to pads situated on the opposite side.

In the 1980’s SMT (Surface Mounted Technology) became very popular and eventually replaced the very reliable through hole soldering technique. SMT components either had smaller leads, or no leads at all which allow them to be smaller. Simultaneously by eliminating component holes in the bare PCB, designers were given more real estate and boards became smaller and high density.

2.  From high temperature solder to low temperature conductive epoxy

The use of high temperature resistant bare board substrate (i.e. very expensive) is more driven by the soldering process than by the application conditions. In both wave soldering and oven reflow processes the temperature rises above 200degC.

In Flexible Printed Electronic, process engineers have replaced the high temperature solder with low temperature conductive epoxy which allowed low temperature material as PET (Mylar) to be used as alternative to expensive Polyimide (Kapton).

A decade ago CMI has developed a unique low temp SMT component assembly process composed of 6 phases.

  1. Chip bonder dispensing
  2. Conductive epoxy dispensing
  3. Pick and place
  4. Thermal curing
  5. Encapsulation
  6. Thermal curing

2017-01-04-T164358353.jpg

2017-01-04-T165717634.jpg

After much reflection and considering the medium-volume-high-mix market it was finally decided to start with dispensing and not screen-printing for the supply of the conductive epoxy and the chip bonder.

Pick-and-place is done using a Juki machine and thermal curing is achieved at 65degC.

Mechanical bonding is enhanced by the subsequent “encapsulation” process and the final thermal curing at 65degC.

In summary, the chip bonder holds the components firmly while conductive epoxy ensures good electrical conductivity between the components and the printed circuit. The encapsulant is here to enhance the mechanical bonding and provide a weathering protection.

3.  Dispensing main parameters

Dispensing is at the centre of the assembly process and CMI has identified 7 parameters to be tightly controlled:

  •  Dispensing time
  • Syringe pressure
  • Retracting height
  • Retracting speed
  • Needle clearance distance
  • Material viscosity
  • Substrate flatness

dispensingstructure.png

The dispensing time and the syringe pressure will together dictate the volume of material to be dispensed.

One of the most undesirable effect is called “tailing”. It is the result of height and speed of the retract phase but also the needle clearance distance.

Finally, the volume consistency will be greatly affected by the material viscosity and substrate flatness.

 4.  Conclusion

Over the years automated programmable dispensing has proven to be a reliable process. Good process control is guaranteeing volume consistency and repeatability.

Without the need for tooling, digital dispensing allows CMI to respond faster to customer demand and to accommodate smaller quantities without increasing NRE (Non-Refundable Expenses).

Digital Dispensing: 7 key parameters to control

1.  Little history of component assembly

I am old enough to remember the time when components had long leads and PCB had holes other than these tiny via holes. It was a time when components were mounted on a bare PCB by inserting the component leads into holes that were drilled on the PCB and soldered to pads situated on the opposite side.

LOPEC (Large-area, Organic & Printed Electronics Convention)

Central Midori will be presenting at LOPEC 2017 in Munich, Germany on March 28th, 2017.LOPEC (Large-area, Organic & Printed Electronics Convention) is the leading trade fair and the most important congress for the printed electronics industry. The conference takes place at Messe München convention Centre and brings together technology and business Pascal Delloue talk will take place on Tuesday March 28th at 16:00 Room 13b and the title is: From Adding to Creating Value.

1