The membrane switch is composed of a multi-layer film, which uses different material properties to play different roles and complete the function of the membrane switch on the electrical product. The circuit layer is the main functional layer of the membrane switch. It is an electronic circuit designed according to the functions required by the circuit product. The conductive silver paste or conductive carbon paste is printed on PVC, PC or PET, and then with the panel layer and the isolation layer. Synthetic membrane switch.

 

At the time of manufacturing a thin film switch circuit layer, screen printing silver paste is prone to the following problems:

 

1. Silver migration on PET tablets;

2. The membrane switch is in use, the silver wire is broken;

3. The rise in circuit resistance.

 

How to understand, prevent and control the above problems? Here is a description of the silver paste ED427, 478 and carbon paste 423 produced by Acheson Ink Co., which is dedicated to membrane switches, flexible circuits and resistance printing. I hope to give you some help for reference:

 

First, the reason and prevention of silver migration on PET tablets

1. Several factors that affect silver migration:

A. The distance between silk screen silver paste circuits (wires);

B. The potential between silk screen silver paste circuits (wires);

C. Existing dielectric;

D. Silver alloy pigment;

E. The integrity of the existing protective layer and its insulating protective layer.

 

2. Prevention and treatment of silver migration:

A. As much as possible, the gap outside the silver circuit (wire) is widened and the voltage (potential) is kept low.

B. An ED423 carbon paste with a thickness of >30 microns or ED452SS blue transparent insulating ink can be applied to the silver circuit.

C. In order to prevent the silver from accelerating migration in a humid environment, a vent hole should be added around the membrane switch during design.

D. In the case of silk screen insulation coating, the insulation coating should avoid dust and pinholes (refer to the sanitary conditions of the silk screen environment), some dust and debris fall into the wire circuit, under certain conditions, causing the electrical performance to decline.

E. Screen printing of ED427 or ED478 silver paste lines, if the insulation coating is insufficiently cured or excess solvent, it will reduce the insulation properties of the insulation layer, thus reducing the migration resistance of silver in humid air media.

 

3. The approach taken for this:

A. Drying after silk screen printing of ED427 or ED478 silver paste shall be carried out in the following three stages of curing:

A.2/3 time (specified temperature);

b. Allow 1-2 hours at room temperature;

c. Infrared drying takes 1/3 time (prescribed temperature). This allows the resistor to reach an optimum value.

B. Using a protective insulating coating, transparent green ink ED452SS, the UV light should be completely solid, that is, the wavelength is 180-420 nm, and the power is 80-120 W/cm. The UV speed is adjusted accurately, and the screen is 160-300 mesh/inch (64-120T/cm, depending on the situation).

 

Second, the silver layer break

1. Silver layer fractures are related to several factors:

A. The insulating ink layer is tightly connected and connected, so that excessive pressure enters the lower silver paste layer, as shown in the figure:

B. Insulating ink layer mode is not ideal (ie process design is not reasonable)

C. The surface of the polyester film is not wetted when screen printing

D. Silver paste is not used and cured

 

2. Prevention and resolution of silver layer fracture:

1Select Acheson Electroday

The 452SS insulating ink is matched to PET and ED427, ED478. Because the silver paste line on the membrane switch is between the two high pressure layers of PET and insulating ink, Electroday 452SS is an ideal cross-bonding insulation layer, which reduces the pressure on the silver paste line.

 

2 insulating ink silk screen printed in semicircular mode

3Dyne pen test: test the surface activity of PET film (the surface activity test after changing the batch number) (Sanhe International British Ketutai PET film can be used as a special film switch sheet).

4 Use the most suitable silver paste thickness (screen printing with 80-100T/CM)

5 After the silk paste is printed, it is first baked with infrared light (80-100 ° C X 30 seconds) and then 140 ° C > 30 seconds. Finally, 150 ° C for 30 minutes in a convection oven. This drying method features:

 

A. The solvent is volatilized from the bottom during drying;

B. Finally, fast curing achieves the best characteristics;

 

3. Precautions for using conductive silver and carbon paste:

1) In order to maintain the freshness and constant solid content of silver and carbon paste:

a. Stir thoroughly before use, so that the upper and lower layers of the whole tank of ink can fully meet the design ratio requirements. At the same time, the silver carbon paste has high viscosity and certain thixotropy, and after sufficient stirring, the silk screen adaptability is improved.

b. Each time the printing is completed, the remaining ink should be placed in a separate clean container (because the screen printing is contaminated), the next time you use the remaining ink, if necessary, add a new ink to it. print.

c. The container is sealed and stored under low temperature drying. At least 4 hours before use, remove it from the low temperature, let it return to room temperature, and stir it before printing. can. If it is true that the viscosity is changed by using the remaining silver and carbon paste, it is necessary to strictly measure the existing solid content when adding the flow agent, calculate the amount of solvent loss, and add it by a precision instrument.

 

2) Control the production process with square resistance (ohm/square/mil):

1 mil = 25 microns = 0.001 inches Measurement method: Calculation steps:

 

(1) L = length of the line under test W = width of the line T = thickness of the line to be tested

(2) L/W = number of squares of the line under test

(3) ohmmeter to measure the reading of the resistance at both ends of the L line (ohm)

(4) Use (3) / (2) = ohm value / per square block

(5) Use (4) / T = square resistance of the line, (ie the resistance of the line at 1 mil thickness, expressed in Ω / (25μ thickness).