Membrane switch screen printing silver paste common problem analysis

Membrane switches are composed of multiple layers of thin films. They utilize different material properties and play different roles to complete the function of membrane switches in electrical products. 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 screen printed on PVC, PC or PET, and then the panel layer and the isolation layer. Synthetic membrane switch.

In the current manufacture of thin film switch circuit layers, screen printed silver paste is prone to the following problems:

1. Silver migration on PET sheet;
2. Membrane switch in use, silver wire breakage;
3. The resistance of the circuit rises.

How to understand, prevent and control the above issues? Here will be a description of the silver paste ED427, 478 and carbon paste 423 produced by Acheson ink company in the United States dedicated to membrane switch, flexible circuit, resistance printing, and hope to give you some help for reference:

I. Causes and Prevention of Silver Migration on PET Sheets

1 . Several factors that affect silver migration:

A. The distance between screen printing silver paste circuits (wires);
B. The potential between screen printed silver paste circuits (wires);
C. Existing dielectrics;
D. Silver alloy pigments;
E. The integrity of the existing protective layer and its insulating protective layer.

2. Prevention and treatment of silver migration:

A. Widen the gap outside the silver circuit (wire) as much as possible, and keep the voltage (potential) low.
B. On the silver circuit, an ED423 carbon paste or ED452SS blue transparent insulating ink with a thickness of >30 μm can be applied.
C. In order to prevent the accelerated migration of silver in a humid environment, vent holes should be provided around the membrane switch.
D. In the case of silk screen insulation coating, the insulation coating should avoid dust and pinholes (referring to the hygienic condition of the screen printing environment), some dust debris falls into the conductor circuit, under certain conditions, causing the electrical properties to decline.
E. Silkscreened ED427 or ED478 silver paste lines, if the insulation layer due to insufficient curing or excessive solvent, will reduce the insulating layer insulation performance, thereby reducing the anti-migration of silver in moist air medium.

3 . The approach taken for this purpose:

A. After drying silkscreen ED427 or ED478 silver paste, the following three stages of curing should be used:
A.2/3 time (specified temperature);
b. At room temperature for 1-2 hours;
c. Infrared drying takes 1/3 of the time (specified temperature). This will allow the resistor to reach its optimum value.
B. The use of protective insulating coating, transparent green ink ED452SS, UV light should be complete, that is, the wavelength of 180-420nm, power 80-120W/cm. The over-UV speed is adjusted accurately. The mesh used is 160-300 mesh/inch (64-120T/cm, depending on the situation).

Second, the silver layer breaks

1. Silver fractures are related to the following factors:

A. Insulating ink layer cross-linked closely, so that excessive pressure into the lower silver layer, as shown:
B. Insulation ink layer pattern is not ideal (that is, the process design is not reasonable)
C. Polyester film screen printing surface does not wet
D. Use of silver paste and curing is not good

2. Silver layer fracture prevention and solution:

1 Use Acheson Electroday

452SS insulating ink matches 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, the use of Electroday 452SS is an ideal cross-coupling insulation layer, thereby reducing the pressure on the silver paste line.

2 Insulation ink screen printing in semicircle mode

3 Dyne pen detection: The surface activity of the PET sheet was tested (after the batch number was changed, the surface activity was detected). (Three-and-international British Ke Tuite PET film can be used as a dedicated film switch sheet).

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

After the silver paste is silk screened, it is first baked in infrared rays (80-100°C for 30 seconds), then 140°C for >30 seconds, and finally in a convection oven at 150°C for 30 minutes. This drying method features:

A. In the drying process, the solvent evaporates from the bottom;

B. Finally, rapid curing to achieve the best characteristics;

3. The use of conductive silver, carbon slurry matters needing attention:

1) In order to keep the fresh and always fixed solid content of silver and carbon slurry:

a. Thoroughly stir thoroughly before use so that the upper and lower layers of the entire ink can fully meet the design ratio requirements. At the same time, the silver carbon paste has a high viscosity and a certain thixotropy. After being fully stirred, the screen printing adaptability is improved.

b. Each time the printing is completed, the remaining ink should be kept in a separate clean container (because the screen printing has been contaminated), the next time the previous use of the remaining ink, if necessary, add a little new ink in it and then print.

c. The container is sealed and stored at low temperature and dry. Remove the cold from the cold at least 4 hours before use, allow it to return to room temperature, and then mix thoroughly before printing. can. If indeed the use of the remaining silver, carbon slurry to change the viscosity, adding flow agent, also need to strictly measure the existing solid content, calculate the amount of solvent loss, and use precision instrument weighing added.

2) Use square resistance (ohm/square/mil) to control the production process:

1 mil = 25 μm = 0.001 inch Measurement Method: Calculation Procedure:

(1) L = Length of the line under test W = Width of the line T = Thickness of the line under test

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

(3) Using an ohmmeter to measure the resistance at both ends of the L line (ohms)

(4) Use (3)/(2) = ohms/squares

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

Examples:

L = 3.00 ft T = 12.5 um/25 um = 0.5 mil W = 0.030 ft Therefore: L/W = 3.00/0.030 = 100 squares, because the end resistance of the L length line = 3 ohms, then 3/100 = 0.030 ohms/square box resistance = 0.030 ohms/0.5 mil = 0.0015 (that is, box resistance with a thickness of 1 mil)

Why use square resistance to calculate the performance of silver, carbon paste and other conductive inks? Because square resistance can explain: the optimal thickness of the ink and its control. Because of the number of ohms/squares/mil, the film thickness, resistance, adhesion, adhesion, and degree of cure can be known.

The best curing of the ink and its control, because the curing is not good, the resistance is high, the block resistance is also too large.

Detection of ink quality: Because the actual measured block resistance value, it should be close to the box resistance value of each batch of Accysen conductive silver and carbon paste. This requires the precipitation of the ink in the construction, the curing speed is in the best state, and the parameters of the block resistance provided by this method can be used to indicate that the quality of the ink has not changed.

Correctly grasp the various process conditions and parameters, using square resistance testing performance, will help reduce the consumption of ink.