Application of solder paste DEK offset printing technology
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The rapid development of electronic technology has promoted the continuous development of SMT surface mount technology. The electronic components are getting finer and finer; the stitch pitch is getting smaller and smaller; the requirements for component mounting strength and reliability are getting higher and higher. At the same time, the public pays more attention to environmental protection, and the call for a large number of lead-containing production processes is getting higher and higher.
The replacement of traditional lead-containing solder paste with lead-free conductive paste to complete component placement is a new SMT technology produced in this context. Offset printing is a critical component of the technology.
Offset printing for surface mount technology (SMT) example 1:
Single-sided PCB patching and mixing wave soldering process
PCBa surface offset printing, placing components, curing, placing the inserted components on the PCBb surface, wave soldering
Offset printing for surface mount technology (SMT) example 2:
Double-sided PCB full mount reflow soldering process
PCBa surface offset printing, placing components, curing, PCBb surface printing solder paste, placing components on PCBb surface, PCBb surface reflow soldering
Offset printing for surface mount technology (SMT) example 3:
Capsule sealing layer
Curing of the PCB offset cover that has been surface mounted
Offset printing process
The so-called offset printing is to print the gelatinous material to a specific flat area, such as a PCB board pad, according to the specified requirements through a screen printing process. Thixotropy is a major feature of the offset process in terms of the impact of process parameter perturbations on its process. In terms of the offset mechanism, the balance between the wet adsorption of the PCB pad, the adhesion of the adhesive, and the surface tension of the offset causes a portion of the ink in the stencil leak to be attracted to the pad. Through the next offset stroke, the stencil leak is filled with the adhesive and wet adsorption is applied to the new pad.
Although the offset printing process and the dispensing process have similarities, they are two different production processes. Compared with the latter, the offset printing process has such characteristics:
* Can control the amount of ink very stably. For PCBs with a pad (pad) pitch as small as 5-10 mils, the offset process can easily and very stably control the thickness of the bond to within 2 ± 0.2 mils.
*Offset printing of different sizes and shapes can be realized by one printing stroke on the same PCB board.
Offset printing
The time required for the PCB is only related to parameters such as PCB width and offset speed, regardless of the number of PCB backings (pads). The dispenser dispenses the glue on the PCB in sequence, and the time required to dispense varies with the number of dots. The more glue points, the longer it takes to dispense.
Most customers who use offset technology are often very experienced in solder paste printing technology. The process parameters related to the offset printing technology can be determined by using the process parameters of the solder paste printing technology.
Next, we discuss how the printing process parameters affect the offset process.
Stencil
Compared with solder paste printing, the thickness of the metal mesh used for offset printing technology is relatively thicker (0.1-2mm); considering that the glue does not have the automatic paste to the PCB when the solder paste is reflowed. The characteristics of the pad polycondensation, the size of the mesh leakage hole should also be smaller, but it is better not to be smaller than the component pin size. Excessive glue will cause short-circuits between component pins, especially when the placement machine is difficult to achieve 100% full patch accuracy. For PCB boards with small pitch chips, special attention should be paid to chip pin shorts.
Printing gap / scraper
Unlike solder paste printing, the printing gap of the machine is usually set to a small value (rather than zero!) during offset printing to ensure that the peeling between the stencil and the PCB follows the blade printing process. The print gap is usually related to the size of the screen. If zero gap (contact) printing is used, a smaller separation speed (0.1-0.5 mm/s) should be used. Scraper hardness is a relatively sensitive process parameter. It is recommended to use a high-hardness scraper or metal scraper because the low-hardness scraper blade will “hollow” the offset in the stencil leak.
Printing direction
When offsetting epoxy glue, it is recommended to use single-direction printing to eliminate misalignment that may be caused by reciprocating printing. The scraper and the foil blade work alternately, the scraper completes the offset stroke, and the file returns the glue to the starting position of the print.
Printing pressure / printing speed
The rheology of the glue is better than that of the solder paste. The offset speed can be relatively high, but it can't be as high as it can't make the glue roll on the leading edge of the blade. Generally, the offset pressure is from 0.1 to 1.0 Kg/cm. The offset pressure is increased to just wipe the glue on the surface of the stencil.
Experience
* Epoxy glue seems to stick to the scraper more easily than solder paste. If a missing print occurs, check that there is glue on the scraper and the flood blade.
To start the offset printing board, first fill the stencil leak with the printing glue. That is, the same PCB board placed in the printing position is printed multiple times. Once the stencil leak is filled with the ink, each time the squeegee completes a printing stroke, most of the stencil in the stencil will be printed on the PCB. And to ensure a very stable amount of printing. For offset printing, keeping the stencil leaks "sticked" by the printing ink itself is the content of the offset printing process, and it is not necessary to make a fuss about it.
* Generally, there is no need to clean the stencil during the offset printing process. If “smear” appears on the back of the stencil, only the “smudged” area should be partially cleaned. And you must use the cleaning agent recommended by the supplier of the adhesive.
*Offset thickness depends to a large extent on the inherent properties of the print. With the same process parameters, different offsets can be obtained with different characteristics.
*Adhesive printing technology should also pay attention to ensure the compatibility of (including metal silver) glue, BCP board and component metal pins under the conditions of production process temperature and humidity.
In the solder paste printing process, the reflow process will "automatically" correct the iE "patch misplacement" within a certain range. But in offset printing technology, the offset printing process determines that engineers should not "predict" this "automatic correction" function. In other words, the offset process is more challenging for engineers.