About DEK Offset Printing Technology (DEK Pumpprinting tm)
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 pin spacing 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 its voice against the large-scale use of lead-containing production processes is increasing. Cosmetic Bag,Makeup Pouch,Travel Makeup Bag,Drawstring Makeup Bag Ningbo Happiness Stationery Industrial & Trading Co Ltd , https://www.bagshappiness.com
The use of lead-free conductive glue instead of traditional lead-containing solder paste to complete component placement is a new SMT technology created under this background. Offset printing is an extremely critical component of the technology.
Example 1: Offset printing for surface mount technology (SMT):
Single-sided PCB placement and insertion wave soldering process
Offset printing on PCBa side-placing components-curing-placing interposer components on PCBb side-wave soldering
Example 2 of Offset Printing for Surface Mount Technology (SMT):
Double-sided PCB full mounting reflow soldering process
PCBa surface offset printing-placing components and curing and PCBb surface printing solder paste-placing components on PCBb side-PCBb side and then solder
Example 3 of Offset Printing for Surface Mount Technology (SMT):
Capsule sealing layer
Once the surface mount PCB offset printing layer is cured
Offset printing process
The so-called offset printing is to print the gel-like material to a specific plane area according to the prescribed requirements through the screen printing process, such as the PCB board pad. In terms of the influence of process parameter disturbance on its process, thixotropy is a major characteristic of offset printing process. As far as the offset printing mechanism is concerned, the balance between the wet adsorption force of the PCB board pads, the adhesiveness of the printing paste and the surface tension of the offset printing makes part of the printing paste in the screen holes leaked into the pad. Through the next offset printing stroke, the screen leaks are filled with printed glue and a wet adsorption force is generated on the new pad.
Although the offset printing process is similar to the dispensing process, it belongs to two different production processes. Compared with the latter, the offset printing process has these characteristics:
* Can control the amount of glue very stably. For PCBs with a spacing of underlays (pads) as small as 5-10 mils, the offset printing process can easily and very stably control the thickness of the printed glue within the range of 2 ± 0.2 mils.
* Offset printing of different sizes and different shapes can be realized by one printing stroke on the same PCB board.
Offset-block
The time required for the PCB board is only related to the parameters such as the width of the PCB board and the speed of the offset printing, and not to the number of PCB substrates (pads). The dispenser is to place glue on the PCB one by one, and the time required for dispensing varies with the number of glue points. The more glue points, the longer it takes to dispense.
Most customers who use offset printing technology are often very experienced in solder paste printing technology. The process parameters of the offset printing technology can be determined by using the process parameters of the solder paste printing technology as a reference point.
Next, we discussed how the printing process parameters affect the offset printing process.
Stencil
Relative to solder paste printing, the thickness of the metal screen used for offset printing is relatively thicker (0.1-2mm); considering that the glue does not have the automatic paste to the PCB board when the solder paste is reflowed Due to the shrinkage characteristics of the pad, the size of the stencil leakage hole should also be smaller, but it is better not to be smaller than the pin size of the component. Too much glue will cause a short circuit between component pins, especially when the placement machine is difficult to achieve 100% complete placement accuracy. For PCB boards with small-pitch chips, special attention should be paid to the short circuit of chip pins.
Printing gap / squeegee
Unlike solder paste printing, the printing gap of the machine during offset printing is usually set to a small value (rather than zero!) To ensure that the peeling between the stencil and the PCB board follows the squeegee printing process. The printing gap is usually related to the screen size. If zero-gap (contact) printing is used, a smaller separation speed (0.1-0.5 mm / s) should be used. Squeegee hardness is a relatively sensitive process parameter, it is recommended to use a higher hardness scraper or metal scraper, because the low hardness scraper blade will "hollow" the offset printing in the screen hole.
Printing direction
When printing epoxy resin glue, it is recommended to use unidirectional printing to eliminate the possible misalignment caused by back and forth printing. The squeegee and flood blade work alternately, the squeegee completes the offset printing stroke, and the squeegee sweeps the glue back to the starting position of the printing process.
Printing pressure / printing speed
The rheology of glue is better than solder paste. The offset printing speed can be relatively high, but it must not be too high to make the glue roll on the front edge of the blade. Generally speaking, the offset printing pressure is 0.1-1.0Kg / cm. The pressure of the offset printing is increased to just scrape off the glue on the surface of the screen.
Talk of experience
* Epoxy glue seems to stick to the spatula more easily than solder paste. If missed printing occurs, check whether glue is adhered to the scraper and flood blade. To start offset printing, fill the screen holes with printing adhesive. That is, the same PCB board placed in the printing position is printed multiple times. Once the screen leaks are filled with printing glue, each scraper will complete a printing stroke in the future, and most of the printing paste in the screen leaks will be printed on the PCB. And to ensure a very stable amount of glue. For offset printing, keeping the screen holes "sticked" by the printing glue itself is the content of the offset printing process, and there is no need to make any fuss about it.
* It is generally not necessary to clean the screen during the offset printing process. If "smearing" appears on the back of the screen, only the areas that are "smeared" need to be cleaned locally. And must use the cleaning agent recommended by the printing ink supplier.
* The thickness of offset printing depends largely on the inherent characteristics of the offset printing. In the case of other process parameters unchanged, the use of different characteristics of offset will get different offset thickness.
* Adopting offset printing technology should also pay attention to ensure the compatibility of (metallic silver) glue, BCP board and component metal pins under the production process temperature and humidity conditions.
In the solder paste printing process, the reflow process will "automatically" correct the "misplacement of the patch" within a certain range. However, in offset printing technology, the offset printing process determines that engineers should not "luxury" this "automatic correction" function. In other words, the offset printing process is more challenging for engineers.