Experience of the most popular lead-free conductiv

2022-08-21
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The rapid development of electronic technology has promoted the continuous development of SMT surface mount technology. Electronic components become more and more sophisticated; The pin spacing is getting smaller and smaller; The requirements for the mounting strength and reliability of components are higher and higher. At the same time, the public pays more attention to environmental protection, and the voice against the large-scale use of lead containing production technology is getting louder and louder. Using lead-free conductive glue to replace traditional lead-containing tin paste to complete component mounting is a new SMT technology produced under this background

offset printing is a key component of this technology. The so-called offset printing is to print colloidal materials to specific plane areas according to the specified requirements through the silk printing process, such as PCB pad. Thixotropy is a major characteristic of offset printing process in terms of the influence of process parameter disturbance on its process. In terms of offset printing mechanism, PCB plastic film blowing machine manufacturers continue to innovate and take on a new look, and enter a new stage of development. The balance of the interaction between the wet adsorption force of the pad, the printing glue viscosity and the offset surface tension makes some of the printing glue in the plate leakage hole be absorbed to the pad. Through the next offset printing trip, fill the plate leakage hole with printing glue and generate wet adsorption on the new pad foaming polyurethane, which is composed of isocyanate and polyol as the main raw materials and additives

although offset printing process and dispensing process have their similarities, they belong to two different production processes. Compared with the latter, offset printing process has some characteristics:

it can control the amount of printing glue very stably. For PCB boards with backing (PAD) spacing as small as 5-10 mils, the offset printing process can easily and stably control the thickness of the printing adhesive within the range of 2 ± 0.2 mils

offset printing of different sizes and shapes can be achieved on the same PCB through one printing stroke. Offset printing - one piece; The time required for PCB is only related to parameters such as PCB width and offset printing speed, but has nothing to do with the number of PCB backing (pads). The dispensing machine places the glue on the B board of the PC (pay attention to the protruding part of the machine) in order bit by bit. The time required for dispensing varies with the number of glue points. The more glue points, the longer the dispensing time

most customers using offset printing technology are often very experienced in solder paste printing technology. The process parameters of solder paste printing technology can be used as a reference point to determine the relevant process parameters of offset printing technology

the shrinkage rate is still large due to the early solidification of the gate. Next, how the printing process parameters affect the offset printing process is discussed

Compared with solder paste printing, the thickness of metal plate used in offset printing technology is relatively thicker (0.1-2mm); Considering that the glue does not have the characteristic of automatically converging to the PCB pad when solder paste reflow soldering, the size of the board leakage hole should also be smaller, but it is better not to be smaller than the component pin size. Too much glue will lead to a short circuit between the pins of components, especially when the mounter is difficult to achieve 100% perfect placement accuracy, the "short circuit" condition is particularly easy to occur. For PCB boards with small spacing chips, special attention should be paid to the problem of chip pin short circuit

the printing gap/scraper is different from solder paste printing. During offset printing, the printing gap of the machine is usually set to a small value (not zero!), In order to ensure that the peeling between the board and the PCB board occurs following the scraper printing process. The printing gap is usually related to the board size. If zero gap (contact) printing is used, a smaller separation speed (0.1-0.5mm/s) should be used. The hardness of the scraper is a sensitive process parameter. It is recommended to use a scraper or metal scraper with higher hardness, because the blade of the scraper with low hardness will "excavate" the offset in the leakage hole of the plate

When printing epoxy resin glue in the direction of

, it is recommended to use single direction printing to eliminate the possible dislocation caused by back and forth printing. The scraper and the flood blade work alternately. The scraper completes the offset printing process, and the brush returns the glue to the starting position of the printing process

printing pressure/printing speed the rheology of glue is better than that of solder paste. The offset printing speed can be relatively high, but it must not be as high as it can not make the glue roll at the front edge of the scraper blade. Generally speaking, the offset printing pressure is 0.1-1.0kg/cm. The offset pressure increases to just scrape the glue on the board surface

experience:

epoxy glue seems to bond to the scraper more easily than solder paste. In case of missing printing, check whether there is glue on the scraper and flood blade. The woman starts to offset the board, first fill the leakage hole of the board with printing glue. That is, the same PCB placed in the printing position is printed many times. Once the board leak is filled with printing glue, after each scraper completes a printing stroke, most of the printing glue in the board leak will be printed on the PCB. And ensure a very stable amount of printing glue. For offset printing, keeping the leakage hole of the board "stuck" by the printing adhesive is the content of the offset printing process, so there is no need to make a fuss about it

there is generally no need to clean the plate in the offset production process. If there is "smearing" on the back of the board, only the "smeared" area needs to be partially cleaned. And the detergent recommended by the printing adhesive supplier must be used

offset printing thickness largely depends on the inherent characteristics of the printing adhesive. Under the condition that other process parameters remain unchanged, different offset thicknesses will be obtained by using different characteristics of printing adhesives

when using offset printing technology, attention should also be paid to ensuring the compatibility of glue (including metallic silver), 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 IE "patch dislocation" within a certain range. However, in offset printing technology, the offset printing production process determines that engineers should not "expect" this "automatic correction" function. In other words, the offset printing process is more challenging for engineers

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