PCB News - Lead-free conductive glue printing leads the new trend of SMT technology

The rapid development of electronic technology has promoted the continuous development of smt surface mount technology. Electronic components are becoming more and more refined; 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 is paying more attention to environmental protection, and the voice of opposing the large-scale use of lead-containing production processes is increasing. Using lead-free conductive glue instead of traditional lead-containing solder paste to complete component placement is a new smt technology produced under this background.

Offset printing is an extremely critical part of this technology. The so-called offset printing is to print glue-like materials on specific flat areas, such as PCBs, according to the specified requirements through the screen printing process. As far as the influence of process parameter disturbance on its process is concerned, thixotropy is a major characteristic of the offset printing process. As far as the offset printing mechanism is concerned, the balance of the interaction between the wet adsorption force of the PCB board pad, the adhesiveness of the printing and the surface tension of the offset printing makes the part of the printing glue in the hole of the stencil to be sucked onto the pad. Through the next offset printing stroke, the stencil hole is filled with the printing glue and the wet adsorption force is generated on the new pad.

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

can control the amount of glue printed very stably. For PCB boards with a substrate (pad) pitch as small as 5-10 mils, the offset printing process can easily and very stably control the thickness of the printed rubber in the range of 2±0.2 mils.

It is possible to realize offset printing of different sizes and shapes on the same PCB board through one printing stroke. Offset printing-one piece; the time required for the PCB board is only related to the width of the PCB board and the offset printing speed and other parameters, and has nothing to do with the number of PCB substrates (pads). The glue dispenser puts the glue on the PCB in sequence, point by point, and the time required for dispensing varies with the number of glue points. The more glue dots, the longer it takes to dispense glue.

Most customers who use offset printing technology are often very experienced in solder paste printing technology. The process parameters related to the offset printing technology can be determined with the process parameters of the solder paste printing technology as a reference point.

Next, I discussed how the printing process parameters affect the offset printing process.

Compared with solder paste printing, the thickness of the metal stencil used in offset printing technology is relatively thicker (0.1-2mm); considering that the glue does not have the automatic orientation of solder paste during reflow soldering Due to the shrinkage of PCB pads, the size of the holes on the stencil 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 the component pins, (smtsh.cn/target=_blank class=infotextkey>Shanghai smt), especially when the placement machine is difficult to achieve 100% full placement accuracy, the "short circuit" situation is particularly likely to occur. For PCBs with small-pitch chips, special attention should be paid to the short-circuit of the 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 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 (0.1-0.5mm/s) should be used. Squeegee hardness is a more sensitive process parameter. It is recommended to use a higher hardness squeegee or metal squeegee, because the blade of a low-hardness squeegee will "knock out" the offset printing in the leakage holes of the stencil.

Direction When offset printing epoxy resin glue, it is recommended to use unidirectional printing to eliminate possible misalignment caused by back and forth printing. The squeegee and the flood blade work alternately, the squeegee completes the offset printing stroke, and the squeegee blows 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 high enough to make the glue roll at the leading edge of the scraper blade. Generally speaking, the offset printing pressure is 0.1-1.0Kg/cm. The offset printing pressure is increased to just scrape off the glue spread on the surface of the screen.
Empirical talk

Epoxy glue seems to be easier to stick to the squeegee than solder paste. If a missing print occurs, check whether there is glue on the scraper and flood blade. The woman started to offset the board, and first filled the holes in the stencil with printing glue. That is, the same PCB board placed in the printing position is printed multiple times. Once the leakage holes of the stencil are filled with the printing glue, each time the squeegee completes a printing stroke, most of the printing glue in the leakage holes of the stencil will be printed on the PCB board. And to ensure a very stable printing volume. For offset printing, keeping the stencil holes "sticked" by the printing glue is itself the content of the offset printing process, and there is no need to make a fuss about it.

Generally, there is no need to clean the screen during the offset printing process. If there is "smearing" on the back of the screen, you only need to clean the "smeared" area locally. And must use the cleaning agent recommended by the printing rubber supplier.

The thickness of the offset printing depends to a large extent on the inherent characteristics of the printing rubber. In the case of other process parameters unchanged, the use of different characteristics of the printing glue will get different offset printing thickness.

The use of offset printing technology should also pay attention to ensure the compatibility of the glue (including metallic silver), the BCP board and the component metal pins under the production process temperature and humidity conditions. In the solder paste printing process, the reflow process "automatically" corrects iE" patch misalignment" within a certain range. But 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.

The above is the introduction of lead-free conductive glue printing to lead the new trend of SMT technology. Ipcb also provides PCB manufacturers and PCB manufacturing technology.