Lead-free conductive offset printing technology experience

The rapid development of electronic technology has promoted the continuous development of SMT surface mount technology. The more and more fine the electronic components are, the smaller the pitch of the pins, and the higher and higher the mounting strength and reliability of components. At the same time, the public is paying more attention to environmental protection and the increasing opposition to the use of lead-containing production processes is increasing. The use of lead-free conductive glue instead of traditional lead-based solder paste to complete component placement is a new SMT technology that has emerged in this context.

Offset printing is an extremely critical part of the technology. The so-called offset printing is through the screen printing process to print the colloidal material according to the specified requirements to a specific planar area, such as a PCB board pad. Thixotropy is a major feature of the offset printing process in terms of the influence of process parameter disturbances on its process. As far as the mechanism of offset printing is concerned, the balance between the wet adsorption force of the PCB pad, the adhesiveness of the printed pad, and the surface tension of the offset is such that a part of the printing paste in the drain hole of the stencil is absorbed onto the pad. After the next offset stroke, the screen drain hole is filled with the sealant and wet adhesion force is generated on 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 features as:

It can control the amount of glue very stably. For PCB boards with substrate (pad) spacing as small as 5 to 10 mils, the offset process can easily and very steadily control the thickness of the paste within 2 ± 0.2 mils.

It is possible to realize different sizes and different shapes of offset printing on one PCB by one printing stroke. Offset—Blocks; PCB board time is related only to PCB board width and offset speed, and is independent of the number of PCB board backings (pads). Dispensers place glue on the PCB little by little. The time required for dispensing varies with the number of spots. 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 it discusses how the printing process parameters affect the offset printing process .

For stencil printing relative to solder paste printing, the thickness of the metal stencil used for offset printing is relatively thick (0.1-2mm); considering that the glue does not have the solder paste automatically reflowed PCB board pad features, the size of the screen drain hole should also be smaller, but it is best not to be less than the component pin size. Too much glue will cause a short circuit between components, 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 shorting.

The print gap/squeegee is different from solder paste printing, where the machine's print gap is typically set to a small value (rather than zero!) to ensure that the peel between the screen and the PCB is followed by a squeegee printing process. The print 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 harder scraper or metal scraper because the low hardness scraper blade will “knock out” the offset in the leaky hole of the screen.

In the case of offset epoxy resin glue, single-direction printing is recommended to eliminate possible misalignment caused by back and forth printing. The squeegee blade and the flood blade alternately work. The squeegee completes the offset stroke and the squeegee returns the glue to the initial position of the printing process.

Printing pressure/printing speed The rheology of glue is better than solder paste. Offset speeds can be relatively high, but must not be as high as they do not allow glue to roll on the leading edge of the blade. In general, the offset pressure is 0.1-1.0 Kg/cm. The offset pressure increases to just scratch the glue on the surface of the screen.

Experience talk:

Epoxy glue seems to bond more easily to scrapers than solder paste. If bleed occurs, check the squeegee and the flood blade for glue. The woman started to offset the board and filled it with the glue. That is, printing the same PCB board placed in the printing position multiple times. Once the stencil leak is filled with the print glue, each time a squeegee finishes a printing stroke, most of the print in the stencil leak will be printed on the PCB. And to ensure a very stable amount of rubber. For offset printing, keeping the stencil's leaky hole “printed” by the adhesive itself is the content of the offset printing process, and there is no need to be overwhelmed by it.

It is generally not necessary to clean the screen during the offset printing process. If “smearing” appears on the back of the stencil, only the “smeared” area needs to be partially cleaned. And must use the cleaning agent recommended by the glue supplier.

Offset thickness depends to a large extent on the inherent properties of the adhesive. With other process parameters unchanged, the use of different characteristics of the printing ink will obtain different offset thicknesses.

The use of offset printing technology should also ensure that (including metal silver) glue, BCP board and component metal pins in the production process temperature and humidity compatibility. In the solder paste printing process, the reflow process will "automatically" correct the 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, offset printing is more challenging for engineers.

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