Basic Introduction of SMT

                             Basic introduction of SMT

1.Basic concept   

 

Surface assembly technology, known in English as “SURFACE MOUNT TECHNOLOGY” referred to as SMT, it is the surface mount components paste, soldered to the printed circuit board pads coated with solder paste, and then the surface mount components accurately placed on the pads coated with solder paste, through the heating of printed circuit board until the Solder paste melting, cooling will realize the connection between the components and the printed circuit.

 

 

 

 

2. Advantages of surface assembly technology:

 

(1) high assembly density, the use of SMT relatively speaking, can make the electronic product size reduction of 60%, 75% weight reduction

 

(2) the reliability of the cream, the general rate of defective solder joints is less than 10 parts per million, than through-hole components wave soldering technology is an order of magnitude lower.

 

(3) good high-frequency characteristics

 

4）Cost reduction

 

(5) to facilitate automated production.

 

 

 

3. Disadvantages of surface assembly technology:

 

1) components on the nominal value can not be seen, maintenance work is difficult

 

2) maintenance and replacement of the device is difficult, and need special tools

 

3) poor consistency of coefficient of thermal expansion (CTE) between components and printed boards. With the special hand in hand with the disassembly equipment and the emergence of new low coefficient of expansion of the printed board, they have no longer become an obstacle to the further development of SMT.

 

 

 

4. Surface assembly process:

 

SMT process has two types of the most basic process flow, one for the solder paste reflow process, the other is the patch - wave soldering process. In actual production, should be based on the type of components and production equipment used as well as product requirements to choose a different process, now the basic process flow is shown below:

 

                     SMT flow

1) Paste - reflow soldering process, the process is characterized by simple, fast, conducive to the reduction of product size.

 

 

 

2) Patch - wave soldering process, the process is characterized by the use of double-sided board space, the volume of electronic products can be further reduced, and still use the through-hole components, inexpensive, but the equipment requirements increase, the wave soldering process defects are more difficult to achieve high-density assembly.

 

 

 

3) mixed installation, the process is characterized by the full use of PCB board space on both sides, is one of the methods to achieve the minimization of the installation area, and still retains the characteristics of through-hole components of low price.

 

 

 

4) Both sides are using solder paste - reflow soldering process, the characteristics of the process can make full use of the PCB space, and to achieve the minimization of the installation area, the process control is complex, stringent requirements, commonly used in intensive or ultra-small electrical products, cell phones is one of the typical products.

 

 

 

We know that, in terms of new materials, solder paste and glue are thixotropic fluid, they cause defects accounted for 60% of the total defects of the SMT, training to master the knowledge of these materials in order to ensure the quality of the SMT SMT also involves a variety of mounting process, such as printing process, dispensing process, paste put process, curing process, as long as any one of these process parameters drift, will lead to the generation of defective products, SMT process SMT process personnel must have a wealth of process knowledge, at any time to monitor the process status, predict the development trend.

 

 

  Solder paste and printing technology   

 

   1. Solder paste   

 

Is the solder powder and has the function of flux paste flux mixed into a paste, usually solder powder accounted for about 90%, the rest is the chemical composition. It is a complex material system, the manufacture of solder paste involves fluid dynamics, metal smelting, organic chemistry and physics and other comprehensive knowledge

 

 

 

(1) for solder paste a class of fluids, due to the long chain structure of macromolecules and entanglement, or the presence of thixotropic agents, their flow behavior is far more complex than the low molecular fluids, such fluids are subjected to external forces, shear and shear rate is no longer proportional to the viscosity of the liquid is no longer a constant, the flow behavior is not subject to the rheological equations, and therefore in the engineering of this type of fluids known as non and tonic fluids.

 

 

 

(2) Solder paste outside the increase in force, the viscosity of the paste decreased rapidly, but fell to a certain level and then began to stabilize. That is, the solder paste in printing, by the scraper's thrust, its viscosity decreases, when it reaches the template window, the viscosity reaches the lowest, so it can be smoothly through the window to sink to the PCB pads, with the cessation of the external force, the viscosity of the solder paste and the speed of the solder paste to come back.

 

 

 

(3) solder paste is composed of solder ball powder and paste flux, solder ball powder particle size is generally controlled in 25UM-45UM, too coarse powder will lead to solder paste adhesion performance deterioration, and in the paste flux, usually contains a certain amount of rosin or other resins, it is to increase the viscosity, and the second is to prevent the welding process into a film of the solder in the second oceanization.

 

 

 

(4) viscosity and solder paste coating method: one is a metal stencil printing, the other is through the dispensing machine drop coating, these two types of methods require different viscosity of the solder paste, the viscosity range is shown in the table below:

 

 

 

(5) The printing performance of solder paste:

 

SMT large-scale production, first of all, the requirements of the solder paste can be smooth, non-stop through the solder paste leakage board or distributor to the PCB, if the printing performance of the solder paste is not good, it will block the leakage board of the eyelets, resulting in the production can not be carried out normally, the reason is that the lack of a flux in the solder paste or insufficient dosage caused by the alloy powder is not the shape of the poor, the distribution of particle size does not meet the requirements can also cause a decline in the performance of the printing.

 

 

 

(6) the adhesion of the solder paste: solder paste after printing placed for a period of time (8H) is still able to maintain sufficient viscosity is necessary.

 

 

 

(7) the collapse of the solder paste:

 

Is to describe the solder paste printed on the PCB and after a certain paste temperature is still a good shape of a term, this phenomenon often leads to reflow after the emergence of “bridge link, flying beads”.

 

 

2. Solder paste printing technology:

 

Solder paste printing is the first process in the SMT, solder paste printing involves three basic elements ----- solder paste, templates and printers, a reasonable combination of the three, the quality of the paste to achieve the quantitative distribution of solder paste is very important. Solder paste has been said before, is now the main description of the module and the printing machine.

 

 

 

1) metal template (stencils) manufacturing methods:

 

A. Chemical corrosion method, due to the existence of side corrosion, so the window wall finish is not enough for stainless steel materials, the effect is poor, so the effect of leakage printing is also poor.

 

B. Laser cutting method, it is the use of microcomputer-controlled CO2 or YAG laser generator, like light painting directly on the metal template cutting window.

 

C. Electroforming method: electroforming method of manufacturing templates is obviously expensive, suitable for use in fine-pitch device welding products.

 

 

 

2) Printing machine:

 

Description of fully automatic printing machine, usually with optical alignment system, through the PCB and template on the recognition of the alignment mark, the realization of the mold reverse window and PCB pads automatically aligned, printing machine accuracy of 0.01MM, but the printing machine of a variety of process parameters, such as squeegee speed, squeegee pressure, demolding speed, the template and the gap between the PCB board still need to be manually set.

 

 

 

(3) factors affecting the printing effect:

 

A. The smoothness of the template window and the diameter to depth ratio.

 

B. The effect of thixotropic properties of solder paste.

 

 

 

4) Solder paste printing process:

 

A. Solder paste preparation: solder paste should be placed in the refrigerator cold (between 0-10 degrees). When used, removed from the refrigerator at room temperature (4 hours) and then open the lid, the conditions of the factory through the machine stirring, about 0.5-1H can be used, it should be noted that the viscosity of the solder paste, the particle size is consistent with the current product requirements. (Viscometer should be used for measurement)

 

 

 

B. Installation and calibration of the stencil, semi-automatic can be through the CCD to help align.

 

C. Printing solder paste: the initial amount is not too much, pay attention to the quality of the environment: no wind, clean, temperature (23 = 3) degrees, relative humidity less than 70%.

 

D. Completion / cleaning templates.

 

 

 

(5) the adjustment and impact of the printing machine process parameters:

 

A. Squeegee speed: generally in the 12-40mm / s

 

B. Squeegee pressure: generally in 0.5KG/25MM

 

C. Squeegee width: the length of the PCB plus 50MM or so for the best

 

D. Printing gap: generally controlled in 0-0.07MM

 

E. Separation speed:

 

F.Squeegee shape and production materials .

 

 

 

(6) solder paste printing defects, causes and countermeasures:

 

A. Misalignment of solder paste graphics:

 

Causes: improper alignment of the steel plate and pad offset; printing machine printing accuracy is not enough

 

Harm: easy to cause bridge connection

 

Countermeasures: adjust the position of the steel plate, adjust the printing machine

 

 

 

B. Solder paste graphics pulled sharp, there is a depression:

 

Causes: excessive squeegee pressure, rubber squeegee hardness is not enough, the window is very large

 

Hazard: the amount of solder is not enough, easy to appear virtual welding, solder joint strength is not enough.

 

Countermeasures: adjust the printing pressure, change the metal scraper to improve the template window design.

 

 

 

C. Too much solder paste:

 

Causes: template window size is too large, PCB template gap is too large

 

Hazard: easy to cause bridge connection

 

Countermeasures: check the size of the template window, adjust the printing parameters, especially the PCB template clearance

 

 

 

D. Uneven graphics, broken points

 

Causes: template window wall luminosity is not good, the printing plate times, failed to wipe off the paste in a timely manner, the thixotropy of the paste is not good.

 

Hazard: easy to cause the amount of solder is not enough, such as false solder defects.

 

Countermeasures: wipe the template.

 

 

 

E. Graphic staining:

 

Causes: the template printing times, not in time to wipe the net, poor quality of solder paste, the plate leaves the jitter.

 

Hazard: easy to bridge.

 

Countermeasures: wipe the template, change the solder paste, adjust the machine.

 

 

 

In short, solder paste printing should pay attention to the parameters of the paste will change at any time, such as particle size / shape, thixotropy and flux properties, in addition, the parameters of the printing machine will also cause changes, such as printing pressure / speed and ambient temperature, the quality of the solder paste printing on the quality of the soldering has a great impact on the soldering process should be carefully treated each parameter in the printing process, and often observe and record the relevant coefficients.

 

 

 

Our product LASCAN L3000 3D Solder Paste Thickness Tester obtains tens of thousands of data sets by laser in the visual range, and then get some basic parameters on the solder paste, such as the maximum value, the minimum value, the average value, area, volume, etc. From the previous content we know that the solder paste thickness tester is the most important parameter in the solder paste printing process. Volume, etc. From the previous content we know that the stencil also has a very important role in the entire printing of the solder paste, face stencil cleaning machine with an air pump to the solution through the rotating arm of the stencil to clean!