Analysis and selection of SMT Mounter

- May 06, 2020-

Analysis and selection of SMT Mounter

 

The automatic component placement machine plays a very important role in the assembly of circuit board, but the assembler does not have enough knowledge about how to select a suitable and functional machine when purchasing such equipment. In this article, we will discuss the functions of the component placement machine one by one. With these knowledge, we can easily choose out Proper component placement machine.

 

1Key differences

 

In the face of the continuous demand for lightweight electronic products, the assembly industry hopes to achieve this requirement by reducing the size of components and increasing the assembly density of circuit boards. In recent years, with the gradual adoption of cladding technology, the goal of miniaturization of components seems to be solved. In fact, there are still some difficulties to be further confirmed. For example, although the pins of cladding technology similar to the traditional surface adhesion technology are low-temperature eutectic solder balls, it is not so that the component placement machine can be easily applied to the cladding technology, the traditional assembly Institute There are still some technical differences between the component placement machine and the use of cladding crystal. These differences mainly lie in whether the component placement machine can still provide the same assembly and alignment quality as in the past when it is used in the components with smaller spacing (less than 0.012 ") and smaller end. The component placement machine must have a more accurate vision and alignment system to meet the needs of cladding crystal technology.

 

In addition, the common cladding spacing is 0.004 "and the diameter of the solder ball is less than 0.002". Before the back welding process, the components are temporarily adhered to the circuit board by flux (Flox). If the components at this time belong to smaller spacing, the amount of flux they have will be less than other components with larger spacing. If the flux is not wet, the components with larger spacing will not have more flux If it can be perfect or can not provide enough adhesion to fix the original part on the welding pad, the welding quality will be poor. The problems mentioned above are very complex when the component placement machine is used in the cladding technology.

 

Generally speaking, a device can be applied to a 0.004 "module with a speed of about 8 sec / part, in which the whole dynamic process includes a suction module, and the price of the positioning function of the mobile and visual system is about $400000 to $500000. The difference lies in the requirements of the selected equipment. Although such a device has been able to meet the current requirements, is it still applicable in the future? This depends on continuous observation of the future development of component forms.

 

2Resolution and accuracy of component placement machine

 

Obviously, resolution and accuracy are very important parameters for component placement machine in the application of fine pitch parts. Generally speaking, accuracy is a function of the precision resolution of visual system and the stability of the equipment in X -, Y -, and z-directions. In short, in the component placement action, before the placement head moves to the set position in the 3-D direction, it is The system must be able to first "see" and confirm whether the component is placed in the correct position.

 

The resolution of all tested component placers must be in the range of 0.00004 "to 0.00008", and the accuracy is about 0.0004 "to 0.002". This result shows that the smaller the data is, the better in the use of small spacing. Other parameters (such as the accuracy under the full speed movement of the component suction nozzle, the standard of the alignment error between the welding pad and the solder ball) will affect the level of the equipment. From this point of view, the distance between components that can be loaded by the current high-speed machine is 0.0034 "to 0.012". We can deduce the minimum distance between components that can be allowed during assembly by the following formula: accuracy * (percentage of 100 / alignment error between pad and solder ball) * 2 = distance that can be used properly ex: 0.001 "* 100 / 25 * 2 = 0.008"

 

If it can be seen from the above example that the component placement machine with an accuracy of 0.001 "is suitable for components with a spacing of 0.008" with a 25% alignment error. If the error tolerance adopted by the supplier is 33%, the component spacing used by the above machine is 0.006 "

3Accuracy and verification of component placement machine

 

The accuracy and continuity of the component placement machine can be verified by the chrome on glass method. The method is to mark the glass by soldering, and then place the glass in the component placement machine. The machine alignment is guided by the substrate and the fiducial of the component placement position. After the whole assembly process is completed, the machine alignment is conducted through optics The inspection of microscope and the measurement of cursor ruler aim at multiple positions, and then judge its accuracy by full speed movement.

 

The test method of chrome on glass is a kind of artificial process, however, it is still a quite effective test method on whether the machine has a certain degree of accuracy. It is quite important that when the machine is just started up, the action potential of this manual alignment accuracy confirmation must be quite accurate, so as not to affect the accuracy of subsequent operation. But not all of these devices will perform similar actions at the beginning, and not all of them are equipped with glass plates with solder scales to help calibrate the accuracy.

 

In addition, the thermal stability of the component placement machine will also affect its accuracy. Therefore, Jabil's overlying crystal component placement machine uses granite abutment and ceramic support beam to reduce this kind of influence. In this case, it is very important for the equipment to conduct self thermal correction when it is brave to handle the components with a spacing of 0.004 "in normal time, although some manufacturers emphasize that its equipment does not need to This kind of correction is done, but the importance of thermal correction is emphasized from time to time in the relevant literary plays.

 

4Placement rate

 

Generally speaking, the speed of placement does not include the steps of circuit board transportation and visual alignment, so the speed published by various manufacturers is quite fast. However, if the above two actions are included in the calculation, the speed will decrease rapidly. For example, if the two items are not included, the general speed is around 2000 PPH, but if the two items are added, the speed will drop sharply to 800 PPH.

 

Looking around, it takes about 2.5 to 8 seconds for the component placement machine to perform the absorption component, solder paste, visual alignment and placement. If the relevant processes such as solder paste, solder paste or conductive adhesive can be separated from the component placement machine, the placement time can be effectively shortened. In addition, the same effect can be achieved if the moving distance of the suction nozzle can be shortened.

5Conveying

 

In general, when the circuit board moves at a high speed in the component placement machine, the components shall adhere to the adhesive flux as much as possible. If the movement, start or stop action of the circuit board is too large, it is likely to cause component displacement and confusion. Therefore, in the process of conveying the circuit board, the conveying system must have individual speed control units in three different areas.

 

In the first area, the system must transport the circuit board to the predetermined position at high speed as much as possible (in this part of components, sufficient viscosity is usually provided by the previously adhered solder paste to cope with high-speed movement). This area is mainly to transfer the flip chip onto the circuit board.

 

The second region, commonly known as the assembly region, moves at a slower rate because the flip chip is assembled at this stage.

 

The third area (often referred to as the breakaway area) is to send the circuit board out of the component placer. Flux plays a very important role in this. Increasing the amount of flux will provide greater adhesion of flip chip, which can transport circuit board at a higher speed, so it can also improve production capacity, but it will also face some problems, that is, if not combined with cleaning system, residual flux will cause poor welding of flip chip.

 

6Alignment of circuit board

 

Due to the different sizes of substrates used in flip chip, the alignment of circuit boards will cause different degrees of problems. The substrates of some component placers are vacuum adsorbed Clamming) method is quite questionable in the use of thinner substrate. It is difficult to meet the flatness requirements of thinner substrate. Although the substrate is quite flat when the vacuum system is operating, when the vacuum is removed, the substrate will return to the original bending. In addition, if the component is temporarily fixed by the adhesive of flux, the substrate will be very flexible after moving It is easy to cause component displacement. Although a hard board can be added to strengthen the substrate, the vacuum system will no longer work. Part of the vacuum system will cooperate with a special tool, and the relative price will be increased.

 

7Adjustable set pressure

 

The downward pressure of the fitting suction nozzle is also a very important parameter of the component placement machine. For the component, a sufficient downward pressure is required to make it accurately placed on the welding pad, but it cannot be too large to make the component slide away from the welding pad. The general downward pressure is 3 to 10g per bump.

 

8Visual system

 

Vision system is one of the most important units in the placement machine of clad crystal components. In addition to recognizing solder joints, the vision system also has the function of inspecting component dimensions to confirm whether the components absorbed by suction nozzle are correct. The parameters that affect the visual system are gain, offset, threshold and visual tools. With this part, the components with small spacing can be effectively processed. When the distance between components is reduced, the solder joint is also relatively forced to be reduced. For example, the height of solder joint of components with a distance of 0.004 "is about 0.002". When the solder joint is reduced to be close to the chip, even if the image is enlarged, it is still very difficult to distinguish, because the color of the chip surface protection layer is quite close to the color of solder joint .

 

Another problem is that the size of flip chip is quite small. In some cases, some standard reference points are larger than chip. To solve this problem, we must control the search area of vision system.

 

9Method of flux application

 

Flux is very important in this assembly process. We hope it has enough viscosity, excellent wetting power, and can reduce the residual amount as much as possible, especially the problem of residual amount will affect the quality of welding, because if the flux remains on the flow path of underfill, it will be adverse to its adhesion, so as to make up for the CTE mismatch The effect will be greatly reduced.

 

There are many ways to apply flux in flip chip technology, including spray, drip and dip.

 

Spray is to spray the liquid flux to the area where the assembly is to be placed by atomizing the spray nozzle.

 

A drip is a drop of liquid flux in the center of the area where the component is to be placed.

 

Dip is to drop the liquid flux onto the turntable, then use the scraper to control its thickness and flatness, and then dip the flux onto the turntable with the suction nozzle. This system can be used in the middle and low speed component placement machine. The system that uses turntable to make flux with fixed thickness must pay special attention to the problem of pollution.

 

If the action of flux loading is separated into another process rather than under the same mechanism as the component placement machine, if it is handed over to the printing machine to do this step, the production capacity will be greatly improved.

10Handling of components

 

There is no special specification for how to deal with bare die components in the industry. Generally, waffle Paks, gel Paks, extended wafer systems, surf tape and other methods are commonly used. Gel Paks and extended wafer systems need to use automatic vacuum processing system, while waffle Paks or waffle Paks tracks do not need to use vacuum devices, but they are more complex than other methods. In addition, the surf tape system needs to use a kind of equipment called flipper feeder, which is similar to the tape and reel feeder used in the traditional SMT process in appearance and function. The disadvantage is that it is quite difficult to put components into surf tape. If we can design a better pocket size In the future, the standard tape and reel will be adopted more widely.

 

All manufacturers provide basic material feeders, but flipper feeders and extended wafer feeders are optional Feeder is a high-speed / low compatibility component processing method, which can be understood, because complete wafer processing is quite difficult.

 

11Conclusion

 

There are some shortcomings in the standard SMT process and flip chip assembly technology, mainly in the accuracy requirements, flip The feature of chip technology is that its size and spacing are quite small, and the component placement machine used in the traditional SMT process may not be published. Because of this, the industry needs to further develop new technology to expand the capacity and characteristics of the equipment. In addition, there are many imperfections in the process and materials of flux, which also depend on the development of new methods and technologies to match with flip chip technology. Under such a premise, it is necessary for equipment engineers to develop newer and more convenient equipment to promote the new process.


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