Master the essence of SMT component mounting process for flexible circuit boards

For FPC, quality people are no strangers. Even if you are not in the FPC factory, you must know the relevant information of FPC. But you can't talk about the knowledge. In our practical application, there are countless examples. The initial application. We are familiar with even mobile products such as mobile phones. When the smartphone was just emerging, FPC was still a very high-tech high-tech. Now it is like a mobile phone. "When the old time, Wang Xieyan flew into the home of ordinary people." Below I will share the process technology of FPCA (FPC Assembly), which is practical and specific, and it is different for individual product categories and equipment. The essence of the flexible circuit board SMT placement technology is all inclusive and will definitely help you.

According to the placement accuracy requirements and the types and quantities of components, the current commonly used schemes are as follows:

Solution 1. Multi-chip placement: Multi-chip FPC is positioned on the carrier half by positioning template, and is fixed on the pallet together with SMT placement.

1. Scope of application:

A, component types: chip components generally larger than 0603, QFQ and other components with a pin pitch greater than or equal to 0.65.

B. Number of components: several components to more than a dozen components per FPC.

C. Mounting accuracy: The placement accuracy is medium.

D, FPC characteristics: the area is slightly larger, there are no components in the appropriate area, each FPC has two MARK marks for optical positioning and two or more positioning holes.

2. FPC fixed:

According to the CAD data of the metal leakage plate, the internal positioning data of the FPC is read to manufacture a high-precision FPC positioning template. The diameter of the positioning pin on the template is matched with the diameter of the positioning hole on the FPC, and the height is about 2.5 mm. There are also two lower pins on the FPC positioning template. A batch of pallets is made based on the same CAD data. The thickness of the pallet is about 2mm, and the warpage deformation of the material after several thermal shocks is small, and the good FR-4 material and other high-quality materials are preferred. Before performing SMT, the pallet is placed on the pallet positioning pin on the template so that the positioning pin is exposed through the hole in the pallet. Put the FPC one by one on the exposed fixed pin, fix it on the pallet with thin high temperature resistant tape, do not let the FPC offset, then separate the pallet from the FPC positioning template, carry out welding printing and placement, and withstand high temperature. The adhesive tape of the tape (PA protective film) should be moderate, and must be easily peeled off after being impacted by high temperature. Moreover, there is no residual glue on the FPC.

It is important to note that the shorter the storage time between the start of the FPC and the placement of the FPC on the pallet, the better.

Solution 2. High-precision placement: Fix one or several FPCs on a high-precision positioning plate for SMT placement

1. Scope of application:

A. Component types: Almost all conventional components, QFP with pin pitch less than 0.65mm are also available.

B, the number of components: dozens of components or more.

C. Mounting accuracy: In comparison, QFP mounting accuracy with a high placement accuracy of 0.5mm pitch is also guaranteed.

D, FPC characteristics: large area, there are several positioning holes, MARK mark with FPC optical positioning and optical positioning mark of important components such as QFP.

2. FPC fixed:

The FPC is attached to the component carrier. The positioning pallets are batch-customized with extremely high precision and high precision, and the difference in positioning between each pallet is negligible. This type of pallet has a small dimensional change and warpage deformation after a high temperature impact of several dozen times. The positioning plate has two positioning pins, one of which has the same height as the FPC, and the diameter matches the aperture of the positioning small hole of the FPC. The other T-shaped positioning pin is slightly higher than the former. One point, because the FPC is very flexible, large in size, and irregular in shape, the role of the T-shaped locating pin is to limit the offset of certain parts of the FPC, ensuring printing and placement accuracy. For this fixing method, the place corresponding to the T-shaped positioning pin on the metal plate can be appropriately treated.

The FPC is fixed on the positioning plate, although the storage time is not limited, but due to environmental conditions, the time should not be too long. Otherwise, the FPC is prone to moisture, which causes warping deformation and affects the quality of the placement.

High-precision placement and process requirements and precautions on FPC:

1. Fixed direction of FPC: Before making steel mesh and pallet, the fixed direction of FPC should be considered first, so that it is less likely to cause poor welding during reflow soldering. The preferred solution is to place the chip elements in the vertical direction, SOT and SOP horizontally.

2, FPC and plastic SMD components are the same as "moist sensitive devices". After FPC absorbs moisture, it is easy to cause warping deformation, and it is easy to stratify at high temperature. Therefore, FPC is the same as all plastic SMD, usually it is moisture-proof and preserved. Must be wetted and dried before. High drying methods are generally employed in factories that are produced at scale. The drying time is about 12 hours at 125 °C. The plastic SMD is 16-24 hours at 80 °C - 120 °C.

3. Preservation of solder paste and preparation before use:

The composition of the solder paste is complicated. When the temperature is high, some components are very unstable and volatile, so the solder paste should be sealed in a low temperature environment. The temperature should be greater than 0 ° C, and 4 ° C - 8 ° C is most suitable. Before use, return to normal temperature for about 8 hours (under sealed conditions), when the temperature is consistent with normal temperature. It can be turned on and used after being stirred. If it is not used at room temperature, the solder paste will absorb the moisture in the air, causing splashing during reflow soldering and causing undesirable problems such as solder beads. At the same time, the absorbed water easily reacts with certain activators at high temperatures, consumes the activator, and is prone to poor soldering. Solder paste is also strictly prohibited to return to temperature at high temperatures (above 32 °C). Use even force when mixing manually. When stirring until the solder paste is thick and thick, use a spatula to pick up it. It can be subdivided naturally, indicating that it can be used. It is better to use a centrifugal automatic mixer, which is more effective, and can avoid the phenomenon that air bubbles remain in the solder paste by manual stirring, so that the printing effect is better.

4. Ambient temperature and humidity:

Generally, the ambient temperature requires a constant temperature of about 20 ° C, and the relative humidity is kept below 60%. Solder paste printing is required to be carried out in a relatively closed space with small air convection.

5, steel mesh

The thickness of the metal drain plate is generally selected to be between 0.1 mm and 0.5 mm. According to the actual effect, when the thickness of the drain plate is less than one-half of the minimum pad width, the effect of the solder paste stripping is good, and the residual solder in the air leakage is small. The area of the leak is generally about 10% smaller than the pad.

Due to the accuracy requirements of the mounted components, the common chemical corrosion does not meet the requirements. It is recommended to use chemical corrosion plus local chemical polishing, laser and electroforming to make metal drain plates. From the comparison of price performance, the laser method is preferred.

1) Chemical corrosion plus local chemical polishing:

The chemical corrosion method is used to manufacture the leakage plate, which is relatively common in China, but the hole wall is not smooth enough. The chemical polishing method can be used to increase the smoothness of the hole wall. This method is relatively inexpensive to manufacture.

2) Laser method:

high cost. However, the processing precision is high, the hole wall is smooth, and the tolerance is small, and it is suitable for printing a solder paste of QFP with a pitch of 0.3 mm.

6, solder paste:

According to the requirements of the product, general solder paste and no-clean solder paste can be selected separately. Solder paste characteristics are as follows:

1) Particle shape and diameter of solder paste:

The shape of the granular paste is spherical, and the proportion of the non-spherical shape cannot exceed 5%. The diameter should be according to the general rule, the diameter of the solder ball should be less than one-third of the thickness of the metal leakage plate, and one-fifth of the minimum aperture width. Otherwise, the excessively large diameter solder balls and irregular particles can easily block the leakage window, resulting in Poor solder paste printing. Therefore, the metal plate with a thickness of 0.1-0.5 mm and the minimum leak window width of about 0.22 mm determine the diameter of the solder ball to be about 40 μm. The ratio of the largest and smallest diameter solder balls cannot exceed 5%. If the diameter of the solder ball is too small, the surface oxide will increase rapidly with the diameter becoming smaller, and the flux will be consumed in the reflow process, which seriously affects the soldering quality. If it is a no-clean solder paste, its deoxidation material is less and the soldering effect will be worse. Therefore, spherical solder paste particles having a uniform size of 40 um are preferred.

2) Solder ratio:

The solder paste with a solder content of about 90%-92% has a moderate viscosity and is not easily collapsed during printing. After reflow soldering, the thickness is approximately 75% of that of the solder, which is sufficient solder to ensure reliable soldering strength.

3) Viscosity:

The flow dynamics of solder paste are very complex. It is obvious that the solder paste should be easy to print and adhere firmly to the surface of the FPC. The low-viscosity solder paste (500Kcps) is prone to collapse and short-circuit, while the high-viscosity solder paste (1400Kcps) tends to remain in the metal leak. Slowly block the leak and affect the print quality. Therefore, 700-900Kcps solder paste is ideal.

4) Thix coefficient: generally choose 0.45-0.60.

7, printing parameters:

1) Type and hardness of the scraper:

Since the speciality of the FPC fixing method is that the printing surface cannot be as flat as the PCB and the thickness and hardness are the same, it is not suitable to use a metal scraper, and a polyurethane flat scraper having a hardness of 80-90 degrees is used.

2) The angle between the scraper and the FPC:

Generally choose between 60-75 degrees.

3) Printing direction:

Generally, it is printed on the left and right or front and back. The most advanced printing machine blade is printed at a certain angle with the conveying direction, which can effectively ensure the printing amount of solder paste on the QFP four-sided pad, and the printing effect is the best.

4) Printing speed:

In the range of 10-25mm / s. Printing too fast will cause the blade to slip, resulting in a missing print. Too slow to cause the solder paste edges to be irregular or contaminate the FPC surface. The blade speed should be proportional to the pad pitch and inversely proportional to the viscosity of the drain plate thickness. The pad leakage of 0.2 mm width is only 10 mm/s when the printing speed is 20 mm/s. Therefore, a moderate printing speed can ensure the amount of solder paste printed in fine printing.

5) Printing pressure:

Generally set to 0.1-0.3kg / cm length. Since changing the printing speed will change the printing pressure, usually, the printing speed is fixed first and then the printing pressure is adjusted, from small to large, until the solder paste is scraped off the surface of the metal leakage plate. Too small a pressure will make the amount of solder paste on the FPC insufficient, and too much printing pressure will make the solder paste too thin, and increase the possibility that the solder paste will contaminate the reverse side of the metal leakage plate and the surface of the FPC.

6) Stripping speed:

0.1-0.2mm/s. Due to the special nature of the FPC, the slower stripping speed facilitates the detachment of the solder paste from the leak. If the speed is fast, the pressure of the air between the FPC and the pallet will change rapidly between the metal leakage plate and the FPC, which will cause an instantaneous change in the gap between the FPC and the pallet, affecting the solder paste from the leak hole. Detachment and the integrity of the printed graphics cause bad. Now, more advanced printing presses can set the stripping speed to be accelerated, the speed can be gradually accelerated from 0, and the stripping effect is also very good.

8, patch:

According to the characteristics of the product, the number of components and the efficiency of the placement, the medium and high speed placement machine is generally used for mounting. Since each piece of FPC has an optical MARK mark for positioning, there is little difference between SMD placement on the FPC and placement on the PCB. It should be noted that after the component patch is completed, the suction force in the nozzle should be changed to 0 in time, and the nozzle can be removed from the component. Although improper placement of this process can cause poor placement when mounting on a PCB, the probability of such a defect on a soft FPC is much greater. At the same time, attention should be paid to the height of the lower stick, and the speed at which the nozzle is removed should not be too fast.

9, reflow soldering:

Mandatory hot air convection infrared reflow soldering should be used so that the temperature on the FPC can be changed more evenly, reducing the occurrence of poor soldering.

1) Temperature curve test method:

Due to the different endothermic properties of the pallets and the different types of components on the FPC, they are heated in reflow soldering, the temperature rises at different speeds, and the absorbed heat is different. Therefore, the temperature profile of the reflow soldering is carefully set, and the soldering quality is large. influential. A more appropriate method is to place two FPC-equipped pallets in front of the test board according to the actual board spacing, and mount the components on the FPC of the test pallet, and test the temperature probe with high-temperature solder wire. Solder the test point and secure the probe lead to the pallet with a high temperature tape (PA protective film). Note that the high temperature tape does not cover the test points. The test points should be selected on the solder joints near the sides of the pallet and on the QFP pins. Such test results can better reflect the real situation.

2) Setting of temperature curve and transmission speed:

Since the weight ratio of solder paste solder used is 90%-92% and the flux composition is less, the whole reflow soldering time is controlled to about 3 minutes. We should adjust the temperature of the reflow soldering zone and the time required for each functional segment. How much to set the heating and conveying speed of each temperature zone of reflow soldering. It should be noted that the transmission speed should not be too fast, so as to avoid jitter and cause poor soldering.

It is known that there is less activator in the no-clean solder paste and the degree of activation is lower. If the conventional temperature curve is used, the preheating time is too long, the degree of oxidation of the solder particles is also high, and there will be too much activator reaching the temperature peak. The zone is depleted and there is not enough activator in the peak zone to reduce the oxidized solder and metal surface. Solder cannot quickly melt and wet the metal surface, resulting in poor soldering. Therefore, for no-clean solder paste, a different positioning curve than conventional solder paste should be used to obtain a good soldering effect. This is often overlooked by some SMT technologists.

1. Reflow soldering curve of conventional solder paste

The recommended temperature profiles for different brands of solder paste are different, and the solder paste temperature curves for different flux ratios are also significantly different. The following is the temperature curve we use:

A. Preheating stage: The temperature is from room temperature to 150 ° C, and the rising speed is about 1-2 ° C /.

B. Insulation stage: the temperature rises from 150 ° C to 170 ° C, and the holding time is 30-60 seconds.

C. Welding stage: The temperature rise rate is about 20C/s, the maximum temperature peak should not exceed 230°C, the area above 200°C should be kept for 20-40 seconds, and the time of 220°C-230°C should be controlled for 3-5 seconds.

D. Cooling stage: The temperature naturally drops after reaching the peak value, and the falling speed is between 2 °C and 5 °C/s. The temperature drop rate is too slow, and it is easy to form a copolymerized metal compound, which affects the welding strength.

2. Reflow soldering curve of no-clean solder paste

A. There are certain differences in the temperature profile of the no-clean solder pastes with different characteristics.

B. When the nitrogen-filled reflow process is used, the temperature profile of the no-clean solder paste is similar to the ordinary solder paste temperature curve under normal reflow conditions, and has a heat retention area of about 150 ° C, which differs only in time.

C. When using ordinary reflow soldering, the temperature profile of the no-clean solder paste is significantly different from the conventional temperature profile. According to the supplier's recommendation, the preheating curve must rise to about 160 °C, the rate of rise is 1.5-2 °C / s, and then rise to the peak at 2-4 ° C, where the obvious insulation area. The maximum temperature peak should not exceed 230 °C, the area above 200 °C should be kept for 20-40 seconds, and the time between 220 °C and 230 °C should be controlled for 3-5 seconds.

summary

SMD placement on the FPC, one of the key points is the fixed FPC, the quality of the fixed directly affects the quality of the placement. Followed by solder paste selection, printing and reflow soldering. In the case where the FPC is well fixed, it can be said that more than 70% of the defects are caused by improper setting of process parameters. Therefore, according to the difference of FPC, the difference of SMD components, the heat absorption of the pallet, the characteristics of the selected solder paste, the different characteristics of the equipment, the process parameters are determined, and the production process is controlled to detect abnormal conditions in time. Analyze and make correct judgments and take necessary measures to control the non-performing rate of SMT production within dozens of PPMs.

GUS Technology has 20 years of SMT professional manufacturing experience and has trained more than 1,000 technicians and trusted Chinese suppliers in more than 50 countries around the world;

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Shenzhen GUS Technology Co., Ltd.