"In the early days of electronic circuit board production, the assembly of vias was completely done by manpower. After the introduction of the first batch of automatic machines, some simple pin components can be placed, but complex components still need to be placed manually.
DoctorofSacredMusic Dr. Sheng Le
Wave soldering. In addition to SMD, there are:
SMC: surface mount component.
There are mainly rectangular chip components, cylindrical chip components, composite chip components and special-shaped chip components.
SMD architecture design office
SMD Architectural Design Office is a world-renowned young architect design office. SMD has always stood at the forefront of the world's architectural design and construction engineering industry. Since its establishment, it has completed design projects, including office buildings, banks and financial institutions, government buildings, public buildings, private houses, medical institutions, religious buildings, airports, entertainment and sports venues, and school buildings.
2 development editor
About twenty years ago, surface mount components came out, which ushered in a new era. From passive components to active components and integrated circuits, they eventually become surface mount devices (SMD), which can be assembled with pick-and-place equipment. For a long time, people thought that all pin components could be packaged in SMD eventually.
3 component editing
classify
There are mainly chip transistors and integrated circuits
Integrated circuits include SOP, SOJ, PLCC, LCCC, QFP, BGA, CSP, FC, MCM, etc.
Examples are as follows:
1. interconnection: providing mechanical and electrical connection/disconnection, consisting of connecting plugs and sockets, and interconnecting cables, brackets, cabinets or other PCBs; However, the actual connection with the circuit board must be through surface mount contact.
2. An active electronic component: In analog or digital circuits, you can control the voltage and current yourself to produce gain or switching effect, that is, you can change your basic characteristics in response to external signals.
B passive electronic component: when an electrical signal is applied, it does not change its own characteristics, that is, it provides a simple and repeatable response.
3. Odd shape: Its geometric factor is strange, but not necessarily unique. Therefore, it must be installed by hand, and its shell shape (in contrast to its basic function) is not standard, such as many transformers, mixed circuit structures, fans, mechanical switch blocks and so on.
parameter
Parameter specifications of various SMT components
Chip resistor, capacitor, etc. : dimensions: 020 1, 0402, 0603, 0805, 1206, 12 10, 20 10, etc.
Tantalum capacitors: dimensions and specifications: Tana, TANB, TANC, Tanso.
Transistors: SOT23, SOT 143, SOT89, etc.
DoctorofSacredMusic Dr. Sheng Le
Melf cylindrical components: diodes, resistors, etc.
SOIC IC: dimensions: SOIC 08, 14, 16, 18, 20, 24, 28, 32.
QFP fine pitch integrated circuit PLCC integrated circuits: PLCC20, 28, 32, 44, 52, 68, 84.
BGA ball grid array package IC: Array pitch specifications: 1.27, 1.00, 0.80.
CSP integrated circuit: the side length of components shall not exceed 1.2 times the side length of internal chips and the array spacing.
There are many methods to evaluate the statistical average diameter of spray particles ejected from nozzles, such as arithmetic statistical average diameter and geometric statistical average diameter, but the most commonly used one is Sautel average, or SMD for short.
The principle is to approximate all fog particles as a sphere with the same surface area and volume and uniform diameter, and the obtained sphere diameter is the Sotel average diameter.
Because this statistical average well reflects the physical characteristics of the subjects, it is the most widely used in practice.
Patch element (8 pieces)
4 feature editing
Electronic products have high assembly density, small volume and light weight. The size and weight of the patch component are only about110 of the traditional plug-in. After SMT, the volume of general electronic products is reduced by 40%~60% and the weight is reduced by 60%~80%.
High reliability and strong anti-vibration ability. The defect rate of solder joints is low.
Good high frequency characteristics. Reduce electromagnetic and radio frequency interference.
It is easy to realize automation and improve production efficiency. Reduce the cost by 30%~50%. Save materials, energy, equipment, manpower, time, etc.
5 inspection editor
smd,sauter meandiameter
Inspection of surface mounted components. The main testing items of components include solderability, pin flatness and usability, which are sampled by the inspection department. Use stainless steel tweezers to hold the components and immerse them in a tin can at 235 5℃ or 230 5℃, and then take them out at 2±0.2s or 3±0.5s to test the solderability of the components. Check the solder joints under a microscope of 2 0 times, and it is required that more than 90% of the solder joints of the components are stained with tin.
As a processing workshop, you can do the following visual inspection:
1. Visually or with a magnifying glass, check whether the soldering end or pin surface of the component is oxidized or free of pollutants.
4. The nominal values, specifications, models, accuracy and overall dimensions of parts and components shall meet the product process requirements.
3. 3. The pins of SOT and SOIC should not be deformed. For multi-lead QFP devices with lead spacing less than 0.65mm, the flatness of lead * * * should be less than 0. 1mm (which can be optically detected by mounter).
4. For products that need cleaning, the marks of components will not fall off after cleaning, and the performance and reliability of components will not be affected (visual inspection after cleaning).
6 theoretical editor
Inspection method: It is expounded that process monitoring can prevent circuit board defects and improve the overall quality.
Inspection can often remind you whether there are too many variables in your assembly process. Even if your manufacturing process can achieve continuous zero-defect production, you need some form of inspection or monitoring to ensure the required quality level. Surface mount is a series of very complex events, including a large number of individual actions. Our trick is to establish a balanced strategy of inspection and monitoring, without 100% inspection. This paper discusses the inspection methods, techniques and manual inspection tools, and reviews the automatic inspection tools and the use of inspection results (number and type of defects) to improve the quality of processes and products.
Inspection is a product-centered activity, while monitoring is a process-centered activity. Both of them are necessary for quality planning, but the long-term goal should be to reduce product inspection and increase process monitoring. Product inspection is passive (defects have occurred), while process monitoring is active (defects can be prevented)-obviously, prevention is much more valuable than passive response to existing defects.
Inspection is actually a screening process because it tries to find unqualified products for repair. The fact is clear that a large number of inspections may not necessarily improve or guarantee the quality of products. The third point of Deming's fourteen points said, "Don't expect a lot of inspections". Deming emphasized that a strong process should focus on establishing stable, repeatable and statistically monitored process objectives, rather than large-scale inspection. Inspection is a subjective activity, and even with considerable training, it is also an arduous task. In many cases, you can ask a group of inspectors to evaluate a weld, but you will get several different opinions.
Operator fatigue is the reason why 100% inspection usually fails to find every manufacturing defect. Besides, it is a costly and worthless operation. It rarely achieves the expected goal of higher product quality and customer satisfaction.
A few years ago, we began to use the word "process monitoring" instead of inspectors, because we wanted to change the concept of workplace from passive reaction to active prevention. Inspectors usually sit at the end of the assembly line to check the products. Ideally, process monitoring activities are the balance between product inspection and process monitoring, for example, confirming the correct process parameters are used, measuring the performance of machines, and establishing and analyzing control charts. Process monitoring plays a leading role in these activities; They help machine operators to accomplish these tasks. Training is a key factor. Process supervisors and machine operators must be familiar with process standards (for example, IPC-A-6 10), concepts of process monitoring and related tools (for example, control charts, pareto chart, etc.). ). Process monitoring also improves product quality and process monitoring. As a key member of the manufacturing team, the monitor encourages the method of preventing defects, rather than the method of finding and repairing defects.
Excessive inspection is also a common problem. In many cases, excessive inspection is only caused by misunderstanding of IPC-A-6 10 process standard. For example, plug-ins, many inspectors also hope that the two sides of the board are perfectly welded with round feet and the through holes are completely filled. However, this is not required by IPC-A-6 10. The quality of inspection fluctuates with the attention and concentration of inspectors. For example, fear (management pressure) may improve the concentration of the workplace, and the quality may improve for a while. However, if batch inspection is the main inspection method, defective products may still be produced and may leave the factory.
Another term that we should avoid is modification. In this industry, many employees believe that repair welding is a normal and acceptable part of the assembly process. This is very unfortunate, because any form of rework and repair is not desirable. Reworking is usually considered undesirable, but it is a necessary information to instill in the entire manufacturing organization. It is very important to establish a manufacturing environment that regards defects and rework as inevitable and the least desirable.
For most companies, manual inspection is the first line of defense. Inspectors use various magnifying tools to observe components and welding points at close range. IPC-A-6 10 establishes some basic amplification criteria according to the pad width of the detection element. The main reason for formulating these guidelines is to avoid over-inspection caused by over-magnification. For example, if the pad width is 0.25~0.50 mm, the desired magnification is 10X, and 20X can be used as a reference if necessary.
Every inspector has a favorite inspection tool; It is best to use a three-lens folding pocket magnifying glass for mechanics. You can carry it with you, and the maximum magnification is 12X, which is just suitable for dense solder joints. Perhaps the most common inspection tool is a microscope, and its magnification range is 10-40 times. However, the fatigue caused by continuous use of microscope usually leads to excessive inspection, because the magnification usually exceeds the guidance of IPC-A-6 10. Of course, it is still useful when it is necessary to carefully check possible defects.
For general inspection, it is best to use a video system equipped with a zoom lens (4-30x) and a high-definition color monitor. These systems are easy to use and, more importantly, less fatigued than microscopes. A high-quality video system costs less than $2,000, and a good microscope is also in this range. The additional benefit of the video system is that more than one person can see the object, which is very helpful when the training or inspector needs a second opinion. Edmund Science has a large number of magnifying tools, from hand-held magnifying glasses to microscopes to video systems.
To sum up, it is a challenge to establish a balanced monitoring strategy between 0- 100% inspections. Starting from this key checkpoint, we will discuss the inspection equipment.
Automation is wonderful; In many cases, it is more accurate, faster and more efficient than inspectors. But depending on its complexity, it can be quite expensive. Automatic detection equipment may dilute people's consciousness and give people an illusion of safety.
Solder paste inspection. Solder paste printing is a complex process, and it is easy to deviate from the expected results. A clearly defined and correctly implemented process monitoring strategy is needed to control the process. At least manually check the coverage area and measure the thickness, but it is best to use automatic coverage, thickness and volume measurement. Use the range control chart (x- bar chart) to record the results.
Solder paste inspection equipment ranges from simple 3 times magnifier to expensive automatic online machine. The first-level tool uses optics or laser to measure the thickness, and the second-level tool uses laser to measure the coverage area, thickness and volume. Both tools are used offline. The third-level tool also measures the coverage area, thickness and volume, but it is installed online. The speed, accuracy and repeatability of these systems are different, depending on the price. The more expensive tools provide better performance.
For most assembly lines, especially for high-mixed production, medium-level performance is the first choice. It is an off-line tool for installing countertops and measuring coverage area, thickness and volume. These tools are very flexible, cost less than $50,000, and usually provide the required amount of feedback. Obviously, automation tools are much more expensive ($75,000-$200,000). However, it is faster and more convenient for them to check the board because it is installed online. It is most suitable for high-volume and low-mixing assembly lines.
Glue check. The distribution of glue is another complicated process, which easily deviates from the expected result. Like solder paste printing, a clearly defined and correctly implemented process monitoring strategy is needed to control the process. It is recommended to check the glue point diameter manually. Use the range control chart (x- bar chart) to record the results.
Before and after the glue dropping cycle, it is best to drop at least two isolated glue spots on the cardboard to represent the diameter of each glue spot. This allows the operator to compare the quality of glue spots during the gluing cycle. These points can also be used to measure the diameter of glue points. Glue spot inspection tools are relatively cheap, and there are basically portable or desktop measuring microscopes. I wonder if there is any automatic equipment specially designed for glue spot inspection. Some automatic optical inspection (AOI) machines can be adjusted to accomplish this task, but they may be overqualified.
First piece confirmed. Companies usually make a detailed inspection of the first circuit board coming off the assembly line to confirm the setting of the machine. This method is slow, passive and inaccurate. It is common that a complex circuit board contains at least 1000 components, many of which are not marked (value, part number, etc. ). This makes the inspection difficult. Verify machine settings (components, machine parameters, etc. ) is a positive method. AOI can be effectively used for inspection of the first board. Some hardware and software vendors also provide feeder setting confirmation software.
Coordinating the verification of machine settings is the ideal role of the process monitor, who guides the machine operators to complete the production line confirmation process with the help of the checklist. In addition to verifying the settings of the feeder, the process monitor should also carefully check the first two boards with existing tools. After reflow soldering, the process monitor should check the key components (dense components, BGA, polar capacitors, etc.) quickly and in detail. ). At the same time, the production line continues to assemble plates. In order to reduce downtime, the production line should be filled with circuit boards before reflow, and the process monitor should check the first two circuit boards after reflow. This may be a bit dangerous, but you can gain confidence by verifying the machine settings.
X-ray inspection According to experience, X-ray is not necessarily mandatory for BGA assembly. However, if you can afford it, this is certainly a good tool at hand. Should be recommended for CSP assembly. X-ray is very good for checking welding short circuit, but it is not effective for finding welding open circuit. The low-cost X-ray machine can only look down, and the inspection of welding short circuit is enough. An X-ray machine that can tilt the inspected object is more suitable for checking the open circuit.
Automatic optical inspection (AOI). Ten years ago, optical inspection was used as a tool to solve everyone's quality problems. Later, the technology was stopped because it couldn't keep up with the pace of assembly technology. In the past five years, it has reappeared as a desirable technology. A good process monitoring strategy should include some overlapping tools, such as ICT, optical inspection, functional testing and appearance inspection. These processes overlap and complement each other, and neither of them can provide sufficient coverage alone.
Two-dimensional (2-D)AOI machines can check missing components, alignment errors, incorrect part numbers and polarity inversion. In addition, three-dimensional (3-D) machines can evaluate the quality of welding points. Some suppliers provide desktop two-dimensional AOI machines for less than $50,000. These machines are ideal for initial product inspection and small batch sample planning. In the higher performance category, the price of 2-D stand-alone or online machines is 75,000-125,000 USD, while the price of 3-D machines is150,000-250,000 USD. AOI technology has a bright future, but processing speed and programming time are still a limiting factor.
Data collection is one thing, but using this data to improve performance and reduce defects is the ultimate goal. Unfortunately, many companies collect a lot of data, but they don't make effective use of it. Reviewing and analyzing data can be laborious, and it is often seen that this work is only carried out by engineering designers, excluding production activities. Without accurate feedback, production will be carried out blindly. Weekly quality meeting may be an effective way for engineering design and production departments to communicate key information and promote necessary improvement. These meetings need a leader and must be well organized, especially in a short time (30 minutes or less). The data presented at these meetings must be user-friendly and meaningful (for example, pareto chart). When the problem is confirmed, an investigator must be appointed immediately In order to ensure the successful conclusion of the meeting, the leaders attending the meeting must make accurate records. Ending means root cause and corrective measures.
7 package editing
Miniature SMD wafer-level CSP package;
Micro SMD is a standard thin product. There is a solder bump on one side of SMD chip. The production process steps of micro SMD include standard wafer manufacturing, wafer re-passivation, deposition of solder bumps on I/O pads, back grinding (only for thin products), protective packaging and coating, testing with wafer selection platform, laser marking, packaging into strips and coils, and finally assembling on PCB by standard surface mount technology (SMT).
Micro SMD is a wafer level chip scale package (WLCSP), which has the following characteristics:
1. The package size is consistent with the die size;
Minimum I/O pin;
13. No underfill material is needed;
4. The distance between connecting lines is 0.5 mm; ;
5. No interposer is needed between the chip and PCB.
Matters needing attention
Precautions for surface mounting:
A. micro SMD surface mounting operations include:
1.Printing flux on PCB;
4. Use standard pick-and-place tools to place components;
3. Reflow soldering and cleaning of solder bumps (depending on the type of flux).
B. the surface mounting advantages of micro SMD include:
1. Shipped in the form of standard coil packaging, which is convenient for operation (in line with EIA-48 1- 1 specification);
4. You can use standard SMT pick-and-place tools;
3. Standard reflow soldering process.
Packing size
Package dimensions of SMD patch assembly:
Metric system: 3216-2012-1608-1005-0603-0402.
English system:1206—0805—0603—0402—0201—01005.
note:
0603 is divided into metric system and English system.
The English system of metric system 0603 is English system 020 1.
The metric system of imperial system 0603 is metric system 1608.
Also note the difference between 1005 and 0 1005.
1005 is also divided into metric system and English system.
The metric system of English system 1005 is metric system 25 12.
The English system of metric system 1005 is English system 0402.
For example, in ProtelDXP(Protel2004) and later versions, there are already packaging libraries for SMD patch components, such as
CC 1005-0402: SMD capacitor package, with metric of 1005 and British of 0402.
CC 13 10-0504: used for patch capacitor, metric 13 10, English 0504.
CC 1608-0603: Capacitor for SMD, metric 1608, British 0603.
CR 1608-0603: used for chip resistor. The package with metric system of 1608 and British system of 0603 is the same size as CC 16-8-0603, but it is easy to identify.
PCB layout
There are two types of SMT packages: solderless shield definition (NSMD) and solder shield definition (SMD). Compared with SMD method, NSMD method can strictly control copper etching process and reduce stress concentration points on PCB, so it should be the first choice.
In order to achieve a higher height from the ground, it is recommended to use a copper-clad layer with a thickness of less than 30 microns. The copper-clad layer with a thickness of more than 30 microns will reduce the effective height from the ground, thus affecting the reliability of welding. In addition, the width of the connecting line between the NSMD pad and the ground pad should not exceed two thirds of the pad diameter. It is recommended to use the gasket sizes listed in table 1:
The layout of PCB with through-hole structure (micro-through-hole) in the pad should conform to the definition of NSMD pad to ensure sufficient wetting area on the copper pad and enhance the welding effect.
Considering the internal structural performance, the circuit board processing method with organic solderability protection (OSP) coating can be adopted, and copper OSP and nickel-gold coating can be adopted:
1. If the nickel-gold plating method (nickel plating and gold plating) is adopted, the thickness shall not exceed 0.5 micron to avoid brittle fracture of the joint;
2. Because the flux has surface tension, in order to prevent the parts from rotating, the printed lines should be symmetrical in X and Y directions;
[13] It is recommended not to use hot air flux coating (HASL) for circuit board processing.
typeset and print
Screen printing process:
1. After electroplating and polishing, the template is cut by laser.
3. When the number of welding bumps is less than 10 and the size of welding bumps is small, the hole should be as far away from the pad as possible to minimize the bridging problem. When the number of welding bumps exceeds 10 or the welding bumps are large, there is no need to offset.
13. Print with Grade 3 (particle size is 25-45 microns) or precision flux.
Component placement
The placement of micro SMD can use standard pick-and-place tools, and can be identified or located by the following methods:
1. The visual system of the package can be found.
The vision system that can locate a single solder bump is slow and expensive.
Other features of micro SMD mounting include:
1. In order to improve the mounting accuracy, it is better to use IC mounter/fine pitch mounter instead of chip shooter.
3. Because SMD solder balls are self-centering, when displacement occurs, it will automatically correct.
3. Although the micro-patch can withstand the placing force as high as 1kg for 0.5 seconds, it should be placed with as little or no force as possible. It is suggested that solder bumps should be placed in the flux on PCB, and the depth should be greater than 20% of the height of flux.
Welding cleaning
Reflow welding and cleaning:
1. Micro SMD can use the industry standard reflow soldering process.
2. It is suggested to use nitrogen for cleaning in reflow soldering.
3. According to the standard of J-STD-020, the micro-patch can withstand three reflow operations at most (the highest temperature is 235℃), which meets the requirements.
4. The miniature SMD can withstand the reflux temperature as high as 260℃ for 30 seconds.
Welding rework
The key factors leading to micro SMD rework are as follows:
The rework process is the same as most BGA and CSP packages.
4. All parameters of rework reflow soldering should be completely consistent with the original parameters of reflow soldering during assembly.
13. The rework system shall include a local convection heater with molding capability, a bottom preheater and a component pick-and-place machine with image overlapping function.
quality detection
The following is the reliability check of solder joints when installing micro SMD on FR-4 PCB, and the mechanical test results. Testing includes using daisy-chained components. Product reliability data are listed separately in each quality inspection report of the product.
Welding quality inspection
Welding reliability quality inspection:
1. Temperature cycle: The test shall be conducted according to IPC-SM-785 "Guide to Accelerated Reliability Test of Surface Mount Welded Parts".
1. packaging shearing: as a part of the production process, it is necessary to collect the shearing data of solder bumps during the packaging process to ensure the tight combination of solder balls and packaging. For solder bumps with a diameter of 0. 17mm, the average package shear force of each solder bump is about 100gm. For solder bumps with a diameter of 0.3mm, the packaging shear force of each solder bump is greater than 200gm. The measured package shear value will be different with different materials and surface mounting methods.
13. tensile test: fix a screw on the back of the component, and pull the assembled 8-weld convex miniature SMD component vertically upward until the component is pulled away from the circuit board. For solder balls with a diameter of 0.17mm, the average tension recorded is 80g per solder ball.
2. Drop test: The object of the drop test is the miniature SMD package with 8 solder bumps mounted on a PCB with a thickness of 1.5mm, and the diameter of the solder bumps is 0. 17 mm ... The first side falls 7 times, the second side falls 7 times, the corner falls 8 times and the horizontal falls 8 times, totaling ***30 times. If the test results show that the impedance in the daisy chain loop increases by more than 10%, the test is considered to have failed.
5. Three-point bending test: A test plate with a width of 100mm is used for three-point bending test, and the midpoint is twisted with a force of 9.45 mm/min. The test results show that even if the torque is increased to 25 mm, the welding bumps will not be damaged.
thermal performance
According to the regulation of IA/JESD5 1-3, the thermal characteristics of miniature SMD package were evaluated by using an inefficient thermal conductivity test board. The performance of SMD products depends on the chip size and application (PCB layout and design).
8 moistureproof editor
Provisions on the management of moisture-proof patch parts:
purpose
In order to ensure that all humidity sensitive devices are effectively controlled during storage and use, the following two points should be avoided:
① The welding quality of parts is affected by moisture.
(2) When wet parts are heated at instantaneous high temperature, cracks will occur between the plastic body and the pin. Slight cracks will cause leakage of the shell, which will make the chip slowly fail after being wetted, affecting the product life, and serious cracks will directly damage the components.
application area
It is suitable for the storage and use of all moisture-sensitive parts.
content
3. 1 Inspection and storage
3. 1.65438+ When packages must be opened, the number of unopened packages should be reduced as much as possible. SMD parts should be put back to the original packaging in time after inspection, and then sealed after vacuumizing with a vacuum machine.
[13] 1.2 All open SMD parts shall be put online as far as possible.
3. 1.3 The storage environment of humidity sensitive parts requires room temperature below 30℃ and relative humidity below 75%.
2. Production and use
3.2. 1 Control the number of packages to be opened according to the production plan. PCB, QFP and BGA shall be used up within 12 hours as far as possible, and SOIC, SOJ and PLCC shall be completed within 48 hours.
2.2 Unpack the unused SMD parts, put them back in the bag, put in the desiccant, and seal them after vacuumizing with a vacuum extractor.
2.3 When using SMD parts, first check the humidity value of the humidity indicator card. If the humidity reaches more than 30%, it should be baked. Generally, the humidity indicator cards for the patch parts in the company are six cycles, and the humidity is 10%, 20%, 30%, 40%, 50% and 60% respectively. Reading: If 20% of the circles turn pink and 40% of the circles are still blue, then 30% next to lavender between blue and pink is the humidity value.
3. Wetting and drying
3. 1 When unsealing, it is found that the humidity of the indicator card is above 30%, so it should be dried at high temperature. Oven temperature: 65438 025℃ 5℃ for 5~48 hours. The specific temperature and time vary from manufacturer to manufacturer. Please refer to the manufacturer's drying instructions.
13.QFP has two kinds of packaging plastic trays: high temperature resistant and high temperature resistant. The high-temperature resistant trays are Tmax= 135, 150 or 180℃, which can be directly baked, and cannot be directly baked in the oven.
9 needs editing
Technical requirements for FPC surface mounting:
With the development of miniaturization of electronic products, a considerable number of patches of consumer products are installed on FPC to complete the assembly of the whole machine because of the assembly space. FPC surface mounting technology has become one of the development trends of SMT technology. The process requirements and precautions of surface mount are as follows.
Traditional SMD installation
Features: the installation accuracy is not high, the number of components is small, the component types are mainly resistor and capacitor, or there are some special-shaped components.
Key process: 1. Solder paste printing: FPC is located on a special printing tray with its shape, and it is usually printed by a small semi-automatic printer or by hand, but the quality of manual printing is worse than that of semi-automatic printing.
4. Mounting: Generally, manual mounting can be used, and individual components with high position accuracy can also be mounted by manual mounter.
3. Welding: Reflow welding process is generally used, and spot welding can also be used under special circumstances.