PCB Printed Circuit Boards And Pcb Assembly 's Reliability Analysis.


Key words: printed circuit board PCB ,PCB design.

Abstract:
PCB is becoming more and more important, and the reliability of assembly has become an important embodiment of the competitiveness of electronic products. Through the analysis of the main factors affecting the reliability of board-level assembly, this paper puts forward the methods and paths to improve the reliability of PCB board-level assembly from five aspects: reasonable selection of components, selection of substrate, solder paste printing and quality control of reflow soldering.



1. Introduction.

With the rapid development of information technology, especially the content and status in modern weapon systems have become the key factors that determine the overall strength of weapons and equipment, and the quality of electronic products directly determines the effectiveness of weapons and equipment on the battlefield. Therefore, it is particularly urgent to improve the assembly quality of electronic products, especially the reliability of PCB board assembly. This paper explains how to improve the reliability of PCB board assembly from five aspects: reasonable selection and design of components, selection and design of substrate, layout and direction design of components, printing of SMT solder paste and quality control of reflow soldering. . The article is extracted from the author of "Smart Electronic Manufacturing": Dr.Mao Lifei of Topscom, published on China electronics Network in 2020.
 
2. Reasonable selection and design of components.

The reasonable selection and design of components is a key link in the board-level assembly of PCB. According to the requirements of process, equipment and overall design, the packaging form and structure of SMC/SMD are selected according to the electrical performance and function of determined components, which plays a decisive role in circuit design density, productivity, testability and reliability [1]. At present, there are many specifications and different structures of SMT components, and there may be a variety of packaging forms for integrated circuits that achieve the same function; in the design of circuit PCB, reasonable choices should be made according to the specifications of components provided by market suppliers and the capacity and precision of existing production equipment.

2.1 rectangular sheet element.

For sheet resistors, chip capacitors and chip inductors, the common problem is that the designed pad does not match the size of the component. The size of the pad is much larger than the outer size of the component, and it is connected by the accumulation of solder during welding, which is easy to cause the component to crack during vibration, while the size of the pad is smaller than the external size of the component, it can not be welded.

2.2 SOP small form factor package and SOJ package.

That is, the miniaturization of DIP packaging integrated circuits, the pins are mainly Euro-wing, J-shaped and I-shaped. The common problems are that there is no one-pin screen mark in PCB design, so its direction can not be confirmed during welding; due to the oxidation of device pins, it is easy to cause virtual welding after reflow soldering.

2.3 PLCC plastic encapsulated lead chip carrier.

The occupied area of the component is small, the pin strength is high and it is not easy to deform, but the maintenance of the solder joint is not convenient. The common problem is that the program is not written before assembly, and the device needs to be removed and rewritten after assembly, but the device can only be welded manually after removal, and the reliability of welding is not reflow soldered at one time.
2.4 QPF square and flat package chip carrier.

Mainly used in ASIC integrated circuits. The most common lead center distances are 0.80mm, 0.65mm and 0.5mm, and the packaging is in the form of pallets. The common problem is that the device is very easy to cause deformation damage of the device pin in the process of screening and transfer, and the non-coplanar of the device pin is inconsistent with the shape and welding strength of the solder joint after welding. in the case of high and low mild vibration shock, it will have a negative impact on the reliability after welding. The article is extracted from the author of "Smart Electronic Manufacturing": Dr.Mao Lifei of Topscom, published on China electronics Network in 2020.
 
2.5 BGA Ball Grid Array package.

There are mainly two categories: PBGA and CBGA, which is an alternative to QPF packaging, which makes more Icano terminals per unit area chip. The common problem is that the device is from screening, disassembly, packaging, re-inspection to transfer and welding, because it is placed in the air for too long, coupled with humid climate, high humidity, ball oxidation and serious moisture absorption of the device, this will have a great impact on the reliability after welding and easily lead to virtual welding.
2.6 through-hole plug-in device.

The common problem is that the pin size of the device is larger than the through hole size of the printed board, which makes it impossible to assemble. When installing through-hole plug-in devices (metal packaging), it should be clear whether the board assembly or off-board assembly and whether the shell is grounded or not in terms of technical requirements. When the pcb board assembly is required, the PCB pad that cannot be aligned to the ground should leave enough space with the ground to avoid short circuit caused by solder flow during welding. For sockets with high density and fine spacing (the spacing is less than 50mil), due to the small spacing, it is easy to cause short circuit on the installation surface of the device after solder flows through the through hole, so the resistance solder should be applied on the mounting surface of the device during PCB processing.

Selection and Design of 3.PCB substrate.

The performance of the substrate is an important part of the PCB module, which will greatly affect the electrical performance, mechanical performance and reliability of the electronic component, so it must be carefully selected.



3.1 substrate material.
It is generally required that the thermal expansion coefficient (CTE) should be as small as possible and the consistency is good, and the substrate must have the heat resistance of 260C / 50s. For single and double panels with lower general requirements, FR-4 copper-clad epoxy glass cloth laminate can be used, which is suitable for plug-in and paste mixed products. When installing fine pitch IC with high power and density, copper clad polyimide glass cloth laminate can be used, which is common in multilayer, double-sided reflow soldering process or electronic products requiring high reliability.
 
3.2 basic process requirements for SMT printed circuit boards.

The warping requirement of SMT PCB is more stringent than that of traditional PCB. The maximum value of upwarping is 0.5mm and that of downward warping is 1.2mm. In terms of process side, according to the maximum value of SMB manufacturing and installation workers, the long edge of PCB is generally within 5mm. In order to ensure the smooth transmission of PCB in the automatic production equipment of SMT, the four corners of PCB should be arc-shaped (< the diameter of 10.0mm). From reinspection to assembly, the vacuum package of PCB board is removed and exposed in the air for a long time, and the pad of PCB board is oxidized in air, which reduces the weldability of PCB board and is easy to cause virtual welding. vacuum packaging should be maintained before assembly.

Layout and orientation Design of 4.PCB components.

The components of the whole circuit board should be evenly distributed, and the power devices should be dispersed on the board in order to prevent the deformation of the PCB caused by the local overheating of the circuit board and the adverse effect on the reliability. High-quality devices should not be placed centrally to avoid high local temperature caused by too large heat capacity (high-quality devices can also be considered for afterloading). The arrangement of the components had better be consistent in order to facilitate the welding and inspection of the components; at the same time, different designs should be adopted for different welding methods.

4.1 Wave soldering mode.

In order to eliminate the "shadow effect", a special design is adopted, such as the components are arranged in the same direction, the connection of the two metallized ends of the sheet components should be perpendicular to the direction of wave soldering, the tall, large sheet elements and small components should be placed alternately, the long axes of SOP and SOT should be arranged parallel to the direction of tin wave flow, and the length of SMC/SMD pads should be extended. At the same time, lengthen the element pad (generally extend 2.0mm) to increase the solder contact area and reduce the phenomenon of virtual welding and missed welding. In order to eliminate the surface tension of molten solder, a special design is adopted, such as the components are arranged in the same direction; for the components with dense solder joints, the stolen tin pad should be used to reduce and eliminate the bridging phenomenon during wave soldering. For single-or double-row multi-pin components (such as SOP, connectors, etc.), the pad area of the last foot of the component along the wave soldering direction can be doubled. For QFP components, the QFP elements can be placed at 45 °and the necessary stolen tin pads can be used for reliable welding of QPF elements with pins greater than or equal to 0.65mm (ordinary wave soldering can only deal with QPF elements with pins greater than or equal to 1.00mm).

4.2 Reflow soldering mode.

In order to eliminate the "Manhattan effect", a special design is adopted, such as the components are arranged in the same direction; the connection of the two metallized ends of the sheet element should be perpendicular to the transmission direction of the reflow solder. avoid element upright or displacement caused by different melting time at both ends of the sheet element and uneven force between the poles. When the SMC/SMD is close to the PCB split slot or process edge, attention should be paid to the configuration direction of the components and the stress degree of the solder joint, so as to prevent the damage of the sheet components caused by splitting the plate or process edge. No through hole is allowed in or at the edge of the pad of flake components to prevent the loss of solder, and the safe distance between the through hole and the pad is greater than 0.635mm. The article is extracted from the author of "Smart Electronic Manufacturing": Dr.Mao Lifei of Topscom, published on China electronics Network in 2020.
 
4.3 reference factors for the design of component pads.

The main factors that should be considered in the design of PCB component pad are as follows: (1) the size of the pad matches the size of the component, which is the basic principle that should be followed in the design of the pad. The sizes of components produced by different manufacturers are slightly different, so we need to make minor adjustments according to different component sizes after establishing standard pad data. (2) under the premise that the wiring density of PCB does not affect the wiring gap and installation density of PCB, the size of the pad should be close to the large tolerance as far as possible. (3) according to the factors of welding process, some preventive measures should be taken in the design of the phenomena such as bridging, warping and shielding effect that may occur in the process of reflow solder. the pad data are modified according to different processes. (4) if the solder joint has good reliability design, there are almost no defects such as virtual welding and bridge connection in the welding process; on the contrary, poor pad design (especially the pad design of QFP, CSP and other devices with fine spacing) will lead to the phenomenon of unreliable solder joint and virtual welding.

5.SMT solder paste printing.

5.1 template.

Template is the basic tool of solder paste printing, which can be divided into three main types: screen template, metal template and flexible metal template.
 
5.2 Solder paste printing.

The quality of solder paste is an important link that affects the production of surface mounting. The selection of solder paste usually considers the following aspects: good printability, good solderability, and low residue. Usually, we use low residue solder paste with Sn63/Pb37 alloy composition. Table 1 shows how to select the appropriate solder paste according to the pin spacing of the components. It can be seen from the table that the larger the pins of the components, the smaller the solder powder particles of the solder paste, and the printing is relatively better. However, it does not mean that the smaller the solder powder particles, the better, because from the welding effect, the solder paste with large solder powder particles is better than the solder paste with small solder powder particles. Therefore, when we choose, we should consider all kinds of factors comprehensively.
Pin spacing / mm1.2710.80.650.50.4.

Tin powder shape aspheric particle diameter / μ m 22 "6322" 6322 "685.3"




5.3 setting of printing process parameters.


5.3.1 Angle and pressure of scraper.

Many printing experiments show that printing with scraper in the direction of 45 °can obviously improve the upward imbalance of different template openings of solder paste, and at the same time reduce the damage to fine spacing template openings. Scraper pressure not only depends on the cylinder stroke, to adjust to the best scraper pressure, we must also pay attention to the parallelism of the scraper. The pressure is generally 30N/mm2.

5.3.2 Printing Speed.
The solder paste rolls forward on the template driven by the scraper. Fast printing speed is conducive to the springback of the template, but at the same time hinders the transfer of solder paste to the PCB pad; while the speed is too slow, the solder paste will not roll on the template, resulting in poor resolution of the solder paste printed on the pad. Usually, the printing speed is 25mm / 30mm for fine pitch IC and 25~50mm/s for large spacing IC.

5.3.3 Printing method.
At present, the most common printing methods are contact printing and non-contact printing. The printing method with gap between template and PCB is non-contact printing, and the gap value is generally 0.5mm to 1.5mm, which has the advantage of being suitable for different viscosity solder paste. The solder paste is pushed into the template hole by the scraper to contact the PCB pad. After the scraper is slowly removed, the template will be automatically separated from the PCB, which can reduce the trouble of template contamination caused by vacuum leakage. With the development of component packaging in the direction of miniaturization and high density, contact printing is widely used because of its high printing accuracy. However, 70% of the welding quality problems are caused by the solder paste printing process. Therefore, in order to improve the reliability of PCB board-level assembly, the quality control of solder paste printing should be strengthened.
 
6. Quality control of reflow soldering.
Reflow soldering is the most difficult step to control in PCBA process, so obtaining a better reflow curve is the key to get good PCB welding.


6.1 preheating phase.

During this period of time, the PCB must be uniformly heated and the flux activity should be stimulated. Generally, the heating rate should not be too fast to prevent the circuit board from being heated too fast and produce large deformation. We try to control the heating rate below 3 ℃ / s, the ideal heating rate is 2 ℃ / s, and the time is controlled between 60 and 90s.

6.2 infiltration stage.
At this stage, the flux begins to volatilize, and the temperature should be kept at 60 ℃ 120 s between 150 ℃ and 180 ℃, so that the flux can give full play to its role. The heating rate is generally 0.3 ℃ / s.

6.3 reflux stage.
The temperature of this stage has exceeded the melting point temperature of the solder paste, the solder paste melts into liquid, and the tin on the pins of the components. In this stage, the time when the temperature is above 183 ℃ should be controlled between 60s and 90s. If the time is too short or too long, there will be problems with the welding quality, in which the time control of the temperature between 210 ℃ and 220 ℃ is very critical, and the best time control is 10 to 20 seconds.

6.4 cooling stage.
At this stage, the solder paste begins to solidify, and the components are fixed on the circuit board. The cooling rate should not be too fast, generally controlled below 4 ℃ / s, and the ideal cooling rate is 3 ℃ / s. Due to the excessive cooling rate, the circuit board will produce cold deformation and stress concentration, which will lead to problems in the welding quality of PCB. When measuring the temperature curve of reflow soldering, the measuring point should be placed between the pin and the circuit board. Try not to use high-temperature tape, but should use high-temperature solder welding and thermocouple fixed to ensure that more accurate curve data are obtained. In short, PCB welding is a very complex process, it is also affected by circuit board design, equipment capacity and other factors, if only take into account a certain aspect is far from enough, we also need to continue to study and explore in the actual production process, efforts to control the factors affecting welding, so that welding can achieve the best results.
7. Concluding remarks. The article is extracted from the author of "Smart Electronic Manufacturing": Dr.Mao Lifei of Topscom, published on China electronics Network in 2020.
 
SMT Pcb circuit design mainly refers to the automatic layout and wiring of CAD and the establishment of component database. The product quality and cost depend on this stage. According to statistics, the proportion of maintenance rework in the manufacturing cost of electronic products is generally as high as 15% to 25%. If the product quality can be controlled in the design and production process, this cost will be greatly reduced and the through rate of the product will be improved. PCB substrate is the structure of the interconnection of components, and it is an important part to ensure the electrical performance and reliability of products; SMC/SMD is the basis of SMT technology; pcb assembly technology and equipment are tools and means to achieve SMT products, which determine production efficiency and quality cost. Therefore, in order to improve the reliability of PCB boards assembly, we must start from three aspects: PCB design, pcb assembly equipment and pcb assembly technology.