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PCB Board Assembly Reliability
Sep 30, 2017

With the rapid development of information technology, especially in the modern weapons system content and status has become the key factor in determining the overall strength of weapons and equipment, and the quality of electronic products directly determine the effectiveness of weapons and equipment in the battlefield play, therefore, At present, it is particularly urgent to improve the assembly quality of electronic products, especially to improve the reliability of PCB board assembly. This paper describes how to improve the reliability of PCB board assembly from the five aspects of rational selection of components, design choice of substrate, layout and direction design of components, SMT solder paste printing and reflow quality control. 

2. Reasonable choice of components design

Reasonable choice of components Design PCB board assembly is a key part of the ring. The SMC / SMD package form and structure are selected for the electrical performance and function of the identified components according to the process, equipment and overall design requirements, which plays a decisive role in line design density, manufacturability, testability and reliability [1]. SMT components are now the size and diverse structure, to achieve the same function of the integrated circuit may exist in a variety of packages; in the circuit PCB design, should be based on market suppliers to provide the components of the specifications and existing production equipment capacity and accuracy , Make a reasonable choice.

2.1 rectangular sheet-like components

For chip resistors, chip capacitors and chip inductors, the common problem is that the design of the pad and the component size does not match. The size of the pad is much larger than the size of the component, and the solder is connected by soldering. It is likely to cause the element to crack at the time of vibration, and the size of the pad is smaller than the external dimensions of the component.

2.2 SOP small outline package and SOJ package

That is, DIP package integrated circuit of the reduced type, the main main European wing, J-shaped and I-shaped. Frequently Asked Questions There is no PCB marking on the printed board design, which can not confirm the direction when welding is caused. Due to the oxidation of the device pins, it is easy to cause Weld after reflow soldering.

2.3 PLCC Shaped Leaded Chip Carrier

Components occupied area is small, the pin is not easy to deformation of high strength, but the welding of the repair is not convenient. The common problem is that the program is not written before assembling. After the device is assembled, it is necessary to remove the program again. However, after removing the device, it can only be soldered manually. There is no one-time reflow soldering in the reliability of welding.

2.4 QPF square, flat package chip carrier

Mainly used in ASIC integrated circuits. The most common lead center distance is 0.80mm, 0.65mm and 0.5mm, in the form of tray. The common problem is that the device in the screening and transport process can easily lead to damage to the device pin deformation, the device pin is not coplanar, after welding the solder joint shape and welding strength is inconsistent at high and low temperature shocks will be welded The reliability of the adverse effects.

2.5 BGA ball grid array package

Mainly PBGA and CBGA two categories, is an alternative to QPF package, so that the unit area of the chip I / O terminals more [2]. The common problem is that the device from the screening, disassembly, packaging, re-inspection to transshipment, welding, due to the time in the air is too long, coupled with the humid climate, the humidity of the large ball oxidation and device moisture absorption, which will be welded Reliability caused a great impact, easily lead to Weld.

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 circuit board, resulting in the inability to assemble. Through-hole insertion device (metal package) installation, the technical requirements should be clear board assembly or board assembly and the shell is grounded. On the requirements of the board assembly, can not be on the ground of the PCB plate should be set aside with enough space to avoid solder flow caused by welding short circuit. For high-density fine pitch socket (spacing less than 50mil), due to the small spacing, welding is likely to cause solder flow through the hole after the installation of the device in the short circuit, the PCB should be processed in the device mounting surface coating Solder solution.

3.PCB substrate selection design

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

3.1 Substrate material

Generally, the thermal expansion coefficient (CTE) is as small as possible and the consistency is good. The substrate must have a heat resistance of 260 ° C / 50 s. For the general requirements of the lower single, double-sided, can be used FR-4 copper-clad epoxy glass cloth board, suitable for plug, paste mixed products. When the installation of fine pitch IC power, the density is large, can be used copper clad polyimide glass cloth board, common in multi-layer board, double-sided reflow process or require high reliability electronic products [3].

3.2 Basic requirements for printed circuit boards

SMT printed circuit board on the warping requirements than the traditional printed circuit board more stringent, upturned maximum 0.5mm, under the Alice for the 1.2mm. Process side, according to SMB manufacturing, installation workers maximum, generally from the long side of the PCB within 5mm [4]. In order to ensure that PCB in the SMT automatic production equipment in the smooth transmission, PCB 4 corners should be used arc (<10.0mm diameter). PCB board from the re-inspection to the assembly, because of its original vacuum packaging was demolished in the air exposure time is too long, PCB board pad in the air oxidation, resulting in reduced PCB solderability, likely to cause Weld, assembly Should be kept in vacuum before packaging.

4. PCB component layout and direction design

The distribution of the components of the circuit board should be uniform, the power device on the board to disperse the distribution, in order to prevent the local board overheating caused by deformation of the printed circuit board and the adverse impact on reliability. The quality of the device do not focus on the placement, to avoid the heat capacity is too large to form a high local temperature (high-quality devices can also consider after loading). The direction of the arrangement of the elements is the best, in order to facilitate the welding and inspection of components; the same time, different welding methods should take a different design.

4.1 wave soldering welding method

In order to eliminate the "shadow effect" to take a special design, such as the direction of the best arrangement of components consistent, chip components of the two metallized ends of the connection should be perpendicular to the direction of wave soldering, high, large chip components and small Of the components should be placed alternately, SOP and SOT long axis should be arranged in parallel with the direction of the flow of tin, SMC / SMD pad length extension, while extending the component pad (generally extended 2.0mm) to increase the solder contact area, Weld and missed welding phenomenon [5]. In order to eliminate the surface tension of molten solder to take a special design, such as the direction of the best arrangement of components consistent; more dense parts of the solder joints, in the wave soldering to reduce and eliminate bridging phenomenon should be used to wear tin solder pad. For single or double row multi-foot components (such as SOP, connectors, etc.), the components can be along the wave soldering direction of the last one foot pad area doubled. For QFP components, the QFP element can be placed at 45 ° and the necessary stolen solder pads can be used to reliably solder QPF components with a pitch greater than or equal to 0.65mm (ordinary wave soldering can only handle foot pitch greater than or equal to 1.00mm Of the QPF components).

4.2 reflow soldering method

In order to eliminate the "Manhattan effect" to take a special design, such as the direction of the best alignment of components; chip components of the two metallized ends of the connection should be perpendicular to the direction of reflow soldering, to avoid the chip at both ends of the melting time is different , The poles caused by the uneven components of the components upright or displacement. When the SMC / SMD close to the PCB segmentation or process edge, should pay attention to the direction of the component and the solder joint stress to prevent the division of the puzzle or process edge caused by the chip components damage. There is no permissible vias in the pads or edges of the chip components to prevent loss of solder and the safety distance between the vias and the pads is greater than 0.635 mm [6].

4.3 Reference factors for component pad design

In the PCB component design should also consider the main factors are the following four points: (1) component size pad size and component size match, which is pad design should follow the basic principles. The size of the components produced by different manufacturers is slightly different, so we need to establish the standard pad data according to the size of different components for minor adjustments. (2) PCB wiring density does not affect the PCB wiring gap and installation density under the premise of the pad size should be as far as possible to the large tolerance near. (3) the welding process factors in the reflow process may occur during the bridge, Alice and wave soldering process may occur in the shielding effect and so on, the design should take some preventive measures, according to the different process on the The pad data is corrected accordingly. (4) solder joint with the reliability of the design of the pad, the welding process almost no Weld, bridge and other problems; the contrary, poor pad design (especially fine pitch QFP, CSP and other devices Of the pad design) will lead to unreliable solder joints, such as Weld and so on.

5.SMT solder paste printing

5.1 template

Templates are the basic tool for solder paste printing and can be divided into three main types: screen formwork, metal formwork and flexible metal formwork.

5.2 solder paste printing

The advantages and disadvantages of solder paste is an important part of the production of surface paste. Selection of solder paste usually takes into account the following aspects: good printability, good solderability, low residue. In general, we use the solder paste alloy composition for the Sn63 / Pb37 low residue type solder paste. Table 1 shows how the corresponding solder paste is selected according to the pin pitch of the component. From the table can be seen, the greater the pin between the components, solder paste powder particles smaller, relatively good for printing. But not to say that the choice of solder paste tin powder particles as small as possible, because from the welding effect, the tin powder particles of solder paste welding effect than tin powder small solder paste good [7]. Therefore, we in the choice of factors from all aspects of comprehensive consideration.

Pin pitch / mm1.2710.80.650.50.4

Tin powder shape non-spherical ball-type ball-type particle diameter / m22 ~ 6322 ~ 6322 ~ 6322 ~ 685.3

5.3 Setting of printing process parameters

5.3.1 Scraper angle and pressure

Several printing tests have shown that the scraper is printed in the direction of 45 °, which can significantly improve the imbalance of the opening of the different molds of the solder paste, while also reducing the damage to the opening of the fine pitch template. Scraper pressure is not only dependent on the cylinder stroke, to adjust to the best scraper pressure, but also must pay attention to scraper parallelism. The pressure is generally 30N / mm2.

5.3.2 Printing speed

The solder paste will roll forward on the template with the squeegee. Printing speed is conducive to the springback of the template, but at the same time will hinder the solder paste to the PCB on the transfer; and the speed is too slow, the solder paste will not roll on the template, causing the pad on the solder paste resolution bad. Usually for a fine pitch IC when the printing speed of 25 ~ 30mm / s, for only a large pitch IC when the printing speed of 25 ~ 50mm / s.

5.3.3 printing method

At present, the most common printing method is divided into contact printing and non-contact printing. Between the template and the PCB there is a gap between the printing method for non-contact printing, the general gap value of 0.5 ~ 1.5mm, the advantage is suitable for different viscosity solder paste. The solder paste is pushed into the template opening by the scraper and the PCB pad. After the blade is slowly removed, the template is automatically separated from the PCB. This can reduce the trouble of template contamination due to vacuum leakage. With the packaging of components to the miniaturization, high-density direction, contact printing because of its high printing accuracy is widely used. And 70% of the welding quality problem is due to the solder paste printing process generated, therefore, in order to improve the reliability of PCB board assembly should strengthen the quality of paste paste printing control.

6. Reflow quality control

Reflow soldering is the most difficult step in PCB assembly process, so obtaining a better reflow profile is the key to getting good PCB welding.

6.1 Preheating phase

In this period of time to make the PCB evenly heated and stimulate the flux activity, the general temperature of the temperature should not be too fast to prevent the board too hot and produce large deformation. We try to control the heating rate below 3 ℃ / s, the ideal heating rate of 2 ℃ / s, time control between 60 ~ 90s.

6.2 infiltration phase

This stage began to volatilize the flux, the temperature between 150 ℃ ~ 180 ℃ should be maintained between 60 ~ 120s, so that the flux can give full play to its role. The temperature of the heating is generally 0.3 ~ 0.5 ℃ / s [8].

6.3 return stage

The temperature of this stage has exceeded the melting point of the solder paste, the solder paste is dissolved in liquid, and the component pin is tin. The temperature in the period of 183 ℃ above the time should be controlled between 60 ~ 90s. If the time is too short or too long will cause the quality of welding problems, which temperature between 210 ℃ ~ 220 ℃ time control is quite critical, generally in control for 10 ~ 20s for the best.

6.4 cooling stage

This stage began to solidify the solder paste, components are fixed on the circuit board, the cooling rate should not be too fast, generally controlled at 4 ℃ / s below, the ideal cooling rate of 3 ℃ / s. Due to excessive cooling rate will cause the board to produce cold deformation and stress concentration, which will lead to PCB welding quality problems. When measuring the temperature profile of the reflow soldering, the measuring point should be placed between its pin and the circuit board. Try not to use high-temperature tape, but should be used high-temperature solder welding and thermocouple fixed to ensure a more accurate curve data. In short, PCB welding is a very complex process, it is also by the circuit board design, equipment capacity and other factors, if only take into account a certain aspect is far enough, we also need in the actual production process Continuous research and exploration, efforts to control the impact of welding the various factors, so that welding can achieve the best results.