SMT丨Process characteristics and detailed production process

SMT丨Process characteristics and detailed production process

1. Current status of surface mount technology SMT

SMT is currently the most popular technology and process in the electronics assembly industry. Since it was introduced to the market in the early 1970s, SMT has gradually replaced the traditional "manual plug-in" wave soldering assembly method, and has become the mainstream of the modern electronic assembly industry. It is called the second revolution of electronic assembly technology. Internationally, this assembly technology has formed a world trend, which has led to changes in the entire electronics industry.
At the same time, SMT also promotes and promotes the development of electronic components in the direction of chipping, miniaturization, thinning, lightweight, high reliability, and multi-function, and has become a symbol of a country's technological progress.

2. The process and characteristics of surface mount technology SMT

SMT (Surface Mount Technology) is the abbreviation or abbreviation of surface mount technology. It refers to the mounting of surface mount devices (SMD) on the surface of PCB (or other substrates) through certain processes, equipment and materials, and soldering, cleaning, testing and final assembly.

1. SMT process

The SMT process is related to both the welding method and the assembly method, as follows: According to the welding method, it can be divided into two types: reflow soldering and wave soldering.
According to the assembly method, it can be divided into three methods: full surface assembly, single-sided mixed assembly, and double-sided mixed assembly.
The main factors affecting the quality of welding: PCB design, quality of solder (Sn63/Pb37), quality of flux, degree of oxidation of the metal surface to be welded (component solder end, PCB solder end), process: printing, pasting, soldering (correct temperature curve), equipment, management.

2. The following discusses several factors that have a greater impact on the quality of reflow soldering.

(1) Influence of solder paste printing quality on reflow soldering process:

According to statistics, under the premise that the PCB design is correct and the quality of components and printed boards is guaranteed, 70% of the surface assembly quality problems are caused by the printing process. Misalignment, edge collapse, adhesion, and less printing that occur during printing are all unqualified and should be reworked. The specific inspection standard should comply with the standard of IPC-A-610C.

(2) Process requirements for mounting components:

In order to obtain the ideal placement quality, the following three elements should be met in the process:
① correct component; ② accurate position; ③ appropriate pressure.
The specific inspection standard should comply with the standard of IPC-A-610C.

(3) Set the process requirements of the reflow soldering temperature curve. The temperature curve is the key to ensure the welding quality.

The heating rate before 160°C is controlled at 1-2°C/s. If the temperature rise rate is too fast, on the one hand, the components and PCB will be heated too fast, which will easily damage the components and cause deformation of the PCB. On the other hand, if the flux in the solder paste volatilizes too fast, it is easy to splash metal components and produce solder balls; the peak temperature is generally set at about 30-40°C higher than the melting point of the alloy (for example, the melting point of 63Sn/37Pb solder paste is 183°C, the peak temperature should be set at around 215°C), and the reflow time is 60-90s.
Low peak temperature or short reflow time will make the welding insufficient and cannot form a certain thickness of intermetallic alloy layer. In severe cases, the solder paste will not melt. If the peak temperature is too high or the reflow time is long, the intermetallic alloy layer will be too thick, which will also affect the strength of solder joints, and even damage components and printed boards.

3. Features of SMT:

(1) High assembly density, small size and light weight of electronic products. The volume and weight of patch components are only about 1/10 of traditional plug-in components. Generally, after SMT is used, the volume of electronic products is reduced by 40% to 60%, and the weight is reduced by 60 _{% %} 80%.

(2) High reliability and strong anti-vibration ability. Solder joint defect rate is low.

(3) Good high-frequency characteristics. Reduced electromagnetic and radio frequency interference.

(4) It is easy to realize automation and improve production efficiency.

(5) Reduce the cost by 30%~50%. Save material, energy, equipment, manpower, time, etc.

3. The development trend of surface mount technology SMT

1. Narrow pitch technology (FPT) is an inevitable trend in the development of SMT

FPT refers to the technology of assembling SMD with pin spacing between 0.635-0.3mm and SMC with length and width less than or equal to 1.6mm and 0.8mm on PCB. Due to the rapid development of computer, communication, aerospace and other electronic technologies, the integration of semiconductor integrated circuits is getting higher and higher, SMC is getting smaller and smaller, and the pin spacing of SMD is getting narrower and narrower. At present, QFP with pin spacing of 0.635mm and 0.5mm has become a communication device in industrial and military electronic equipment.

2. Miniaturization, multi-pin and high integration are the inevitable trend of the development of SMT packaging components

Surface mount components (SMC) are developing towards miniaturization and large capacity. At present, it has developed to a specification of 01005; surface mount devices (SMD) are developing in the direction of small size, multiple pins, and high integration. For example, BGA, which is currently widely used, will develop in the direction of CSP. FC (flip-chip) applications will be more and more.

3. Green lead-free soldering process is a new trend in the development of SMT process

Lead (Pb), is a toxic metal that is harmful to the human body. And it is very destructive to the natural environment. Due to the requirements of environmental protection, especially the introduction of ISO14000, most countries in the world have begun to prohibit the use of lead in soldering materials, that is, lead free soldering. In 2004, Japan banned the production or sale of electronic production equipment that uses soldering materials containing lead. In 2006, Europe and the United States banned the production or sale of electronic production equipment soldered with lead materials. The use of lead-free soldering is the general trend, and some large-scale electronic processing enterprises in China will accelerate the development of lead-free soldering in China.

Four, SMT detailed production process

1.1 Reflow soldering process

Reflow soldering refers to a soldering process that realizes the mechanical and electrical connection between the solder end or pin of the surface mount component and the PCB pad by melting the solder paste that is pre-printed on the PCB pad. The process flow is: printing solder paste – SMD – reflow soldering, as shown in the figure below.

1.1.1 Solder paste printing

Its purpose is to apply an appropriate amount of solder paste evenly on the pads of the PCB to ensure good electrical connection and sufficient mechanical strength when the solder pads corresponding to the SMD components and the PCB are reflow soldered.
How to ensure that the solder paste is evenly applied to each pad? We need to make the stencil. The solder paste passes through the corresponding openings of each pad on the stencil, and under the action of the scraper, the tin is evenly coated on each pad. An example of a stencil diagram is shown in the figure below.

The schematic diagram of solder paste printing is shown in the figure below.

The printed PCB with solder paste is shown in the figure below

1.1.2 SMD

This process is to use the placement machine to accurately mount the chip components to the corresponding position on the surface of the PCB printed with solder paste or adhesive.
SMT machines can be divided into two types according to their functions:
A high-speed machine: suitable for mounting small and large quantities of components: such as capacitors, resistors, etc., and can also mount some IC components, but the accuracy is limited.

B Universal machine: suitable for placing heterogeneous or high-precision components: such as QFP, BGA, SOT, SOP, PLCC, etc.
The equipment diagram of the placement machine is shown in the figure below,

and the PCB after placement is shown in the figure below

1.1.3 Reflow soldering

Reflow soldering is a literal translation of English Reflow Soldering, which is to realize the mechanical and electrical connection between the solder end of the surface mount component and the PCB pad by melting the solder paste on the pad of the circuit board to form an electrical circuit.
Reflow soldering is a key process in SMT production, and reasonable temperature curve setting is the key to ensure the quality of reflow soldering. Improper temperature curves will cause soldering defects such as incomplete soldering, virtual soldering, vertical components, excessive solder balls and other soldering defects on the PCB, which will affect product quality.

The equipment diagram of the reflow soldering furnace is shown in the figure below.

After the reflow furnace, the PCB completed by reflow soldering is shown in the figure below.

1.2 Wave soldering process

Wave soldering is generally a soldering process for plug-in devices. The molten liquid solder, with the help of the pump, forms a solder wave of a specific shape on the liquid surface of the solder tank, and the PCB with components inserted passes through the solder at a specific angle and a certain immersion depth on the conveyor chain. The process of spot welding is realized by wave crest, as shown in the figure below.
The general process flow is: device insertion-PCB loading-wave soldering-PCB blanking-DIP pin trimming-cleaning, as shown in the figure below.

1.2.1 THC Insertion Technology

1. Component pin forming

DIP devices need to reshape the pins before insertion
(1) Reshaping of manually processed components:

Bent pins can be shaped with tweezers or a small screwdriver, as shown in the picture below.

(2) Reshaping of machined components: The machine reshaping of components is done with a special plastic machine. Its working principle is that the feeder uses vibration feeding to feed materials, (such as plug-in triodes) and dividers are used to position the triodes. The first step is to bend the pins on the left and right sides; the second step is to bend the middle pin backwards or forwards. As shown below.

2. Component insertion

Through-hole insertion technology is divided into manual insertion and automatic mechanical equipment insertion
(1) manual insertion and welding, the components that need to be mechanically fixed should be inserted first, such as cooling racks, brackets, clips, etc. of power devices, and then Then insert the components that need to be soldered and fixed. Do not directly touch the component pins and the copper foil on the printed board with your hands when inserting.

(2) Mechanical automatic plug-in (referred to as AI) is a relatively advanced automatic production technology in the assembly of contemporary electronic products. The insertion of automatic mechanical equipment should first insert those components with a lower height, and then install those with a higher height. The valuable key components should be inserted at the end. To be close to the welding process. The assembly sequence of PCB components is shown in the figure below.

1.2.2 Wave soldering

(1) Working principle of wave soldering

Wave soldering is a process of forming a solder wave of a specific shape on the surface of the molten liquid solder by means of pump pressure. When the assembly assembly with components inserted passes through the solder wave at a fixed angle, a solder joint is formed in the pin soldering area. technology. During the process of conveying the components by the chain conveyor, they are preheated in the preheating zone of the welding machine (the preheating of the components and the temperature to be reached are still controlled by the predetermined temperature curve). In actual welding, it is usually necessary to control the preheating temperature of the component surface, so many devices have added corresponding temperature detection devices (such as infrared detectors). After preheating, the components enter the lead bath for soldering. The tin tank is filled with molten liquid solder, and the nozzle at the bottom of the steel tank sprays the molten solder into a wave crest of a fixed shape, so that when the component welding surface passes through the wave, it is heated by the solder wave, and at the same time, the solder wave also wets the soldering area and proceeds Extended filling, finally realizing the welding process. Its working principle is shown in the figure below.

Wave soldering uses the principle of convective heat transfer to heat the soldering area. The molten solder wave acts as a heat source, on the one hand, it flows to scour the pin pads, and on the other hand, it also plays a role of heat conduction, and the pin pads are heated under this effect. In order to ensure that the soldering area heats up, the solder wave usually has a certain width, so that when the soldering surface of the component passes through the wave, there is sufficient time for heating and wetting. In traditional wave soldering, a single wave is generally used, and the wave is relatively flat. With the use of lead solder, it is currently in the form of double waves. As shown below.

The lead of the component is solid, and the solder immersion provides a path for the metalized through hole. When the pin touches the solder wave, the liquid solder climbs up along the pin and the hole wall by virtue of the surface tension. The improved capillary action of the metallized vias facilitates the climb of the solder. After the solder reaches the pad of the PCB, it spreads out under the action of the surface tension of the pad. The rising solder expels the flux gas and air in the hole, thereby filling the hole and finally forming a solder joint after cooling.
(2) The main components of the wave soldering machine

A wave soldering machine is mainly composed of conveyor belt, heater, tin bath, pump, flux foaming (or spraying) device, etc. It is mainly divided into flux adding area, preheating area, soldering area and cooling area, as shown in the figure below.

1.2.3 The main difference between wave soldering and reflow soldering

The main difference between wave soldering and reflow soldering is that the heating source and solder supply method in soldering are different. In wave soldering, the solder is pre-heated and melted in the tank, and the pumped solder wave plays a dual role of heat source and solder supply. The molten solder wave heats the PCB's vias, pads, and component leads, while also providing the solder needed to form the solder joints. In reflow soldering, solder (solder paste) is pre-quantitatively distributed on the soldering area of ​​the PCB, and the function of the heat source during reflow is to remelt the solder.

1.3 Introduction of selective wave soldering process

Wave soldering equipment has been invented for more than 50 years. It has the advantages of high production efficiency and large output in the manufacture of through-hole component circuit boards. Therefore, it used to be the most important soldering equipment in the automated mass production of electronic products. However, there are certain limitations in its application:
(1) The welding parameters are different.

Due to different characteristics of different solder joints on the same circuit board (such as heat capacity, pin spacing, tin penetration requirements, etc.), the required soldering parameters may be quite different. However, the characteristic of wave soldering is that all the solder joints on the entire circuit board are welded under the same set parameters, so different solder joints need to "make up" each other, which makes it difficult for wave soldering to fully meet the requirements of high-quality circuit boards. welding requirements;
(2) higher operating costs.

In the actual application of traditional wave soldering, the whole board spraying of flux and the generation of tin slag have brought higher operating costs; especially in lead-free soldering, because the price of lead-free solder is 3 times that of lead solder Above, the increase in operating costs brought about by the generation of tin slag is amazing. In addition, lead-free solder continuously melts the copper on the pad, which will change the solder composition in the tin pot over time, which requires regular addition of pure tin and expensive silver to solve it; (3) Maintenance and maintenance are troublesome
.

The residual flux in production will remain in the wave soldering transmission system, and the generated tin slag needs to be removed regularly, which brings more complicated equipment maintenance and maintenance work to users; for reasons such as these, selective wave soldering should be
used And born.
The so-called PCBA selective wave soldering still uses the original tin furnace, the difference is that the board needs to be placed in the tin furnace carrier/tray (carrier), which is what we often call the furnace fixture, as shown in the figure below Show.

Then expose the parts that need to be wave soldered and dip them in tin, and cover the other parts with a carrier, as shown in the figure below. This is a bit like putting on a life buoy in a swimming pool. The place covered by the life buoy will not get water. If you change it to a tin stove, the place covered by the vehicle will naturally not get tin, and there will be no Problems with re-soldering or dropping parts.

1.4 Through-hole reflow soldering process introduction

Through-hole reflow soldering is a reflow soldering process for plug-in components. It is mainly used for the manufacture of surface mount boards with a small number of plug-ins. The core of its technology is the application method of solder paste.

1.4.1 Process Flow Introduction

According to the solder paste application method, through-hole reflow soldering can be divided into three types: tubular printed through-hole reflow soldering process, solder paste printed through-hole reflow soldering process, and formed tin sheet through-hole reflow soldering process.
1. Tubular printed through-hole reflow soldering process

Tubular printed through-hole reflow soldering process is the earliest application of through-hole component reflow soldering process, mainly used in the manufacture of color TV tuners. The core of the process is the tubular printing machine for solder paste, and the process is shown in the figure below.
2. Solder paste printing through-hole reflow soldering process

Solder paste printing through-hole reflow soldering process is the most widely used through-hole reflow soldering process at present. It is mainly used for mixed PCBA with a small number of plug-ins. The process is fully compatible with conventional reflow soldering processes and does not require special process equipment. The only requirement is to be The soldered plug-in components must be suitable for through-hole reflow soldering, and the process is shown in the figure below.

3. Formed tin sheet through-hole reflow soldering process

Formed tin sheet through-hole reflow soldering process is mainly used for multi-pin connectors. The solder is not solder paste but formed tin sheet. Generally, the connector manufacturer directly adds it, and only heats it during assembly.

1.4.2 Through-hole reflow soldering design requirements

1. PCB design requirements

(1) Suitable for boards with PCB thickness less than or equal to 1.6mm.

(2) The minimum repayment of the pad is 0.25mm, and the molten solder paste is "pulled" once, without forming tin beads.

(3) The stand-off of components should be greater than 0.3mm

(4) The proper length for the lead wire to protrude from the pad is 0.25~0.75mm.

(5) The minimum distance between 0603 and other fine-pitch components and pads is 2mm.

(6) The opening of the steel mesh can be expanded by a maximum of 1.5mm.

(7) The hole diameter is the lead wire diameter plus 0.1~0.2mm. As shown below.

1. Stencil window opening requirements

Generally speaking, in order to achieve 50% hole filling, the stencil window must be expanded. The specific amount of expansion should be determined according to factors such as the thickness of the PCB, the thickness of the stencil, and the gap between the hole and the lead.
Generally speaking, as long as the external expansion does not exceed 2mm, the solder paste will be pulled back and filled into the hole. It should be noted that the expanded area cannot be suppressed by the component package, or must avoid the package body of the component, and form tin beads on one side, as shown in the figure below.

1.5 Conventional PCBA assembly types and process flow introduction

1.5.1 Single-sided mounting

The process flow is shown in the figure below

1.5.2 Single-sided insertion

The process flow is shown in Figure 5 below

The forming of the device pins in wave soldering is one of the least efficient parts of the production process, which brings the risk of electrostatic damage and prolongs the lead time, and also increases the chance of error.

1.5.3 Double-sided mounting

The process flow is shown in the figure below

1.5.4 Mixed on one side

The process flow is shown in the figure below

. If there are few through-hole components, reflow soldering and manual soldering can be used.

1.5.5 Double-sided mixed packing

The process flow is shown in the figure below.

If there are many double-sided SMD components and few THT components, reflow soldering or manual soldering can be used for plug-in components. The process flow chart is shown in the figure below.