The Most Complete PCB Assembly (PCBA) Process Tutorial

 

Step 1: Solder Paste Printing

Solder paste printing is the first step in PCBA processing. We apply solder paste to the area of the printed circuit board where components need to be assembled. This step is accomplished by applying solder paste on the stainless steel template, which corresponds to the circuit board, and the solder paste drips from the holes of the template onto the PCB. 96.5% tin is gray and it contains 3% silver and 0.5% copper. In order to protect the environment, FS Technology uses lead-free solder paste. The solder paste will be heated and melted in step 3 and form a strong joint

Step 2: Pick and Place of Components

The second step of the PCBA processing flow is to place the components on the PCB, which is completed by a high-speed SMT placement machine. At design time, the PCB designer creates a file, which is used to control the SMT device. The file identifies the X, Y coordinates of the PCB assembly, and thanks to this information, the pick and place machine accurately places the SMD components on top of the solder paste through the vacuum nozzle.

At the beginning, humans completed the picking and placement of components by manual soldering, but this is obviously not good for large-volume orders. In order to ensure fast turnaround, we use SMT assembly machines, which can ensure the consistency and accuracy of PCBA.

Step 3: Reflow oven melts and cures solder paste

We need to use a reflow oven to melt and solidify the solder paste. It is best to use equipment with ten temperature zones for this process. The PCB is transferred to the reflow oven through the docking station. The oven can generate a maximum temperature of 250 degrees Celsius, which is enough to completely melt the solder paste. The melted solder paste will form a joint at the junction of the PCB and the component, forming a strong solidified solder joint after entering the cooler, which will create a permanent joint between the SMD component and the PCB.

Step 4: QC Check

Due to the influence of many factors, the soldering quality of the circuit board may be poor. Since the circuit board is an essential component in electronic equipment, no matter how small the error is, it will lead to a circuit board short. Therefore, we need to use a variety of detection methods to ensure its quality.

Manual Inspection

Almost all formal PCBA processing companies will require operators to conduct detailed and comprehensive PCB manual inspections. For a well-equipped assembly plant, the purpose of manual inspection is just a guarantee. Different components are installed on the PCBA board, while SMD patch components are smaller, and visual inspection of the circuit board may cause inaccurate detection due to fatigue of technicians. This detection method is more friendly to DIP components.

AOI Optical Inspection

In high-volume assembly orders, AOI inspection is considered effective. This detection method refracts light through the PCBA solder joints, and observes the solder joints from multiple angles to ensure consistency with the standard board. The biggest advantage of this detection method is that it is fast and can process batch orders in a short time.

X-Ray Inspection:

X-ray machines allow operators to observe the internal structure of printed circuit boards to find defects in their inner layers. This kind of testing equipment requires customers to pay extra fees, and usually we use it in complex and advanced circuit boards. If your assembly plant does not have skilled inspectors, please do not attempt this method of PCB inspection as it may result in rework or scrapped PCBs.

Step 5: Component Soldering

Component introduction

The PCB itself does not function and provides a carrier for electrical connections to electronic components. Various components are installed on different PCBA boards, including DIP components and SMD components.

DIP components are a larger and more common circuit board component that are mounted to a circuit board through through hole assembly. These holes are classified as plated through holes (PTH) and non-plated through holes (NPTH). The operator passes the pins of the DIP component through the holes in the PCB. These holes are connected to other holes and vias with copper traces. When these DIP components are inserted and soldered on the PCB, they make electrical connections with other holes in the same PCB as the designed circuit.

SMD components are the most popular electronic components in modern electronic equipment. The IC chip we often say is one of the SMD components. These components are soldered on the surface of the PCB in the form of patches. With the development of miniaturization trend, SMT assembly technology has gradually replaced DIP assembly.

Welding type

The three most common soldering processes are manual soldering, wave soldering and reflow soldering.

Manual soldering is a way for electronics enthusiasts to make their own PCBA boards. Electronic enthusiasts insert the pins of the DIP component into the holes of the PCB, and then solder the pins on the soldering surface to death by a welding machine.

Reflow soldering is usually used in the SMT assembly process, which can easily complete the soldering of SMD components and ensure that these components are not damaged. When you choose a PCBA processing factory, please pay attention to whether the reflow soldering equipment used by the service provider is in ten temperature zones.

Wave soldering is an upgraded version of manual soldering, which can complete the soldering work automatically. Once the PTH components are placed on the PCB, the PCB is placed on a conveyor belt and moved to a dedicated oven. Here, a wave of molten solder splashes onto the bottom layer of the PCB, where the component leads are located. This will solder all pins at once.

Step 6: PCB Functional Test

When the PCBA is manufactured we need to test it. Functional test is an essential PCB test, which refers to checking whether the assembled PCBA can operate according to the design.

If any parameter of the PCBA shows unacceptable results, discard or scrap the PCBA following company standard procedures. The testing stage is very important because it determines the success or failure of the whole PCBA process.

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Important components of printed circuit boards

If you've seen what a printed circuit board looks like before, you know that there are many components on a printed circuit board. The one-stop PCB assembly company installs these pcb components on the bare pcb board in various ways. These components are an important part of the circuit board. FS Tech will introduce you to several particularly important PCB components.

PCB component one: varactor diode

The varactor diode is specially designed according to the principle that the capacitance can change with the change of the applied reverse voltage. It is more capable than conventional diodes. This special diode is generally used in high-frequency modulation circuits such as mobile phones. It can modulate low-frequency signals to high-frequency signals and transmit them. During operation, the voltage of the varactor diode is generally added to the negative electrode, so that the internal junction capacitance of the varactor diode changes with the change of the modulation voltage.

When the varactor diode fails, the following two situations mainly occur:

1. When leakage occurs: the high-frequency modulation circuit will not work or the modulation performance will deteriorate.
2. When the variable capacitance performance becomes poor: the operation of the high-frequency modulation circuit is unstable, and the information received by the other party will be distorted.

When one of the above situations occurs, FS Technology recommends that you replace the varactor diode of the same type. We have strong pcb component procurement ability, if you want to assemble this special pcb original to your pcb board, FS Technology is happy to serve you. We're pretty good at dealing with these varactors.

PCB Component 2: Transistor

In PCB components, it is usually represented by "Q" plus a number, for example: No. 17 transistors are represented by Q17.

Features of transistors

There are 2 PN junctions inside, and this pcb component has amplification capability. There are two types of transistors: NPN type and PNP type. Generally speaking, on an assembled PCB you will find two types of transistors, NPN and PNP, to make them complement each other. The so-called pair tube in the OTL circuit is paired with the PNP type and the NPN type. The components used in the PCB board used in the phone are usually: A92, 9015, A42, 9014, 9018, 9013, 9012 and other models.

PCB component three: FET

It can be extremely low noise and a high degree of input impedance. A FET is a voltage-controlled element, while a transistor is a current-controlled element. When your PBC board only allows a small current to pass, FS Technology recommends that you should choose a field effect transistor. The transistor is selected when the voltage is low and the signal is poor. We have a strong ability to purchase PCB components, whether it is a special transistor or a field effect transistor can be purchased for you. Among our customers, most electronic equipment manufacturers choose field effect transistors, because using field effect transistors as the input stage of the entire electronic equipment can obtain performance that is difficult to achieve with ordinary transistors.

More knowledge of important PCB components can be found in FS Technology's PCBA blog.

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PCB-FS technical drawing

1. The circumference of the circle should be very obvious.

According to the performance of the circular signal, the vehicle characteristics, vehicle grades, and local signs (called instrument grade points) are divided.
2) In the FS technique, the grinding process and the grinding edge should not freeze, there should be at least 3 L marks and the points showing the diagonal marks are not equal between the centers.
3) If the part is on both sides, write the part on both sides.
4) Write as many points as you need to collect on the baord. Do not mark on boards without spaces.
5) Mark on the BGA the closest QFP connected between the battery hagati ≤ 0.5 mm and 0.8 mm apart.
 

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Basic knowledge of PCB board making-FS Technology

PCB concept

PCB can be said to be common or uncommon in our lives. It is an important electronic component and a support for electronic components. All electronic products in our lives contain PCB boards. The full name of PCB is Printed Circuit Board, which is called "printed" circuit board because it is made by electronic printing.

The role and function of PCB in various electronic devices
1. Pad: Provide mechanical support for fixing and assembling various electronic components such as integrated circuits.​​
2. Wires: wires used to connect various electronic components, which can help circuit boards achieve wiring and electrical connection (signal transmission) or electrical insulation between components. Provide the required electrical characteristics.
3. Green oil and silk screen: Provide solder mask graphics for automatic assembly, and provide identification characters and graphics for component insertion, inspection, and maintenance.

Development history of PCB technology

Since the advent of PCB, if we look at the development of PCB assembly technology, FS Technology has divided it into three stages

THT-Through Hole Technology Stage

The role of metallized holes:

(1). Electrical interconnection---signal transmission
Literally, the electrical appliances are directly connected or indirectly connected through transformers and the like
(2). Supporting components---pin size limits the reduction of through hole size
a. Rigidity of the pins
Pin, also called pin. It is the wiring from the internal circuit of the integrated circuit (chip) to the peripheral circuit, and all the pins constitute the interface of this chip. A segment of the end of the lead, which is formed by soldering to form a solder joint with the pad on the printed board. Pin can be divided into bottom, toe, side and other parts.
b. Requirements for automated instrumentation

There are three ways to increase density

(1) Under the regulations (aperture ≥ 0.8mm) to reduce the size of the device hole, why is there a regulation? This is because of the rigidity of the component Pin pins and the limitation of the insertion precision.
(2) Control line width/spacing: 0.3mm—0.2mm—0.15mm—0.1mm
(3) Increase the number of layers of the circuit board: the better the technology, the more layers can be added, FS technology can achieve all the layers, 64 layers - 12 layers - 10 layers - 8 layers - 4 layers - double-sided - single noodle,

SMT-Surface Mount Technology Stage PCB

1. The role of the via hole: it only plays the role of electrical interconnection, the aperture can be as small as possible, and the hole can also be plugged.
2. The main way to increase the density
①. The size of the via hole decreases sharply: 0.8mm—0.5mm—0.4mm—0.3mm—0.25mm
②. The structure of the via has undergone essential changes:
a. Advantages of buried blind hole structure: increase wiring density by more than 1/3, reduce PCB size or reduce the number of layers, improve reliability, improve characteristic impedance control, reduce crosstalk, noise or distortion (due to short lines, small holes )
b. Hole in pad eliminates relay holes and connections
③ Thinning: Double-sided: 1.6mm—1.0mm—0.8mm—0.5mm
④PCB flatness:
The warpage of the PCB board substrate and the coplanarity of the land surface on the PCB board.

PCB warpage is the combined result of residual stress due to mechanical, thermal, etc.

Surface coating of connection pad: PCB surface coating (plating) coating technology The surface coating (plating) coating technology described in this article refers to the solderability for electrical connections other than solder resist (solder mask, solder mask) Or contactable coating (plating) coating. These surface coatings protect the fresh copper surface. Mainly ASL, electroless NI/AU, electroplating NI/AU…

CSP - Chip Scale Packaging Stage

Compared with traditional packaging, advanced packaging improves the integration density and interconnection speed of chip products, lowers the design threshold, and optimizes the flexibility of function matching. For example, flip-chip interconnects the chip and the substrate, shortens the interconnection length, and realizes the enhancement of chip performance and the improvement of heat dissipation and reliability. CSP has begun to enter into a drastic change and development, promoting the continuous development of PCB technology, and the PCB industry will move towards the laser era and nano era.

FS Technology's PCB surface coating technology

In order to ensure that the copper surface of the solder pad on the PCB is not oxidized and polluted before soldering, the circuit boards produced by FS Technology must be protected by a surface coating (plating) layer, and the surface coating (plating) layer must meet the necessary and sufficient conditions to achieve the purpose.

Copper is second only to silver as an excellent conductor and a metal with good physical properties (such as ductility, etc.). In addition, the reserves are quite abundant and the cost is not high. Therefore, copper is selected as a conductive material for PCB. However, copper is an active metal, and its surface is easily oxidized to form an oxide layer (copper oxide and cuprous oxide). This oxide layer often causes solder joint failures and affects reliability and service life. According to statistics, 70% of PCB failures come from the solder joints. The main reasons are: (1) due to the surface contamination of the pad, oxidation and other components of incomplete soldering, virtual soldering, etc.; (2) due to the mutual between gold and copper For diffusion, a diffusion layer is formed or an intermetallic compound is formed between tin and copper, thereby causing failures such as interface looseness and embrittlement. Therefore, the copper surface of the PCB used for soldering must be protected by a solderability protective layer or a solderability barrier layer in order to reduce or avoid failure problems.

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