PCB Fabrication

PCB fabrication and assembly

Publish Date:2020-11-06

Up to 80% of the final product cost depends on how it is designed (the rest is usually paid for by management fees and capital costs). Of course, it is very important to reduce the product cost in design to produce a successful and cost-competitive final product. Manufacturing and assembly design is a formal method of checking the cost of product components and assembly, which aims to reduce costs before starting actual production. This paper will first discuss the manufacturing and design of the assembly concept in general, and then discuss in detail the PCB design related to manufacturing and assembly design in the following items. Finally, the last article concludes this series with the most common PCB design issues.
Before continuing to describe, it is necessary to discuss how the term “manufacturing design” is used in more general terms and more specifically in PCB manufacturing. In general, design for manufacturing and design of assembly can refer to the simplification and optimization of prototype or conceptual design to prepare for its manufacturing. When these terms are used to discuss PCBs, they usually mean examining potential manufacturing problems more directly. The first entry in this series will use the previous definition, as we discuss concepts in a broad sense, and the second and third will use the latter definition as we shift our focus to PCB manufacturing and assembly.

PCB fabrication and assembly

Overview of PCB fabrication and assembly design
In general, the purpose of discussing manufacturing and assembly design is to determine how to design products that can be manufactured and assemble them in the most cost-effective manner. Manufacturing design (DFM) involves reducing overall production costs, and more obviously, assembly design (DFA) involves reducing material inputs, indirect capital costs, and reducing labor. Both of them focus on the application of standards to reduce production costs and also seek to shorten the product development cycle. The combination of the two methods is often referred to as manufacturing and assembly design (DFMA). Two types of analysis will be discussed in the following sections, as they are closely related and the two terms are often used interchangeably.
The PCB design of general rule PCB fabrication and assembly conforms to
After the first conceptual design was created, DFMA analysis began. The conceptual design may involve creating prototypes or developing new versions of the product. After you create a conceptual design, you can check the bill of materials (BOM) for that design through DFMA analysis. DFMA adheres to the following rules:
Reducing the number of components in PCB design is a direct goal and has obvious advantages. It will reduce design costs and assembly complexity, although not obvious, but it has great benefits. For example, when picking and placing machines to fill PCB components, they are limited to the number of components they can support in a single pass. Note that picking up and placing the number of components used by the machine when assembling circuit boards can result in an insignificant cost reduction. For example, if the design requires a 20K resistor and a 10k resistor has been used in the design, it may actually be cheaper to use two 10K resistors in series, which can reduce the number of times the machine is picked up and placed. Similarly, looking for standard ICs that can integrate part of your design into a single IC can speed up assembly time and transfer some of the test requirements to the IC manufacturer. Therefore, paying attention to the number and type of PCB components may be the most important step to reduce the overall PCB production cost. In short, if the final design does not require parts, eliminating them will reduce BOM costs, procurement costs, processing time, test time, and assembly labor input.

Development of modular design

PCB fabrication and assembly

If these modules can be used in a variety of different products, consider splitting the PCB design into functional modules. Increasing the number of specific modules ordered from the manufacturer can significantly reduce the cost per unit of that module. It is also worth noting that using modules can reduce the cost and complexity of testing completed components by simplifying the testing process. Smaller systems themselves are easier to test and repair than larger systems. Obviously, the cost benefits you can get from modular design applications must be weighed against the increased interconnection costs associated with using multiple modules. Other advantages of modular design include ease of design updates, standardization of subsystems across multiple products, and simple troubleshooting of product subsystem design failures.
Strive to use standard components
Using standard components can greatly reduce the time and cost of design and development. There is no doubt that specifying a complex custom solution will greatly increase the upfront cost of any product and may make the design infeasible. Using more common components can also simplify production.
Product supply chain and reduce component supply problems. Another advantage of preferring standard components is that their footprints are easier to verify before they are used in PCB design.
Rely more on components

PCB fabrication and assembly

As long as electrical components have multiple uses in design, PCB designers should use them. For example, using an enclosure that can also be used as a radiator in a design can significantly save design costs. Another example of a dual-purpose device is to use support as a ground connection from the PCB to the PCB housing through the mounting holes connected on the PCB.
Design modules used in a variety of products
The use of standard parts in a range of products can reduce processing costs and achieve mass purchase costs. This concept can also be extended to product modules. If a module can be used for more than one product, higher production can reduce the cost of the module and ultimately the cost of the finished product.
Design easy to manufacture
Choosing PCB materials that need less processing in the manufacturing process can greatly simplify product manufacturing. Avoiding operation (e.g. the shell must be painted with the appropriate housing material) eliminates the entire manufacturing step and reduces the cost of the product. In addition, ensuring that design components do not produce excessive tolerances eliminates time-consuming and expensive part rework during assembly.
Reduce and avoid fasteners if possible
When PCB is to be assembled, as, with all products, it is more expensive to use fastener mounting components rather than press-fit mounting technology. To take advantage of this, try reducing the use of fasteners in assembly. One method is to use a surface-mount version of the power IC and integrate heat dissipation into the design of the circuit board. For example, switching from the TO-220 version of IC with an external heat sink to the D2Pak version using PCB as the integrated heat sink can save a lot of money in the final design.
minimize assembly orientation
If possible, install all parts along one shaft starting from the same side of the assembly. This is often referred to as a “top-down” component, where all components are installed from the top to the final component. The time required to rotate and rotate the product in the assembly process can be saved by using this single side assembly process. Therefore, as with all design decisions, PCB design engineers must weigh whether to better produce smaller PCBs with components placed on both sides of a circuit board, rather than designing a larger PCB, where the components are only placed on one side of the board (part has the ability to handle single-sided PCB assembly and double-sided PCB assembly).
· maximize component layout acceptance
Engineers should design PCB to reduce component installation errors. This can be achieved by using components with higher dimensional tolerances (higher pin spacing) or avoiding problems such as tombstones. The failure rate of components can be greatly reduced by using parts designed with high horizontal placement tolerance. In addition, using a rigid and predictable size infrastructure can also improve the rate at which components are placed correctly. In addition, machine vision type feedback systems and other forms of feedback enable placement automation processes to significantly increase production.
? minimize repositioning and handling during PCB assembly
Any time the PCB is repositioned during assembly increases the time required to assemble components on the PCB. It is easy to understand that as long as both sides of the PCB and components are installed on the front and back of the PCB, relocation will occur. If possible, use all surface mount components on one side of the board. The reflow soldering procedure may be used to limit the soldering steps of a single soldering component to the surface.
Advantages of PCB design for PCB fabrication and assembly
Fewer parts to be processed
It can reduce the cost of the bill of materials.
Treatment costs can be reduced to some extent.
Labor and energy input can
The overall manufacturing time can be shortened, thus greatly improving the manufacturing efficiency.
The lower the complexity, the higher the reliability.
The products can be more competitive.
Higher profits will be achieved.

Daily problems and process matters of quick turn PCB fabrication chip sorted out by IDEASPCB.
1、 What defects often appear in SMT adhesive

  1. SMT SMT components offset a bad phenomenon of high-speed SMT machines. The main reasons are the offset in the X-Y direction when the printed circuit board moves at high speed. This phenomenon is easy to occur on components with a small area of SMT adhesive. The reason is that the adhesive force is not good.
    The reason is that the glue amount under the components is inconsistent (for example, there are two glue points under the IC, one is larger and the other is smaller), the glue strength is not balanced when it is heated and cured, and the end with less glue amount is easy to offset.
  2. The causes of the missing parts are very complex
    The adhesive force is not enough. The wave crest is impacted before welding. There are many residues on some components. A colloid is not resistant to high-temperature impact.
    When patch adhesives are mixed, patch adhesives’ chemical composition from different manufacturers is very different, and the mixed-use is easy to produce many adverse effects.
    It is difficult to cure, the adhesive force is not enough, and the parts are welded off seriously.
    The solution is: thoroughly clean the screen, scraper, dispensing head, and other parts that are easy to cause mixing, and avoid mixing different brands of patch adhesive.
  3. The thrust strength requirement of the 0603 capacitors is 1. 0 kg, the resistance is 1. The thrust strength of 5kg, 0805 capacitor is 1. 5kg, resistance is 2. 0 kg, which means the strength is not enough.
    Reasons for insufficient thrust:
    The glue is not enough, the glue is not solidified, the PCB or components are polluted, and the glue itself is brittle and has no strength.
  4. A 30ml syringe adhesive needs to be hit by air pressure tens of thousands of times before Using it up. Therefore, the chip adhesive itself must have excellent thixotropy. Otherwise, it will cause instability of adhesive points and too little adhesive, which will lead to insufficient strength and cause components to fall off during wave soldering.
    On the contrary, too much glue, especially for small components, is easy to stick to the pad and hinder the electrical connection.
  5. The screen used for printing is not cleaned regularly. What should clean it with ethanol every 8 hours? The colloid has impurities. The mesh opening is unreasonable and too small, or the dispensing pressure is too small, so the designed amount of glue is insufficient. There are bubbles in the colloid. If the dispensing head is blocked, clean the dispensing nozzle immediately. If the preheating temperature of dispensing head is not enough, What should set the dispensing head temperature at 38 ℃.
  6. Wire drawing refers to the phenomenon that the adhesive can not be disconnected when dispensing. The adhesive is connected in a filiform way in the direction of dispensing head movement. There are many wires, and the adhesive is covered on the printed pad, which will cause poor welding. Especially when the size of the nozzle is large, this phenomenon is more likely to occur.
    The drawing of SMT adhesive is mainly affected by the drawing property of its main component resin and the setting of spot coating conditions.
    (1) Increase the dispensing stroke, reduce the movement speed, but it will reduce your production beat.
    (2) The lower the material’s viscosity and thixotropy, the smaller the drawing tendency, so try to choose this adhesive.
    (3) Adjust the thermostat temperature a little higher and forcibly adjust it to low viscosity and high thixotropy adhesive. At this time, We should also consider the storage life of the adhesive and the pressure of the dispensing head.
  7. The fluidity of the adhesive will cause collapse. The common problem of collapse is that it will cause collapse if it is placed too long after spot coating. If the adhesive is extended to the printed circuit board pad, it will cause poor welding. Moreover, for the components with relatively high pins, the collapsed chip adhesive can not touch the components’ main body.
    It will cause insufficient adhesive force, so it is difficult to predict the adhesive collapse rate, which is easy to collapse, so it is difficult to set the initial coating amount.
    2、 What should be paid attention to when processing quick turn PCB fabrication
  8. Precautions for SMT paste processing:
    (1) Storage temperature: 5 ℃ – 10 ℃ in refrigerator is recommended. Do not lower than 0 ℃.
    (2) Out of the warehouse principle: must follow the first in first out, do not cause solder paste in the freezer storage time is too long.
    (3) Thawing requirements: take out the solder paste from the freezer and thaw it naturally for at least 4 hours. Do not open the bottle cap when thawing.
    (4) Production environment: the recommended workshop temperature is 25 ± 2 ℃, and the relative humidity is 45% – 65% RH.
    (5) Used old solder paste: it is recommended to use up the solder paste after opening the lid within 12 hours. If you need to keep it, please fill it in a clean empty bottle, and then seal it, and put it back in the freezer for storage.
    (6) Paste amount on the steel wire: the paste amount on the steel wire should not exceed 1 / 2 of the scraper’s height when printing and rolling. Observe frequently, add frequency and add less.
  9. Precautions for printing in quick turn PCB fabrication process:
    (1) Scraper: the scraper is made of steel scraper, which is conducive to the forming and stripping of solder paste printed on the pad.
    Scraper angle: 45-60 degrees for manual printing; 60 degrees for machine printing.
    Printing speed: manual 30-45mm / min; printing press 40mm-80mm / min.
    Printing environment: temperature 23 ± 3 ℃, relative humidity 45% – 65% RH.
    (2) Steel mesh: the thickness of steel mesh and the shape and proportion of opening are selected according to the products’ requirements.
    QFP / chip: the center spacing is less than 0. 5 mm, and 0402 chip needs laser drilling.
    Inspection of steel mesh: the tension test of steel mesh shall be conducted once a week, and the tension value shall be more than 35n / cm.
    Clean the steel mesh: when printing 5-10 pieces of PCB continuously, wipe it with dust-free screen paper once. Do not use rags.
    (3) Cleaner: IPA solvent: use IPA and alcohol solvent when cleaning steel mesh. Do not use solvent containing chlorine because it will damage the composition of solder paste and affect the whole quality.
    Also, ideas pub would like to emphasize that SMT Mounter is a high-tech product. Safe and correct operation is essential for both machines and personnel. The most basic thing for the placement machine’s safe operation is that the operator should have the most accurate judgment and follow the following basic safety rules.
    To improve quick turn PCB To improve the efficiency and speed of the fabric, the machine operators should receive the operation training according to the correct method; the power should be turned off when checking the machine, replacing parts or repairing and internal adjustment (What must maintain the machine by pressing the emergency button or power supply; “reading coordinates” and the machine should be in your hands when adjusting to stop at any time, to ensure that the “interlocking” safety device stops at any time Machine, machine safety detection, etc. can not skip, short circuit, otherwise, it is easy to personal or machine safety accidents; in the production process, only one operator is allowed to operate a machine. During operation, all parts of the body, such as hands and heads, are out of the machine’s working range. The machine must be properly grounded (not connected to the neutral line). Do not use the machine in a burning gas or filthy environment.

We all know that circuit boards are widely used, and many devices need surface mount PCB assembly.
We can even say that PCB is the backbone of an electrical appliance. But most people don’t know much about surface mount PCB assembly. Do you know the performance characteristics of the circuit board? What are the methods of the circuit board line?
With these questions, let’s follow the professional PCBA factory idea PCB to look at the following idea PCB about “circuit board features and related knowledge introduction.”

PCB fabrication and assembly

performance characteristics of PCB in surface mount PCB assembly

  1. Internal circuit board structure
    The circuit board of the air conditioner is usually referred to as PCB in technical data. Generally, the circuit boards of air conditioners are in the indoor units. However, due to air conditioners’ different sizes and functions, some external units of air conditioners also have circuit boards, and some internal units of air conditioners have multiple circuit boards. Connectors usually connect the circuit boards.
    (1) Single board circuit air conditioner
    Air conditioner single-board circuit refers to the air conditioner circuit composed of a circuit board, common in hang up. The circuit board of the general single-board circuit air conditioner mainly includes a CPU control circuit, power supply circuit, signal drive circuit, wind direction control circuit, internal fan control circuit, etc.
    In addition to this main circuit board, there is usually a small circuit board in the room. The main circuits in the small circuit board are the indicator circuit and remote control receiving circuit. Several wires are connected between the main circuit and the small circuit board through connectors.
    (2) Double board circuit air conditioner
    The air conditioner’s double board circuit refers to that the air conditioner circuit is mainly composed of two circuit boards. The control circuit is usually called the computer board; the other is the power circuit and signal drive circuit, the power driver board. Connectors connect the two circuit boards with multiple wires. The main functions of the connecting wires are the DC power supply and control signal.
    (3) Remote control and indication circuit board
    Common air conditioners have remote control and indication functions. Generally, the remote control receiving and indication function circuit is made into a separate small circuit board connected with the main control circuit board by several wires. The circuit board mainly has remote control receiving head and several light-emitting diodes commonly used on the hook.
    The remote receiver’s function is to receive the signal from the remote controller and send it to the CPU after conversion.
    Led generally has two red and green or three red, green, and yellow, mainly used to indicate the power status, compressor working status, and another related working status.
    (4) Air conditioner circuit board with the display screen
    The air conditioner circuit board with a display screen is more common in the cabinet machine, and a few hanging machines also have a display screen. Generally, the internal unit comprises two electric boards. The control board is composed of a control circuit, power supply circuit, signal drive circuit, and the other is the air conditioner circuit board with the display screen.
    The circuit board of the air conditioner with the display screen, the internal circuit mainly includes a display circuit, a remote control receiving circuit, key matrix circuit, etc., which are connected with the main control circuit board through several wires. Air conditioner circuit board with display, some air conditioner control circuit will also do on the display board.
    The common display screen generally has a led digital tube display, LCD liquid crystal display, VFD fluorescent display, etc.
  2. External circuit board structure
    In addition to the air conditioner’s internal unit, the external unit of the air conditioner will also be equipped with an external circuit board according to the different control circuit structure.
    [two PCB stepping methods of surface mount PCB assembly]
    In today’s society, electronic products’ coverage rate is increasing, and all electronic products need surface mount SMT PCB assembly service. Unlike in the 1970s and 1980s, TV was hard to see, which can’t be compared with now. There is a direct connection between electronic products and circuit boards. Without surface mount SMT PCB assembly service, there will be no electronic products. But how does the circuit board come into being? The PCBA professional manufacturer IDEASPCB introduces the wiring method of the circuit board.
    The first method is manual wiring.
    The first step is to ensure the level and size of the circuit board.
    The second step is to place the package and arrange the package.
    Third, according to the circuit board’s schematic diagram, you can directly use the place / interactive routing menu for manual wiring.
    The fourth step is to modify the circuit board, save or print files.
    Manual routing is more difficult to operate and less efficient, but it is generally better than automatic routing.
    The second method is automatic routing.
    The first step is to use the schematic editor to design the schematic, conduct an electrical check (ERC), and generate the schematic network list.
    Second, enter the PCB environment, use the circuit wizard to determine the number of layers, size, and other circuit board parameters.
    The third step is to use the design/netlist menu to call in the network table. Note: at this time, it is easy to see the common errors that the encapsulation in the network table and the component encapsulation in the encapsulation library do not conform to each other.
    The fourth step is to arrange the components and distribute them reasonably on the circuit board. Automatic placement of components or manual placement of components notes that this action often has to be arranged many times to achieve the effect.
    The fifth step is to set up automatic routing rules and automatic routing.
    The sixth step is to finish the decoration. Save or print out the circuit board file.
    The automatic wiring method is easy to operate and has high efficiency, which is usually worse than manual wiring.
    There are two different wiring methods; each has its own advantages. See which occasion to use, you can use different wiring methods according to different situations.
    After the PCB design is completed, PCB production will be carried out, and then surface mount PCB assembly will be carried out to complete the entire PCB assembly. Finally, a functional test will be conducted on the finished PCB assembly. Only if the test is qualified can it be used, and if it is not qualified, Who can not use it. However, there are many problems to be considered when testing PCBA. First of all, we should know the main content of PCBA, surface mount SMT. What are the basic contents of PCB assembly during testing?
  3. ICT test mainly includes circuit on-off, voltage and current value and fluctuation curve, amplitude, noise, etc
  4. FCT test needs IC program burning, simulate the function of PCBA board, find the problems in hardware and software, and equip with necessary production fixture and test rack
  5. Fatigue test is mainly to sample PCBA board and perform the high-frequency and long-time operation of the function, observe whether there is a failure, judge the probability of failure in the test, to feedback the working performance of PCBA board in electronic products
  6. In a harsh environment, the PCBA board is mainly exposed to the temperature, humidity, drop, splash, and vibration of the limit value to obtain the test results of random samples to infer the reliability of the whole PCBA board batch
  7. The aging test is mainly to power on PCBA board and electronic products for a long time, keep them working, and observe any failure. After the aging test, Who can sell electronic products in batches?
    The process flow of surface mount PCB assembly is complex. In production and processing, there may be various problems due to improper equipment or operation, which can not guarantee that the products produced are qualified. Therefore, a PCBA test is needed to ensure that each product will not have quality problems