PCB Fabrication

Quick turn PCB fabrication

Publish Date:2020-11-06

There are two main methods for quick turn PCB fabrication: chemical mode and physical mode. Nowadays, the most popular PCB manufacturing methods in high school are heat transfer printing and physical engraving. The former belongs to the field of chemistry and the latter belongs to the field of physics.
Heat transfer printing.
First of all, ordinary laser printer is used to print PCB images on special heat transfer paper. Then, the PCB image that has been printed on the heat transfer paper is transferred to the copper-clad laminate (CCL) through the heat transfer device. Finally, the high-precision PCB is produced after the subsequent process (including etching and PCB drilling). In the high school laboratory, heat transfer printing can be used to produce high-precision PCB and can be completed in an hour. When dual-sided PCB manufacturing is involved, the outstanding problem is that the misalignment that has not been successfully solved by the current manufacturing method is not used. Therefore, the double-sided PCB manufactured by heat transfer printing method can not achieve the best work on electronic products.

Quick turn PCB fabrication

Thermal transfer printing adheres to the following PCB manufacturing process:
1。 The bottom PCB image is printed on a piece of heat transfer paper by an ordinary laser printer.
2。 The image of the top PCB is printed on another sheet of heat transfer paper by a normal laser printer.
3。 Two pieces of heat transfer paper are tightly attached to the cut double-sided CCL. The transparent tape can be used to fix the position of paper and plate to avoid dislocation.
4。 Heat transfer printing is carried out by sticking CCL and heat transfer paper in the heat transfer equipment. Then the CCL with PCB image is obtained after cooling.
5。 Then etching and PCB drilling are performed.
The process can be summarized as follows:
Laboratory quick turn PCB fabrication method
In all stages of heat transfer printing of quick turn PCB fabrication, the application of step 3 can accurately lead to the alignment of two heat transfer sheets. However, when step 4 is involved, misalignment often results in heat transfer equipment. Therefore, it is difficult to successfully manufacture double-sided PCB unless several tests are carried out.
· physical sculpture.
According to the basic principle of physical engraving, physical engraving conforms to the milling principle, milling out extra or unnecessary parts of CCL. Application equipment is actually a small CNC drilling and milling machine, also known as a circuit board engraving machine. In the process of double-sided quick turn PCB fabrication, the board should be turned over to mill on the other side of the board after milling on one side. The overturning process may cause dislocation, which should be corrected by software and hardware to reduce the reject rate of dual-sided PCB.
Comparison of thermal transfer printing and physical engraving.
According to the comparison of the two methods, heat transfer printing is better than physical engraving in the following aspects: 1. CNC drilling and milling machine has a higher cost and takes up more laboratory space. However, compared with NC drilling and milling machine, heat transfer machine has the characteristics of low cost and small space occupation.
2。 When using heat transfer printing, it can complete double-sided PCB in one hour. Although there may be multiple thermal transfer printing dislocations, it is possible to produce dual-sided PCB within 3 hours at most. However, CNC drilling and milling machines take more time, at least four hours. In addition, CNC drilling machines and milling machines can only process one PCB at the same time, while heat transfer printing can produce multiple double-sided PCBs at the same time.
An improved heat transfer printing method.

Quick turn PCB fabrication

In order to solve the problem of dislocation in traditional heat transfer printing, this paper will provide an improved quick turn PCB fabrication method, the following steps.
1。 The bottom PCB image and mirror PCB image are printed on the same heat transfer paper, and the holes of the bottom PCB image and the top PCB image are linearly symmetrical.
2。 The heat transfer paper is folded along the symmetry line and covered on the well-cut double-sided CCL.
3。 Then the double-sided CCL with heat transfer paper is put into the heat transfer printing equipment for heat transfer printing. The folding part first enters the heat transfer equipment, and after cooling, the CCL printed with a PCB image can be obtained. Etching and PCB drilling.
By printing the top and bottom PCB images on the same heat transfer paper, the above process can effectively solve the misalignment problem of quick turn PCB fabrication in traditional heat transfer printing. Therefore, reliable double-sided PCB can be manufactured smoothly at one time.
PCB image printing on the same sheet of heat transfer paper can be performed based on any PCB design software with different holding. All in all, this improved PCB manufacturing heat transfer printing method is the most effective for electronic professionals or engineers who can be used in the laboratory to produce dual-sided PCB.

Quick turn PCB fabrication
   Some companies spend a lot of time in product design and development. They have to find factories that can quickly turn PCB fabrication because they need to submit finished products to customers as soon as possible. PCB production is usually high-speed, and the main thing is to improve the placement time. Rapid PCB sample placement processing is a common placement technology in the electronic industry; SMT technology can mount smaller and lighter components to make the circuit board achieve high precision and bring certain convenience to life. Do you know the common problems of quick turn PCB prototype assembly? What problems should we pay attention to when reprocessing?

Daily problems and process matters of quick turn PCB fabrication chip sorted out by IDEANSPCB.
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 adhesive storage life 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, IDEANSPCB 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.