Circuit board modification process analysis

Due to design changes, manufacturing and other reasons, modification work is almost inevitable in many electrical and electronic product development units. The so-called modification refers to the modification of the function of the product in order to meet the new standard. Such as changes in circuit connections or components due to changes in design schemes and technical status, that is, the increase or decrease of wires, the increase or decrease or change of components, and the corresponding structural changes brought about.

The aerospace standard QJ2940A-2001 stipulates: "The modification of the printed circuit board assembly refers to the change of the connection characteristics. The change of this characteristic is achieved by cutting off the printed wires, adding components, and cutting and adding wires (leads) connections. of.” Modifications to printed circuit board assemblies are usually due to design changes and requirements for reliability and safety. For some products, this kind of modification is helpless, and in many cases it is inevitable. For example, when a printed circuit board needs to be changed in design, there are usually two options: one is to realize the design change through modification. The advantages of this approach are obvious: short cycle, low cost, and easy implementation. The disadvantage is that the modification and the risks brought by the modification must be fully demonstrated, such as the fixing method of the modified components, the quality of the solder joints, and the safety distance. Refitting is often accompanied by many unexpected situations, such as the impact on peripheral devices, the performance of modified parts in various environmental experiments, etc. The second is to re-board and re-weld. The advantage of this method is that the risk is controlled, and it can completely solve a series of problems caused by modification; the disadvantage is very obvious: all components must have enough margin (for many expensive components, this is actually very difficult) Do it); the environmental tests that have been completed must be carried out again, and these two points and the delay caused by these two factors are unbearable for many research and development units.

1. Modification of printed circuit board assemblies

The modification of the printed circuit board assembly usually includes the addition of components, the modification of the connection of components, and the cutting of the printed wires in the inner layer of the multilayer printed circuit board.

1. The principle of circuit board modification

Generally speaking, the quality of modified products will not decrease, there should be no hidden dangers, and even the product quality will be better. According to the relevant regulations in QJ2940A-2001, the printed circuit board modification should follow the following principles: within any 25 cm2 area on any printed circuit board assembly, the total number of modifications should not exceed 2; only when the printed circuit board does not change Components can be added only when the structural size is limited; the component wires should be covered with corresponding insulating materials; the extension of the lead wires should be limited to prevent subsequent vibration problems; the first bonding point on the extended wire should be away from the component to the wire The distance between the solder joints should not be greater than 15mm.

2. Common circuit board modification methods

(1) Addition of components: The traditional method of adding components can usually be divided into: installing additional components on the soldering surface of the printed circuit board, installing additional components on the component surface of the printed circuit board, installing additional components on the terminal block There are several methods of adding components, using adhesive mounting to add components, and adding components through lead mounting of adjacent components. The above methods need to be flexibly selected according to the actual situation.

(2) Modification of component connection: The modification of component connection generally includes the following methods: welding after winding and connecting with extended component leads; welding of component leads and vertical leads installed in existing holes; installation of double In-line package components, soldered or not soldered on the cut-off leads (this method can only be used if the leads that need to be cut do not exceed one-third of the number of leads on each side of the dual-in-line package ); Installed on the cut-off lead with welded or unsoldered wire connector; Add a wire to the metallized hole inserted with a flat cross-section lead; Add a wire to the lead of the flat package device; Insulation of the lead of the component.

2. Case analysis of actual circuit board modification

1. Problems to be considered in circuit board modification

In actual work, it is necessary to install 4 pull-up resistors on a welded printed circuit board (the printed circuit board has been subjected to conformal coating and glue dispensing reinforcement). Before the modification, we mainly considered the following factors.

(1) The packaging form of adding resistors: There are two types of resistor packaging options: plug-in and surface mount. Among them, the plug-in resistor is convenient to lead directly, but it is relatively large in size and relatively heavy in weight. After installation, it is necessary to consider the way of its mechanical reinforcement. If the mechanical reinforcement is not proper, it is easy to cause problems in the subsequent mechanical vibration test. Surface-mount resistor leads are relatively inconvenient, but they are small in size, light in weight, and easy to mechanically strengthen. Therefore, the way to solder the lead wires is a problem that needs to be considered.

(2) Selection of cement: The mechanical reinforcement of the added components is a process link that must be considered. Generally speaking, mechanical reinforcement and cementation can be considered in two ways. Since mechanical reinforcement usually requires punching holes to destroy the printed circuit board, the method of bonding is often used to mechanically reinforce the attachment. There are two main types of common adhesives to choose from—silicone rubber and epoxy resin glue. Among them, the bonding strength of silicone rubber is not as good as that of epoxy resin, but the release stress is small during the curing process, and its own elasticity is relatively large, which has a certain mechanical shock absorption effect; the bonding strength of epoxy resin is high, but the release stress during the curing process The stress is also large, and improper use is easy to break the metallized hole and damage the bonded components. Both have advantages and disadvantages, and you need to choose according to the actual situation. For example, when the components are dense and there are many metallized holes, it is better to use silicone rubber for bonding; when the space is relatively sufficient and there are not many metallized holes, it is better to use epoxy resin for bonding.

(3) Selection of the bonding position: The bonding position should be selected in a place that is spacious, has fewer metallized holes, and is as far away from the frame as possible; the bonding position should not interfere with other components, mechanical installation frames, and heat dissipation frames. At the same time, the cemented position should also take into account the distance between the lead-out wire and the target welding point of the lead-out wire. The closer the distance, the better.

(4) Selection of the routing path: the routing path from the resistor lead-out line to the target point is also a problem that must be considered. Generally speaking, lead-out wires should not cross other component bodies, pads and solder joints. If it is necessary to cross, reliable insulation measures must be taken. At the same time, the solder joints at both ends of the wire should not be subjected to obvious wire stress, and the connecting wire should be preformed after the wire path is determined to ensure that its own stress is minimized.

2. Determination of circuit board modification plan

The determination of the modification plan should start from the actual situation of the printed circuit board assembly to be modified, and consider various factors in a comprehensive and balanced manner. Here, we adopted the modification plan of "adding a small resistance board" - using the small resistance board made to weld the patch 0805 package resistor, the schematic diagram of the resistance board is shown in Figure 1. HY914 two-component epoxy resin glue is used between the resistance board and the modified board.

The main advantages of adopting the method of "adding resistance boards" are as follows: First, the solder joints of adding components are reliable. On the resistance board, the resistance element installed can be normally welded on the pad, and the lead-out wire is welded on the lead-out wire pad. Add unreliable factors. Second, the bonding between the resistance board and the modified circuit board. Since the two circuit boards are both FR4 base materials, after the bonding is completed, the mismatch of thermal expansion coefficients between materials is greatly reduced. This greatly reduces the risk of component damage caused by inconsistent thermal expansion coefficients among printed circuit board substrates, components, and adhesives (the traditional modification method is to directly paste components on the surface of the printed circuit board) .

3. Circuit board modification process

The printed circuit board refitting process is divided into 14 processes, including pre-installation preparation, resistance board welding, and cleaning, as shown in Figure 3.

Circuit board modification process flow chart

(a) Preparation before installation. Before installation, the tools and materials needed for modification should be ready for use.

(b) Resistance plate welding. The welding of the resistance board adopts OK 037 soldering iron head, the welding time is 2~3 s, and the welding process uses R-type flux.

(c) cleaning. For cleaning, use a dust-free cloth dipped in alcohol to scrub manually. After scrubbing, the solder joints should have a bright appearance and good wetting, and the printed circuit board should have no residues such as rosin and solder residue.

(e) Inspection of solder joints.

(e) Bonding: HY914 epoxy resin glue is first used for the bonding of printed circuit boards, and glue is applied on both sides of the bonding surface. The glue should be evenly applied and the amount of glue should be appropriate. 4 h. After HY914 is completely cured, apply GD414 one-component room temperature vulcanized silicone rubber evenly around the resistance plate. The glue should cover the edge of the resistance plate and cure for 21 hours.

(f) Conductor treatment: Raychem 0.15 mm2 silver-plated conductors are used for the conductors. Measure the appropriate line length from the lead-out point of the resistance board to the target point, strip the head, and tin-coat it for later use. When measuring the length of the wire, the routing path should be fully considered, and try to avoid placing the cable above the component body, above the solder joint, above the component lead, above the metal ground hole, etc. If it is really impossible to avoid the above positions, necessary insulation measures must be taken to prohibit direct contact of the wires with the above positions.

(g) To remove paint. The original three anti-paint is DBSF6101, and the thinner of the paint is V (toluene): V (n-butanol): V (butyl acetate) = 2:1:1 mixture. Here, its thinner is selected as the paint remover. Use a dust-free cloth to dip a small amount of paint remover and repeatedly wipe the surface of the target solder joint to achieve the paint removal effect. The wiping time should be controlled within 15 minutes. Long-term immersion will cause the pads of the printed circuit board to loosen, which will affect the reliability of the electrical connection.

(h) Lead wire installation. Solder one end of the connecting wire to the lead-out pad of the resistor board, and solder the other end to the target soldering point.

(i) cleaning. After the soldering is completed, wipe off the flux residues on the solder joints and their surroundings.

(j) Inspection of solder joints. Check whether the solder joints are mis-welded, and under a magnifying glass of 5 times, whether the solder joints are bright and well wetted.

(k) Touch-up paint. Repaint the three anti-paint on the resistance board, lead solder joints, etc., and let it dry naturally for 24 hours.

(l) Wire bonding. Carry out dispensing reinforcement treatment on the outlet position of the lead-out line, and glue the lead-out line every 1 cm.

(m) Inspection. Check whether the connection relationship is correct, whether the adhesive colloid is completely cured, whether there are missing paint parts, whether there are solder residues, rubber particles and other redundant substances.

(n) Take pictures. Panoramic and partial multi-angle pictures of the modified printed circuit board should be taken for future reference.

3. Circuit board test verification

After the modification is completed, test verification work is required. The printed circuit board has been subjected to board-level temperature cycle test and random mechanical vibration test. Here, the board-level test conditions and temperature cycle test conditions are selected as follows: temperature range -40~75°C; heating rate 10°C/min, cooling rate 5°C/min; high temperature for 2 h, low temperature for 1.5 h; cycle times 10 times . The mechanical test conditions are: 20~80 Hz, +3 dB/oct; 80~350 Hz, 0.04g2/Hz (g is the acceleration of gravity); 350~2 000 Hz, -3 dB/oct; the total root mean square value is 6.06 ; Loading direction, Z direction; Vibration time, 5 min before the temperature cycle and 15 min after the temperature cycle.

After the above-mentioned temperature and mechanical environment tests, the printed circuit board, especially the modified part of the printed circuit board, was normal in appearance inspection, and the power-on test was normal, which further proved that the modification method is reliable and effective.

Summarize

The modification of printed circuit board assembly is a job with strong uncertainty, complex and changeable state, and various process implementation routes. This requires the craftsman to brainstorm, fully consider various pros and cons, select the best from the best, and come up with the best process implementation plan. In addition, refitting is a last resort. When conditions such as development progress and funds permit, priority should be given to rearranging printed circuit boards to ensure product quality.

The above is a printed circuit board modification process method introduced by Shenzhen Zuchuang Microelectronics Co., Ltd. for you. If you have software and hardware function development needs for smart electronic products, you can rest assured to entrust them to us. We have rich experience in customized development of electronic products, and can evaluate the development cycle and IC price as soon as possible, and can also calculate PCBA quotations. We are a number of chip agents at home and abroad, including MCU, voice IC, Bluetooth IC and modules, wifi modules. We have hardware design and software development capabilities. Covering circuit design, PCB design, single-chip microcomputer development, software custom development, APP custom development, WeChat official account development, voice recognition technology, Bluetooth wifi development, etc. It can also undertake the research and development of smart electronic products, the design of household appliances, the development of beauty equipment, the development of Internet of Things applications, the design of smart home solutions, the development of TWS solutions, the development of Bluetooth audio, the development of children's toys, and the development of electronic education products.