Key Design Points of Wave Soldering Jigs
2024-08-26 17:21
In the wave soldering process, soldering issues of mixed-assembly circuit boards are of great importance, and wave soldering jigs serve as an ideal solution. So, what practical wave soldering jig designs can effectively address this problem? Today, we will specifically introduce the design process and key points of wave soldering jigs.
The miniaturization and lightweight of electronic products have driven the transition of electronic components from through-hole components to surface-mount components. However, some through-hole components cannot be converted to surface-mount components due to various factors, so the use of mixed-assembly circuit boards will persist. To handle the soldering of such boards, neither expensive selective wave soldering equipment nor manual soldering can solve the problem of low efficiency. Therefore, the use of wave soldering jigs is an ideal solution.
1.Functions of Wave Soldering Jigs
(1) Can address the issue of deformation of simple circuit boards with large external dimensions when passing through the soldering furnace.
(2) Can resolve the problem of low production efficiency for small-sized circuit boards.
(3) Can tackle the issue where circuit boards fail to pass through the furnace due to the absence of process edges, or when surface-mount components on the reverse side of the board are too close to the board edges (resulting in contact with the wave soldering chain claws).
(4) Can handle the problem that mixed-assembly circuit boards cannot pass through the furnace directly.
(5) Can prevent contamination of gold fingers and test points.
(6) Can address the issue of avoiding unnecessary insertion of through-hole components (i.e., preventing insertion of components that do not require mounting).
(7) Can help maintain the cleanliness of the circuit board surface.
(8) Can protect special sensitive devices from damage.
2. Design of Wave Soldering Jigs
2.1 Material Selection for Wave Soldering Jigs
To ensure a long service life of wave soldering jigs, the jig material must not only have sufficient strength (to facilitate precision machining and resist deformation) but also be able to withstand high temperatures and harsh process conditions. Therefore, the material of wave soldering jigs must possess the following characteristics:
(1) High strength and suitability for precision machining.
(2) High temperature resistance and good dimensional stability.
(3) Good thermal conductivity, enabling rapid and uniform heat transfer to the circuit board when passing through the furnace.
(4) Corrosion resistance (resistant to corrosion by flux and cleaning agents).
(5) Good thermal shock resistance.
(6) Compliance with anti-static requirements.
(7) When used for environmentally friendly products, it must also meet environmental protection requirements.
(8) Moisture resistance
(9) Excellent electrical insulation
Currently, the two most widely used materials are fiberglass boards (FR-4) and Durostone.
(1) Fiberglass board (FR-4): Composed of fiberglass materials and high heat-resistant composite materials, it does not contain asbestos which is harmful to the human body. It features high mechanical and dielectric properties, good heat and moisture resistance, excellent processability, low price, but a short service life.
(2) Durostone: A type of synthetic stone, it is an environmentally friendly board made from more than 55% fiberglass, combined with eco-friendly resins and adhesives, mixed, pressed and vibrated under vacuum conditions. It has a long service life.
2.2 Composition of Wave Soldering Jigs
Commonly used wave soldering jigs mainly consist of substrates, solder bars, screws for compaction and fixation, springs, and other components. In addition to the above parts, more complex wave soldering jigs may also include some auxiliary tools to ensure positioning or prevent unevenness, such as positioning pins, cover plates, cushion blocks, and pressing blocks.
2.3 Design of Wave Soldering Jigs
3.1 Design of the Substrate
The shape requirements of the wave soldering jig substrate are determined by the external dimensions of the PCB it carries internally. Generally, the external dimensions of the jig are equal to the external dimensions of the PCB plus 40 millimeters on one side. For example, if the PCB is small in size, multiple PCBs can be carried together in one jig. Typically, the distance between two adjacent PCBs is designed to be 30mm. Regarding the shape design of the substrate, standardized design should also be considered, so that different products can be loaded into the furnace in batches.
The cavity of the substrate needs to be designed according to the position of the through-hole components in the PCB layout.
In principle, it is only necessary to open through-holes at the corresponding positions of the jig during the wave soldering process. As long as the maintenance of surrounding equipment is not affected, the holes should be made as large as possible, and the rest of the areas should be well protected. For surface-mount components that require protection, the depth, length, and width of the slots should be determined based on the height and shape dimensions of the surface-mount components.
After slots are made on the substrate, the design of the substrate’s reverse side is also particularly important. The edges of the slots on the reverse side should be chamfered, and using a 120° chamfering tool yields the best results. For slots where surface-mount components and through-hole components are relatively close to each other, excessive chamfering should be avoided to prevent damage to the substrate slots, which would render the substrate scrapped. When there are high-profile surface-mount components on the reverse side, the reverse side of the substrate should be thinned, and solder flow channels should be designed to improve soldering quality.
3.2 Design of Solder Bars
Wave soldering jigs are generally designed in a rectangular shape; therefore, solder bars are divided into long solder bars and short solder bars, with 2 types each. Solder bars typically adopt the 10x10mm standard. The length of a long solder bar is equal to the length of the jig’s long side minus 10mm, while the length of a short solder bar is equal to the size of the jig’s short side minus 30mm.
3.3 Design of Fastener Positions
Fasteners are generally arranged at the opposite ends of the jig, with at least 2 fasteners on each side. When the jig has a large external size, the fasteners need to be evenly distributed in 4 directions. The position of each fastener should not conflict with surrounding components, and there should be no conflict within the 360-degree rotation range centered on the fixing screw of the fastener.
3.4 Design of Auxiliary Tools
When the through-hole components on the PCB have high requirements, it is necessary to make small cushion blocks, which are placed under the components during furnace passage. For through-hole components that tend to float upward during furnace passage, small pressing blocks need to be designed. When these pressing blocks are placed on top of the components during furnace passage, compression springs are fixed to the solder barrier strips near the components to press the anti-floating blocks together, ensuring that the anti-floating blocks do not fall off during furnace passage.
Regarding requirements, when there are many through-hole components or they are relatively scattered, a large number of small auxiliary tools may be required. In this case, we can make a cover plate: first, integrate the mounting holes of multiple cover plates into the positioning pins on the jig; then press the cover plate onto the through-hole components; finally, arrange two compression springs on the solder barrier strips on both sides to press the cover plate tightly. This ensures that the cover plate functions to compress components and allow for slight floating.
3.5 PCB File Design Requirements for Using Wave Soldering Jigs
To ensure wave soldering jigs achieve their intended functionality and service life, PCB files must be reviewed during the PCB design phase. The key review points are as follows:
(1) Surface-mount components and through-hole components on the solder-side of the PCB should be classified and grouped as much as possible. This prevents uneven distribution of surface-mount components and through-hole components, which would increase the complexity of jig design and compromise the jig’s performance and service life.
(2) Larger surface-mount components should be designed on the PCB whenever possible. When the PCB passes through wave soldering, the height of surface-mount components should not exceed 3.5mm. Otherwise, the jig will be too thick, leading to relatively higher jig weight and cost.
(3) Appropriate gaps should be reserved between the pins of through-hole components and their surroundings to allow solder to flow. The distance between surface-mount components and the pins of through-hole components should be at least 4mm to avoid placing large components near the pins of through-hole components. If this requirement is not met, the jig manufactured under such conditions will create a large "wave shadow" during wave soldering, resulting in cold soldering and missing soldering.
The above are the design requirements and key points for wave soldering jigs.
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