Design of wave soldering fixtures PCb wave solder frame

In the wave soldering process, the soldering problem of mixed circuit boards is very important. Wave soldering fixtures are an ideal solution. Are there any useful wave soldering fixture designs that can handle this problem well? Today we will introduce in detail the design process and design key points of wave soldering fixtures.

The miniaturization and lightweighting of electronic products have promoted the transformation of electronic components from through-hole components to chip components. However, some through-hole components cannot be converted into chip components due to various factors, so mixed circuit boards will always exist. To deal with the welding of this kind of board, whether you choose expensive selective wave soldering equipment or manual welding, you cannot solve the problem of low efficiency. Therefore, using wave soldering fixtures is an ideal solution.

 

1. The role of wave soldering fixtures

(1) Ability to deal with the problem of furnace deformation of simple circuit boards with large shapes

(2) Can solve the problem of low production efficiency of small-scale circuit boards

(3) It can solve the problem that the circuit board has no process edge or the chip components on the back of the circuit board are too close to the edge of the board (wave soldering chain claws) and cannot pass the furnace.

(4) Can solve the problem that mixed circuit boards cannot be passed through the furnace directly

(5) Ability to deal with gold fingers and test point contamination issues

(6) Able to solve the problem of hole plugging without plug-in installation

(7) Able to solve the problem of cleaning the surface of the circuit board

(8) Able to deal with damage problems of special sensitive equipment

 

2. Design of wave soldering fixtures

 

2.1 Selection of wave soldering fixture materials

In order for the wave soldering fixture we designed to have a long life, the material of the fixture must not only have sufficient strength to facilitate precision machining and not be easily deformed, but must also be able to withstand high temperatures and harsh process conditions. Therefore, the materials of wave soldering fixtures must have the following characteristics:

(1) High strength and easy for precision machining.

(2) High temperature resistance and good dimensional stability

(3) It has excellent thermal conductivity. When passing through the furnace, the heat can be transferred to the circuit board quickly and evenly.

(4) Corrosion resistance (resistant to corrosion by flux and cleaning agents)

(5)Good thermal shock ability

(6) Comply with anti-static requirements

(7) When used in environmentally friendly products, environmental protection requirements must also be met.

(8)Moisture resistance

(9) Good electrical insulation

 

The two most widely used materials at present are fiberglass boards (FR-4) and synthetic stone.（Durostone Ricocel material）

(1) Fiberglass board (FR-4): It is composed of glass fiber material and high heat-resistant composite material, and does not contain asbestos ingredients that are harmful to the human body. It has high mechanical properties and dielectric properties, good heat resistance and moisture resistance, and excellent processability. Cheap price and short service life.

(2) Composition stone(Durostone): Composition stone is an environmentally friendly stone, which is made of more than 95% natural stone powder, plus a small amount of polyester and adhesive, mixed under vacuum, pressurized, and vibrated into shape. More expensive, longer service life

2.2 Composition of wave soldering fixture

Commonly used wave soldering fixtures mainly include: substrate, tin retaining bar, press buckle and fixing screws, springs, etc. In addition to the above parts, more complex wave soldering fixtures will also have some auxiliary tooling to ensure the position or the concave and convex, such as positioning posts, cover plates, pads, pressure blocks, etc.

 

2.3 Design of wave soldering fixtures

23.1 Design of substrate

The shape requirements of the wave soldering fixture substrate are determined by the shape dimensions of the PCB board carried inside. Generally, the shape size of the jig is equal to the shape size of the PCB board plus 40 mm on one side. If the PCB board is smaller in size, it can carry multiple PCB boards at the same time. Generally, the distance between two boards is designed to be 30 mm. Regarding the shape design of the substrate, we should also consider standardized design to facilitate the centralized passing of different products.

The substrate cavity needs to be designed based on the orientation of the plug-in components in the PCB drawing.

As a general principle, it is only necessary to open holes at the corresponding positions of the fixture for the equipment being welded during wave soldering. Open them as large as possible without affecting the maintenance of surrounding equipment, and protect the rest of the holes. For chip components that need to be protected, the depth, length, and width of the slot should be determined based on the height and appearance dimensions of the chip component.

After the substrate is grooved, the design of the reverse side is also particularly important. The angle should be made at the grooved contour on the back side. When making the angle, a 120-degree chamfering tool works best. Regarding the slots where the patch and plug-in are relatively close, when chamfering, do not pour it too heavily to avoid damage to the substrate slot and resulting in scrapping. When there are higher-height chip components on the back side, the back side of the substrate should be thinned and tin flow grooves should be designed to improve the welding quality.

23.2 Design of tin retaining strips

Wave soldering fixtures are generally designed in a rectangular shape, so the tin retaining bars are divided into long tin retaining bars and short tin retaining bars, a total of 2 types, 2 pieces of each type. The tin retaining strip generally adopts the 10x10 mm standard. The length of the long tin retaining strip is equal to the length of the long side of the fixture minus 10 mm. The length of the short tin bar is equal to the short side of the jig minus 30 mm.

23.3 Design of buckle orientation

The press buckles are generally arranged at the opposite ends of the jig, with at least 2 on each side. When the shape of the jig is large, the press buckles need to be evenly distributed in 4 directions. The position of each press buckle should not conflict with the surrounding components. And there is no conflict within the 360-degree rotation range around the center of the press buckle fixing screw.

 

23.4 Design of auxiliary tooling

When there are lifting requirements for the plug-in components on the PCB board, we need to make some small pads and place them under the equipment when passing through the furnace; for plug-ins that are easy to float when passing through the furnace, we need to design some small pressure blocks to pass the furnace. Press the top of the equipment during the furnace, and fix the pressure spring on the tin retaining bar near the equipment to press the pressure floating block to ensure that the pressure floating block does not fall off during the furnace.

When there are many plug-in devices that need to be considered for pressure floatation or they are relatively scattered, making small tooling may require a large quantity. We can make a cover plate and take into account the mounting holes of multiple cover plates and install them into the positioning posts on the jig. Then press it on the plug-in component, and then design two compression springs on the tin retaining bars on both sides to press the cover plate to ensure that the cover plate plays a role of pressure and floating.

 

23.5 PCB file design requirements using wave soldering fixtures

In order to make the wave soldering fixture achieve its due function and life, it is necessary to review the documents during PCB design. The key points of the review are as follows:

 

(1) SMD components and through-hole components on the PCB solder surface should be classified and separated as much as possible to prevent the irregular distribution of SMD components and through-hole components, which would increase the difficulty of making the jig and affect the use effect and service life of the jig.

 

(2) Larger chip components should be designed on top of the PCB as much as possible. The height of the chip components during wave soldering of the PCB should not exceed 3.5mm. Otherwise, the thickness of the fixture will be too thick, making the weight and cost of the fixture relatively high.

 

(3) Leave appropriate gaps between the pins of through-hole components and around them so that solder can flow. The distance between patch components and the pins of through-hole components should be at least 4mm to prevent placing large objects near the pins of through-hole components. For chip components, because the jig produced in this way will have a large "shadow" of the wave peak when it is wave soldered, resulting in virtual soldering and missing soldering.

The above are the design requirements and design points of the