CN108463062A - Surface mount device reprocesses the loading method of printed board soldering paste - Google Patents

Abstract

The invention discloses a solder paste loading method for surface mount device reworked printed boards, aiming to provide a solder paste loading method with good operability, excellent quality and high reliability. The invention is realized through the following technical scheme : Prepare a printed board solder paste loading tooling that is supported between the printed board and the pressure plate by two spring columns, and use the surrounding columns with suction cups to pass through the pressure plate to absorb the stencil on the printed board, the stencil Fix a lamination plate connected by a pressure plate on the top, put the solder paste on the stencil; and compress the spring, so that the lamination plate on the pressure plate will press the solder paste into the mesh on the stencil; keep the position of the pressure plate , Lift the pressure plate, the suction cup drives the stencil to rise, and the solder paste is formed by reflow soldering on the PCB pad of the printed board, and the solder paste passes through the stencil to open holes and leaks to the printed circuit board welding area components On the pad, connect the pad to the corresponding pad on the printed circuit board PCB to complete the solder paste loading.

Description

Loading Method of Solder Paste for Reworked Printed Boards of Surface Mount Devices

technical field

The invention relates to a method for reloading solder paste on a printed board for surface mount components that need to be re-welded due to bad soldering in surface mount components, so as to meet the quality requirements for solder paste loading in re-welding of surface mount components.

Background technique

There are many types of surface mount devices SMD (Surface Mount Device). The chip pins are J-shaped and bend into the chip. It is difficult to control the flow of solder during soldering. Bent or folded leads, wrong polarity, wrong or skewed solder pin positioning, excessive solder or solder joint bridging or weak soldering, etc. When soldering, use certain tools to accurately place the non-lead surface mount components on the PCB pads that have been printed with solder paste or glued, and then go through wave soldering or reflow soldering to make the components and the circuit board well established. mechanical and electrical connections. Surface mount is to accurately install surface mount components to a fixed position on the PCB. The equipment used is a placement machine, and the surface mount reflow soldering is to melt the solder paste, so that the surface mount components and the PCB board are firmly bonded together. The equipment used is a reflow oven. The electrical and mechanical connection with the PCB is realized by printing solder paste on the PCB pad and solder joints formed by reflow soldering, which puts forward some new requirements for the PCB pad design and surface mount process. Printed stencil design, post-soldering inspection, rework, etc. are all areas that should be considered during the surface mount process. Quad Flat Non-leaded Package QFN (Quad Flat Non-leaded Package) is a leadless package that is square or rectangular. The bottom of the package has a pad that is horizontal to the bottom surface, and there is a large-area exposed pad in the center for heat conduction. , around the periphery of the package around the large pad there are conductive pads for electrical connection. There are two types of conductive pads: one is only exposed on the bottom side of the package, and the other part is packaged in the component; the other pad has a part exposed on the side of the package. The electrical connection to the PCB is achieved by printing solder paste on the PCB pad, forming solder joints through reflow soldering, and connecting the QFN pad to the corresponding pad on the PCB. Due to the different configurations of device pins, the conductive pads of QFN components cannot be tinned on the side, and only one side can form solder joints with the board. Therefore, the solder accumulated on the side cannot be used as a direct basis for judging whether the soldering is good. As an auxiliary means of judgment to determine whether the component has ever been tinned. Since the solder joints of QFN are under the package body, and the thickness is relatively thin, it is impossible to detect the lack of tin and open circuit of QFN solder joints. It can only be judged by the external solder joints. However, due to the different pin configurations of the devices, It is an unspecified part in the IPC standard. In the absence of more methods for the time being, we will rely more on the test stations in the post-production stage to judge whether the welding is good or bad. The rework of QFN to QFN, because the solder joint is completely at the bottom of the component package, any defects such as bridging, open circuit, solder ball, etc. need to remove the component, so it is somewhat similar to the rework of BGA. QFN is small in size and light in weight, and they are used on high-density assembly boards, making rework more difficult than BGA. At present, QFN rework is still an urgent need to develop and improve in the entire surface mount process. In particular, it is indeed difficult to use solder paste to form a reliable electrical and mechanical connection between QFN and printed boards. At present, there are three feasible methods of applying solder paste: one is to print solder paste with a small maintenance screen on the PCB; Printed directly on component pads. The above methods all require very skilled rework workers to complete the task. The choice of rework equipment is also very important. When the surface mount device has poor soldering after soldering, the device needs to be removed and re-soldered, and the corresponding stencil template is manufactured according to the package type and size of the reworked device, and the solder paste is loaded on the reworked printed board of the surface mount device. Align the stencil with the reworked part of the printed board, use anti-static tape to fix the stencil, place the stirred solder paste on the stencil, manufacture a scraper frame and a scraper with the same dimensions as the stencil, and the scraper reciprocates Movement, pushing the solder paste through the opening of the stencil and leak printing onto the printed circuit board 1, completing the transfer of printing ink from the screen to the printed device. Then remove the antistatic tape, and separate the printed circuit board 1 from the stencil by hand to complete the printing of the solder paste. This method of solder paste loading for surface mount device rework printed boards poses three problems:

1. The pressure and angle of the squeegee cannot be precisely controlled to affect the amount of solder loading. The force of the scraper is too large and the angle between the scraper and the printed circuit board is too large, which will cause the relative movement of the printed circuit board 1 and the stencil template, causing the position of the solder paste to shift, resulting in poor printing, and short circuit or short circuit after soldering. The scraper will take away part of the solder paste, resulting in insufficient solder paste; the scraper is too small, the amount of solder paste will increase, resulting in a short circuit after soldering.

2. The speed of the squeegee cannot be precisely controlled to affect the amount of solder loading. The speed of the scraper is fast, the stencil and the printed circuit board cannot be fully bonded, the amount of solder paste increases, and it is easy to cause a short circuit. Low squeegee pressure can cause missed prints and rough edges; high squeegee pressure or a soft squeegee can blur solder paste onto pads and can damage the squeegee, stencil, or screen. Double-thickness stencils allow the proper amount of solder paste to be applied to fine-pitch component pads and standard-solder surface-mount component pads. This involves using a squeegee to force the solder paste into the small holes in the stencil. Using a metal squeegee can prevent changes in the volume of the solder paste, but the design of the holes on the stencil needs to be modified to avoid applying too much solder paste to the fine-pitch pads.

3. The speed of separating the printed circuit board and the stencil cannot be accurately controlled to affect the amount of solder loading. There is a certain adhesion between the solder paste and the inner wall of the stencil opening, and the separation of the stencil is too fast, which will take away part of the solder paste, change the shape of the printed solder paste, and easily cause bridging and short circuit after soldering; and Since the stencil is separated manually, it can only be operated by one person. The process is to remove the anti-static tape on one side and separate the side of the stencil. During the separation process, the shape of the solder paste on the side that is separated first changes, and the solder paste It accumulates in the middle of the pad, and it is easy to short circuit after soldering.

In short, solder paste printing is the first and most critical process of surface mount processing, and the existing method of solder paste loading for surface mount device rework printed boards can easily lead to defects such as short circuits, open circuits, and bridging after soldering, and cannot guarantee soldering quality , It is necessary to change the method of solder loading to ensure that the solder is uniform, the thickness is consistent, the shape meets the requirements, and the position meets the requirements. In fact, solder paste loading has become a technical bottleneck for surface mount device rework.

The invention is an improvement to the traditional solder paste printing method.

Contents of the invention

The present invention aims at the disadvantages of solder paste loading for reworked printed boards of existing surface mount devices, and provides a solder paste with good operability, excellent quality, high reliability, uniform solder paste, appropriate amount of solder paste, and more convenient soldering. The paste loading method is used to improve the solder paste loading and printing ability of surface mount device reworked printed boards and the reliability of long-term operation of electronic equipment.

In order to achieve the above-mentioned purpose of the present invention, a kind of method for surface mount device repair printed board solder paste loading is provided, it is characterized in that comprising the following steps:

Prepare a printed board solder paste loading tooling that is supported by two spring columns 3 between the printed board and the press plate 7, and use the surrounding columns 5 with suction cups to pass through the press plate 7 to suck the stencil on the printed board 2. On the top of the stencil template 2, fix a laminated board 8 connected by a pressure plate, and place the solder paste on the stencil template 2; the laminated board 8 is aligned with the corresponding pad of the printed circuit board 1; The stencil 2 and the printed circuit board 1 are adsorbed on the suction cup, and the spring 6 is compressed, so that the laminating plate 8 on the pressure plate 7 presses the solder paste into the mesh on the stencil 2; keep the position of the pressure plate 7, lift it up The pressure plate 7, the suction cup drives the stencil 2 to rise, and forms solder joints by pressing and brushing solder paste on the PCB pad of the printed circuit board through reflow soldering. On the pad of the component, connect the pad to the corresponding pad on the printed circuit board PCB to complete the solder paste loading process.

Compared with the prior art, the present invention has the following beneficial effects: by replacing the existing solder paste printing technology by vertical pressing, the loading quality of the solder paste on the reworked printed board of the surface mount device is ensured.

Strong maneuverability. The present invention adopts two column springs to support the printed board and the printed board solder paste loading tooling to replace the repair solder paste printing of the mounted device, and replace the existing solder paste with a single process to complete the original multiple independent processes that are observed by eyes Printing reduces the uncertainty of operation, and the operability is better than that of the prior art.

Improves the uniformity of solder paste application. The present invention uses solder paste to press down vertically instead of solder paste printing, with uniform strength, accurate welding on the pad, no missing soldering, uniform thickness of solder paste on the pad, smooth and uniform solder joints, and no need for bridging and short circuits Squeegee pressure, scraper angle and other process parameters will not cause relative movement between the stencil and the printed circuit board, ensuring the shape of the solder paste, avoiding quality defects such as bridging, short circuit, and open circuit after soldering, and improving adhesion. The repair quality of installed devices. The uniform loading of the solder paste is realized, and the quality of the solder paste loading for the rework of the mounted device is ensured.

Easy to demould. The present invention adopts four suction cups to suck the four corners of the stencil template and lift them up at the same time, which will not cause the stencil detachment in the existing method to affect the thickness of the solder paste, resulting in uneven thickness of the solder paste, and it is convenient for the stencil template to detach from the printed circuit board . Not only can it be used to desolder the components that need to be replaced, but it can also melt the solder and solder the newly mounted components. It solves the problem that the traditional method of repairing solder paste for mounted devices cannot ensure the quality of solder paste printing.

The method provided by the present invention is more refined, and the solder paste loading effect is better.

Description of drawings

In order to understand the present invention more clearly, the present invention will now be described through the embodiments of the present invention with reference to the accompanying drawings, wherein:

Fig. 1 is a schematic diagram of the structure of a solder paste loading tool for reworked printed boards of surface mount devices according to the present invention.

In the figure: 1 printed circuit board, 2 stencil template, 3 spring column, 4 solder paste, 5 column, 6 spring, 7 pressure plate, 8 pressure film plate, 9 pressure plate nut.

Detailed ways

See Figure 1. According to the present invention, the printed board solder paste loading tooling supported on the printed board and formed is passed through the pressing plate 7 with four columns 5 with suction cups and fixed with nuts, and the pressing film plate 8 is screwed on the pressing plate 7 through fastening nuts 9 Above, the spring 6 passes through the spring column 3 and is installed on the pressure plate 7; the steel mesh template 2 is installed on the printed circuit board 1, and is aligned with the pad; the suction cup of the spring column 3 loaded with solder paste is adsorbed on the printed circuit board 1, the suction cup of the column 5 is adsorbed on the stencil template 2; add the solder paste 4 on the stencil template 2; press the pressure plate 7, compress the spring 6, and make the pressure plate 8 on the pressure plate 7 press the solder paste 4 into In the mesh hole on the stencil template 2, keep the position of the pressure plate 7, lift the pressure plate 7, the suction cup drives the stencil template 2 to rise, and form solder joints by embossing the solder paste 4 on the PCB pad of the printed board through reflow soldering, The solder paste 4 passes through the opening of the stencil 2 and is leak-printed on the solder pads of the components of the printed circuit board 1 , and connects the solder pads with the corresponding solder pads on the PCB to complete the loading of the solder paste.

Claims (1)

1. A method for surface mount device rework printed board solder paste loading, is characterized in that comprising the following steps: Prepare a printed board solder paste loading tool that is supported by two spring columns (3) between the printed board and the pressure plate (7), and use the surrounding columns (5) with suction cups to pass through the pressure plate (7). The stencil template (2) located on the printing board, a lamination plate (8) connected by a pressure plate is fixed above the stencil template (2), and the solder paste is placed on the stencil template (2); the lamination plate (8) ) to the corresponding pad of the printed circuit board (1); attach the stencil template (2) with solder paste (2) together with the printed circuit board (1) to the suction cup, and compress the spring (6) so that the pressure plate (7) The pressure film plate (8) presses the solder paste into the mesh on the stencil template (2); keep the position of the pressure plate (7), lift the pressure plate (7), the suction cup drives the stencil template (2) to rise, and passes Press and brush solder paste on the PCB pads of the printed board to form solder joints through reflow soldering. The solder paste passes through the stencil (2) to open holes and leaks onto the pads of the components in the soldering area of the printed circuit board (1). The pad and the corresponding pad on the printed circuit board PCB are connected to complete the solder paste loading process.