Solution and frequency bad quality in reflow oven

- Mar 17, 2018 -

Solder ball in reflow oven.

Reflow-oven.jpg

1.The mechanism of formation of welding solder reflow

There in the solder reflow (or solder ball), often between side between reservoir and rectangular chip component at both ends of the pin or fine pitch. In the process of component placement, solder paste is placed between the pins and pads of chip components. With PCB passing through reflow oven, the solder paste melts into liquid, and if the wetting is not good with the pad and device pins, the liquid solder particles can not be polymerized into a solder joint. Some of the liquid from the solder welding outflow, forming tin. Therefore, the wettability of solder and pad and device pin difference is the fundamental reason for the formation of tin.

The paste in the printing process, the template and the pad to offset, if excessive offset will lead to the overflow of solder paste welding plate, prone to solder ball after heating. The Z axis in the process of patch pressure is an important cause of the solder balls, often goes unnoticed, part of mount machine because of Z shaft is based on the thickness of the element to position, which will cause components attached to the PCB moment will be squeezed into the outer bud tin welding phenomenon, this part of the tin will significantly lead solder. This case produces ball size slightly larger, usually as long as the re regulation of Z axis can prevent the solder balls.

2, Reason analysis and control method

There are many reasons for the poor wettability of solder, and the main reasons and solutions related to the related processes are mainly analyzed.

(1) the recirculation temperature curve is not set properly. The reflux of the solder paste is related to the temperature and time, and the solder paste will not be reflux if not enough temperature or time is reached. The temperature rises too fast, the time is too short, the solder paste internal moisture and solvent is not completely volatilized, reach the reflow temperature range, caused by moisture, solvent boiling with solder. It is proved that it is ideal to control the rising speed of the temperature in the preheating area at 1~4 /S.

(2) if the total in the same place of solder ball, it is necessary to check the metal template structure design. Template opening size corrosion precision, pad size is too large, and the surface of soft material (such as copper template), will cause the printing paste outline is not clear connected to each other, this is the case in the fine pitch device pad printing, which will inevitably lead to a large number of pins after reflow between solder balls. Therefore, the suitable template material and template making process should be selected to ensure the quality of solder paste printing according to the different shape and center distance of the plate.

(3) if the patches to reflow time, due to oxidation of solder particles in the solder paste, flux, lower metamorphic activity will lead to no reflow solder paste, solder production. The use of longer working solder paste (at least 4H) will reduce this effect.

(4) in addition, the solder paste printed board is not cleaned adequately, which will make the solder paste on the surface of the printed board and through the air. When the reflow welding is attached to the components, the printing solder paste is deformed. The reason is caused by the ball. Therefore, we should speed up the sense of responsibility of the operators and technicians in the production process, strictly comply with the process requirements and operation rules, and enhance the quality control of the technological process.

Two. A problem of element

One end of the piece element is welded on the weld plate and the other end is raised. This phenomenon is called the Manhattan phenomenon.  The main cause of this phenomenon is the uneven heat of the two ends of the component, and the melting of the solder paste has successively reached. In the following case, the two ends of the component will be heated inhomogeneous.

(1) the design of the element arrangement is not correct. We envisage a reflow limit line across the width of the furnace in the reflow soldering furnace, and once the solder passes through it melts. One end of the rectangular element through the first chip reflow soldering line, solder melting, metal surface infiltration element end with liquid surface tension; and the other end is not reached 183 degrees the temperature of the liquid phase, not only the melting solder paste, flux adhesion, the surface tension is much smaller than the reflow solder paste, so the element end does not melt end upright. Therefore, both ends of the components should be kept in the reflow soldering line at the same time, so that the solder paste on both ends can be melted at the same time, forming a balanced liquid surface tension and keeping the location of the components unchanged.

(2) the preheating of the printed circuit components is not sufficient in the process of gas phase welding. Gas phase is the use of inert liquid steam condenses on the component pins and PCB welding disk, releasing heat and melting solder paste. The gas phase welding is divided into the balance zone and the steam zone. The welding temperature in the saturated steam zone is as high as 217 degrees. During the production process, we found that if the preheating of the welded components is not enough, and the temperature changes above 100 degrees, the gasification force of gas phase welding is easy.

The sheet elements with less than 1206 package sizes float to produce the vertical sheet phenomenon. We preheated 1 to 2min of the welded components in the high and low temperature box at 145~150 degrees, and finally slowly entered the saturated vapor zone to eliminate the phenomenon of chip formation. 

(3) the influence of the design quality of the weld plate. If the pad size of chip components or different asymmetry, will cause the amount of paste printing is not the same, a small pad for a quick response to temperature, solder paste on the melt, a large pad is opposite, so when the solder melting on small pads in the solder paste under surface tension components will straighten up. The width or gap of the weld plate is too large, and there may be a vertical phenomenon. It is a prerequisite for solving the defect to design the weld plate according to the standard and standard strictly.

Three. Bridge connection (welding) between components

Bridging is also one of the common defects in SMT production. It will cause short circuit between components, and bridge connection must be returned.