Graping - causes and methods of prevention.
The article presents the mechanism leading to the formation of a grapping defect and methods to minimize its occurrence.
Basic causes of grapping
The primary cause of grapping is the strong oxidation of the outer layer of solder beads in the paste deposit. The oxide layer causes the solder (even in the molten state) to retain its spherical shape due to the stronger surface tension of the solder oxide. The solder on the inside actually melts and becomes liquid, but the outer surface keeps its spherical shape.
Why are oxides formed in large amounts on the outer layer of the solder balls?
It depends on the activity of the flux and the ratio of the area and volume of the deposit. In addition, because oxides form exponentially at elevated temperatures and have more time to form under longer profiles, the length and temperature of the profile are also important.
© Indium Corp.
'Graping' is a phenomenon in which small amounts of solder do not melt and form clusters of frozen balls resembling bunches of grapes
The degree of activity of the flux is determined at the stage of determining the formula of its composition. In turn, as the size of the deposits decreases, the ratio of the surface area to the volume of the deposit increases steadily. The greater the surface ratio, the more oxides can be formed proportionally. Moreover, the smaller the deposit, the less flux is available, and it is the flux that is responsible for removing the oxides. It should also be taken into account that a certain portion of the flux is released from the mass of the deposit onto the solder mask.
In addition, high temperature long profiles favor the formation of increased amounts of oxides on the outer layer of the deposit. When all of these factors are present in one process, there is a situation where the flux no longer has sufficient strength to remove the oxides and keep them away from the deposit. Since the outer layer of the paste is most exposed to air, this layer loses its activity first and is covered with oxides.
How to minimize the phenomenon of grapping
Kester in its study 'Graping - What, Why and How to Eliminate?' writes: 'The best way to minimize grapping is to reduce the amount of oxides formed in the reflow soldering process, i.e. to reduce the temperature or residence time of the system in the soak zone. In other words, shorten the profile and / or lower its temperature '. This approach is also confirmed by Fritz Byle from the consulting company Astronautics: 'There are two approaches. First, you can work on reducing the plate's exposure to high temperature through the reflow process. Whether this is possible depends on the capabilities of the reflow oven and the location of the largest and heaviest components. The best approach is to shorten the preheat or temperature rise steps. ' If the described profile changes are not possible in a given process, you can also find solder paste on the market that exhibits activity at extended times and temperatures.
Mike Scimeca of FCT Assembly also refers to these two possibilities, adding a third option:
'1. Use more active solder paste. Graping is caused by oxides on the surface of the brazing powder. The activity level of the solder paste should be high enough to cope with the oxide present in the solder powder.
2. High temperature profiles, especially with long preheating, can form more oxide than the solder paste flux can handle. The problem can be solved by a solder paste with a higher level of activity or by modifying the reflow profile and applying a linear temperature rise step […].
3. Another option is to increase the volume of solder paste deposit on the graping pads. Graping is becoming more common as solder paste is imprinted with smaller apertures, typically with a surface ratio of less than 0.6, and transfer efficiency decreases as the surface ratio decreases. Small amounts of solder paste can lead to a grapping defect. Both the enlargement of the aperture and the application of an appropriate nano-coating on the template can help. '