User:Laurasmithhp/Graping

Graping is a phenomenon marked by the appearance of un-reflowed solder particles on top of the solder mass. The solder that is partially coalesced looks like a cluster of grapes which is where the phenomenon’s name is derived.

Causes
Graping occurrence has increased in recent years. The viscosity of the flux decreases with every degree the temperature increases. Lead-free reflow soldering temperatures are higher which results in more graping. Graping also occurs with increased surface oxidation. The increased surface oxidation is the result of smaller printed paste deposit volumes that cause a diminished surface area to flux ratio of the solder particle resulting in flux exhaustion. While solder paste can be manufactured using any size range however there has been a move towards finer particle sizes especially for fine feature stencil printing. Finer particle sizes places added pressure on the solder paste flux to remove surface oxides which leaves the outside of the joint not fully coalesced producing the irregular surface finish known as graping.

Resolutions
The graping phenomenon can be resolved utilizing proper solder materials in addition to correct reflow profile settings. There are solder powders that are created to provide a tighter distribution range as well as have a high oxidation barrier that not only improve the paste release from the stencil but also provide an ideal surface area to volume ratio. These solder powder characteristics help to eliminate the graping phenomenon. The future of solder paste flux formulations provides sufficient activity paired with re-oxidation mitigation capabilities. This pairing means that graping can be resolved as it occurs which is ideal for miniaturization processes.

There are also steps to take when setting up your reflow profile to stay clear of the graping phenomenon. The objective is to reduce the amount of heat exposure to the solder paste during the reflow process. Opt for a ramp to peak profile as opposed to a soak profile. Adjust the belt speed to decrease the total time in oven to a recommended ramp rate of 1°C/second from ambient to peak. Utilize a peak temperature ranging from 235°-240°C. Cut back the time above liquidus (TAL) to 40-60 seconds.