SMT007 Magazine

SMT007-July2019

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54 SMT007 MAGAZINE I JULY 2019 pastes showed a smaller drop in transfer effi- ciency after a pause of 16 hours on the sten- cil. It is not recommended to print solder paste after a 16-hour pause and 24 hours total time on the stencil. All of the solder pastes tested responded well to pauses in the printing of one, two, and four hours, and showed good printability through eight hours on the stencil. Based on this work, the recommended sten- cil life for all of the solder pastes tested would be eight hours. Environmental conditions, an additional rate of fresh solder paste, and other factors will affect this stencil life. Reflow Performance (PR Test Board) The wetting or spread of the no-clean solder pastes was consistent regardless of solder pow- der size. The wetting or spread of the water-sol- uble solder pastes decreased with decreasing solder powder size but was higher overall than the no-clean solder pastes. The solder balling performance for the no-clean and water-solu- ble Type 3, 4, and 5 solder pastes was accept- able for most electronic assemblies. The sol- der balling performance for the no-clean and water-soluble Type 6 solder pastes was exces- sive, as was seen in the IPC solder balling test. The graping performance for the no-clean and water-soluble Type 3, 4, and 5 solder pastes was very good. The graping was unacceptable for both of Type 6 solder pastes. Use of Type 6 solder powder in a solder paste may lead to excessive solder balling and graping. Voiding Performance All of the no-clean solder pastes showed identical low voiding behavior. The water-sol- uble solder pastes showed increasing voiding as the solder powder size decreased. Voiding is affected by solder powder particle size and a host of other factors. Voiding behavior may change with a change in solder powder size, and the process may require adjustments to minimize voiding. Stability of the Solder Pastes The tack force of all of the solder pastes was stable through 48 hours but dropped over 72 hours. The stability of the tack force is well beyond a normal amount of time between printing and reflow. The reflow performance after a 24-hour hold was stable for Type 3, 4, and 5 solder pastes. Type 6 solder pastes gave questionable reflow performance both before and after the 24-hour hold. A 24-hour hold prior to reflow is not recommended but shows that Type 3, 4, and 5 solder pastes were envi- ronmentally stable. More extreme environ- mental conditions will change the stability of the solder pastes. Heat Aging of the Solder Pastes The viscosity of the no-clean Type 3 and 4 solder pastes increased with heat aging, but they were easily printable. The viscosity of Type 5 and 6 no-clean solder pastes increased significantly but they were printable by adjust- ing the print parameters. The viscosity of all of the water-soluble solder pastes increased dra- matically with heat aging and the pastes were not printable. The print performance of the no- clean solder pastes was essentially unchanged with heat aging. Heat aging caused failures in the IPC solder balling results for the no-clean Type 5 and 6 solder pastes, and the water-sol- uble Type 4, 5, and 6 solder pastes. The tack force was stable before and after heat aging for all of the solder pastes. Only the no-clean solder pastes were printed and reflowed after heat aging. Heat aging caused the wetting performance to decrease for all of the no-clean solder pastes. Solder balling per- formance for the no-clean solder pastes wors- ened a little with heat aging. Graping perfor- mance was unchanged through heat aging of the no-clean solder pastes. These results show that the reactivity of the solder pastes increases as the solder powder size decreases. This indicates that the shelf life is shorter for solder pastes made with the smaller solder powder sizes, especially Type 5 and 6 solder powders. Summary This work shows major differences in solder paste performance for no-clean and water-sol- uble solder pastes using SAC305 Type 3, 4, 5, and 6 solder powders. When solder pastes with

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