1. IPC-SM-782 3.7.2 Soldermask Issues Rework of Surface Mount Assemblies due to soldermask related defects is cited as a major cause of problems by assembly people. Following are two classes of assembly problems caused by improperly applied soldermask: 1) soldermask on the component mounting lands; 2) insufficient soldermask coverage of unrelated circuit features in close proximity to the component mounting land.

2. IPC-SM-782 3.7.2 Soldermask Issues Regarding soldermask on the lands and assuming that the solderability requirements of the component leads and board mounting lands have been met, the solder paste composition, viscosity, and aging are within limits, and the thermal profile of the reflow oven is correct, the only other variable which could be detrimental to good solder joint integrity is soldermask material on the lands.

3. IPC-SM-782 3.7.2 Soldermask Issues If there is any soldermask (even if invisible to the naked eye) on the lands during reflow, solder joint integrity can be lost through improper surface wetting.

4. IPC-SM-782 3.7.2 Soldermask Issues The second class of assembly defects is created when there is insufficient soldermask coverage of circuitry in close proximity to the component mounting land leading to solder shorts or bridging. The majority of SMT designs incorporate very small geometries of 0.15 - 0.2 mm [0.006 - 0.008 in] conductors and spacing on the outer layers of the printed board.

5. IPC-SM-782 3.7.2 Soldermask Issues Designing a soldermask to cover a 0.15 mm [0.006 in] conductor and only half of the 0.15 mm [0.006 in] space between the conductor and the land is easy enough to accomplish during design. However, photo imaged soldermask will overcome most of the above issues (see Section 3.7.4).

6. IPC-SM-782 3.7.2 Soldermask Issues Close examination of solder bridging on Printed Board Assemblies will reveal that most of the bridging actually occurs between the component leads above the surface of the PB, and not between the lands on the board.

7. IPC-SM-782 3.7.2 Soldermask Issues Even if a soldermask has sufficient resolution and registration to provide a solder dam between lands, it still may not prevent bridging between the leads. Soldermask should not, and can not, be expected to compensate for deficiencies in the soldering process if the assembly uses Fine Pitch components.