Tom
10-23-2002, 02:45 PM
IPC Advanced Study Guide Page Reference: Pages 33 & 34, Section 1.4
The use of solder mask defined lands requires that the solder mask overlap the copper land, therefore the land diameter is somewhat larger in order to achieve the required land diameter for attachment. The solder joint is narrower but will give a higher standoff of the component from the board. The surface mount lands adhere to the board more because of the larger surface area of copper and the overlapping solder mask.
Although there are some advantages to this practice, the major disadvantage is that the approach is less reliable. The solder mask opening creates an area of high stress. It has been shown that the solder ball contains cracks because of the stress that the edge of the solder mask opening places against the ball after the reflow operation. The following figure shows a photomicrograph of a collapsed ball and how the characteristics are maintained.
The solder mask relief around the land and the manner in which the ball secures itself around the land for proper attachment is preferred. The image on the right side is preferred.
The use of solder mask defined lands requires that the solder mask overlap the copper land, therefore the land diameter is somewhat larger in order to achieve the required land diameter for attachment. The solder joint is narrower but will give a higher standoff of the component from the board. The surface mount lands adhere to the board more because of the larger surface area of copper and the overlapping solder mask.
Although there are some advantages to this practice, the major disadvantage is that the approach is less reliable. The solder mask opening creates an area of high stress. It has been shown that the solder ball contains cracks because of the stress that the edge of the solder mask opening places against the ball after the reflow operation. The following figure shows a photomicrograph of a collapsed ball and how the characteristics are maintained.
The solder mask relief around the land and the manner in which the ball secures itself around the land for proper attachment is preferred. The image on the right side is preferred.