Tombstone is one end of a pin less component (such as a capacitor or resistor) that is lifted and stands on top of its other end, as shown in figure 1.Tombstone is also known as the Manhattan effect, suspension bridge effect.It is caused by unbalanced wetting at both ends of the component during reflow, resulting in an unbalanced surface tension applied to the molten solder at both ends.
So why is there such a tombstone phenomenon in PCB prototype fabrication?
(1) pad spacing and size,chip element terminal size and heat distribution of PCB play an important role in the influence of tombstone.Improper spacing between two pads of a chip element produces tombstone.FIG. 2 is an experimental result of 60Sn/40Pb gas phase welding of chip resistor 0805 for reference only.Too small spacing causes the chip component to drift over the molten solder, creating more tombstone.Too large spacing can easily cause either end to tilt up from the pad.On the other hand, the critical overlap between the chip element and the pad also results in more tombstone, due to the easy separation of either end of the pad.Therefore, simply considering the tombstone only, the best interval between the pads can be set slightly shorter than that of two metal layers, as shown in figure 3.
The size of the pad also affects the tombstone.If the welding pad extends too long beyond the chip element terminal, which will reduce the wetting angle, and the tension of molten solder on the element terminal will be increased, so that the tombstone rate will be higher.If the pad is too wide, the chip element will inevitably drift and make the control force between the two ends of the chip element out of balance, which will also create tombstone easily.
The metal size of chip element terminal is another factor that influences the tombstone.If the width and area of the metal terminals under the chip element are too small, they will reduce the tension under the chip element (counteracting the drive of the tombstone) and thus aggravate the tombstone.
(2) Chip components deviation, which is the most common reason besides pad design.Hot air reflow welding is convection heating, first heating the surface.Because the chip element has a certain heat capacity, thus the always exposed solder paste is first heated and melted.If the chip element is deviated along the length direction, the one end with a larger area of solder paste is melted first to generate surface tension and pull the components up, as shown in figure 4.
(3) Different heating temperature speed at the two solder ends of components will also lead to an increase in the phenomenon of tombstone.As shown in figure 10-53, if a pad is connected to a hole in the plane layer, the pad will have a lower heating rate and will melt then .For chip components, if the solder melt at both ends is out of sync, it will lead to tombstone.
(4) contamination or oxidation of the solder end of components or coating of PCB solder pad may lead to inconsistent weldability at both ends, which is also an important factor of tombstone.
(5)Too fast melting speed of solder alloy will also produce tombstone.Therefore, it is useful to use delayed non-eutectic solder paste when ordinary solder paste encounters serious tombstone problems.Delay wetting can be achieved by using a wide range of paste when solidifying.For example, using 62Sn/36Pb/2Ag produced fewer tombstone than using 63Sn/37Pb.
(6) Too thick solder paste printing may also be a problem.Higher printing thickness results in more tombstone because a large amount of molten solder will float the components.
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