Effect of component placement offset on tombstone defect

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Supervisor:
Dr. Krammer Olivér
Department of Electronics Technology

The tombstone effect became a typical failure phenomenon by the reduction in the size of passive components. Appearance of the failure, vapour phase soldering such as soldering technique was again to the fore, because with the development of the soldering furnace the defect formation can be reduced. The improvement was that by moving the soldering oven tray we can create a soaking time in the reflow profile, thus reducing the failure occurring. This type of soldering is called Soft vapor phase soldering. During the soldering process, it is essential that the melted solder what position offset can still corrects. In this thesis I examine the effect of different placement position offsets after soldering.

In my work I carried out literature survey about the various steps of the soldering, and in relation to these emerging problems. I reviewed the vapor phase soldering in details; especially the soft vapor phase soldering, and vacuum vapor phase soldering. After examining the techniques of soldering, I studied the tombstone effect like the typical failure of soldering, and I described its two-dimensional kinetic model

In the first step of my experiment, I designed a specific PCB with 180 passive components, and with 9 different pad gaps. After applying the solder paste onto the PCB, the placement machine placed the passive resistors in 5 groups with position offsets of 0, 100, 200, 500, 700 μm. The components were soldered by vapor phase soldering; afterwards, I analyzed the solder joints with a stereomicroscope.

In my measurements, I got the result that at 400–1000 μm pad gaps the melted solder can completely self-align components with 0–200 μm position offsets. For greater pad gaps the positional offset greatly reduced the number of components without failed joints after the soldering process.

In the case of 400 to 700 μm pad gaps and 500 μm position offset, the yield was around 50%, and it decreased further at higher pad gaps. In the case of 700 μm position offset, regardless of the pad gaps, the yield was approximately 0%.

The number of tombstone effects increases as well by increasing pad gaps as by increasing positional offsets. Lower than 800 μm pad gap, there is only one case (500 μm pad gap and 200 μm position offset) where tombstone effect appeared. In the case of 1000 μm pad gap, typically at 500–700 μm position offset the failure rate is about 25–40 %, in the case of 1000–1200 μm pad gap, and at 200–300 μm position offset caused 20–25% failure rate, and at 1400 μm pad gap, every position offset caused at least 30% tombstone effect.

Based on the results, my recommendations are that: at 400–1000 μm pad gap up to 200 μm position offset is allowed, correction is not necessary in components position; in the case of 1100 μm pad gap, the 100 μm position offset need to be corrected, and pad gaps above 1100 μm are not recommended.

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