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Causes and countermeasures of
2021-09-29
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Author:Kavie

In the reflow soldering process of the surface mount process (smt mount), the chip components will have the defect of desoldering due to the uplifting, which is called the "tombstone" phenomenon (that is, the Manhattan phenomenon).

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  . The "tombstone" phenomenon occurs in the reflow soldering process of CHIP components (such as chip capacitors and chip resistors). The smaller the component, the more likely it is. The reason is that when the solder paste on the pads at both ends of the component is reflowed and melted, the surface tension on the two solder ends of the component is unbalanced. The specific analysis has the following 7 main reasons:

  1) Uneven heating Uneven temperature distribution in the reflow oven Uneven temperature distribution on the board surface

  2) The problem of the component The shape and size of the soldering end are different The solderability of the soldering end is different The weight of the component is too light

  3) Substrate material and thickness Poor thermal conductivity of substrate material Poor uniformity of substrate thickness

  4) The shape and solderability of the land. The heat capacity of the land varies greatly. The solderability of the land varies greatly.

  5) Solder paste The uniformity or activity of the flux in the solder paste is poor. The thickness of the solder paste on the two pads is different. The solder paste is too thick

  Poor printing accuracy, serious misalignment

  6) Preheating temperature The preheating temperature is too low

  7) Poor placement accuracy and serious component deviation.

   "tombstone" phenomenon is the result of the mixed effect of the above various factors. The following is a simple analysis of the above main factors.

  Welding method incidence

   GRM39(1.6×0.8×0.8mm) GRM40(2.0×1.25×1.25mm)

  Gas phase heating 6.6% 2.0%

   Infrared hot air reflow soldering 0.1%0

  Welding method Incidence rate

   GRM39(1.6×0.8×0.8mm) GRM40(2.0×1.25×1.25mm)

  Gas phase heating 6.6% 2.0%

   Infrared hot air reflow soldering 0.1% 0

Warm-up period: Table 1 is the experimental statistical results of tombstone phenomenon in infrared heating and gas-phase heating reflow soldering. In the test, 1608 and 2125 chip capacitors are used. The test is infrared and hot air reflow soldering and gas-phase heating reflow without preheating. Soldering, it is obvious from the table that the occurrence rate of tombstone phenomenon is much higher than that of the former. This is because the gas phase heating has no preheating zone, which makes the temperature rise very fast. As a result, the probability that the solder paste at both ends of the component will not melt at the same time greatly increase.

The preheating temperature and time are very important. We have carried out experimental statistics for the preheating time of 1-3 minutes and the preheating temperature of 130-160 degrees. The results can be clearly seen that the higher the preheating temperature, The longer the warm-up time, the lower the incidence of "tombstone" phenomenon.

   The high preheating temperature we tested was 170 degrees, which was slightly higher than the preheating temperature during normal production. It was found that when the preheating temperature rose from 140 degrees to 170 degrees, the incidence of "tombstone" phenomenon was greatly reduced. This is because the higher the preheating temperature, the smaller the temperature cutoff at both ends of the component after reflow soldering, and the closer the melting time of the solder paste at both ends will be. However, the longer the solder paste is exposed to a higher preheating temperature, the more serious the deterioration of its flux will be, and the worse the flux will be, the more likely it is to produce soldering defects.

The experimental results of the relationship between the pad size and the tombstone phenomenon show that when B and C decrease, the incidence of tombstone phenomenon decreases, but when C is less than 0.7mm, as C decreases, The incidence of component displacement defects has increased significantly. See schematic

  In the test, it was found that the pad pitch was reduced from 2.8mm to 2.0mm, and the incidence of "tombstone" phenomenon was reduced by 90%, which was only one-tenth of the original. This is because after the size of the pad is reduced, the amount of solder paste applied is correspondingly reduced, and the surface tension of the solder paste when it is melted also decreases. Therefore, in the design, under the premise of ensuring the strength of the solder joint, the size of the pad should be as small as possible. Solder paste thickness When the thickness of the printing template is 20 um, the incidence of tombstone phenomenon is much greater than when the thickness of the stencil is 100 um. This is because 1. Reducing the thickness of the steel mold means reducing the amount of solder paste, and the surface tension of the solder paste when melting is reduced accordingly. 2. Reduce the thickness of the steel mold to make the solder paste thinner, the heat capacity of the entire pad is reduced, and the probability of the solder paste on the two pads being melted at the same time is greatly increased.

   Mounting accuracy Under normal circumstances, the component deviation generated during mounting is automatically corrected by pulling the component due to the surface tension when the solder paste melts during the reflow process. We call it "adaptive". However, when the deviation is severe, pulling will cause the component to stand up, causing tombstone phenomenon. This is because: 1. The heat transfer from the soldering end of the component to the solder paste is uneven, and the end with less solder paste will melt first when heated. 2. The adhesion between the two ends of the component and the solder paste is uneven.

The substrate material test used three different substrates. It was found that the tombstone phenomenon occurred in the paper-based epoxy board, followed by the glass epoxy board, and the alumina ceramic board was low. This is because of the thermal conductivity and heat capacity of different materials. different.

   Solder Paste Due to the difference in flux composition, activity and metal content in solder paste, the occurrence of "tombstone" phenomenon is also different.

  Component weight The smaller the component weight, the higher the defect rate.

   Of course, there are many other influencing factors, such as severe offset, vias on the pad, inconsistent pad design, uneven solder coating, etc.

With the continuous improvement of placement precision, more and more smaller components such as 0603, 0402, 0201, etc. are used, but the tombstone phenomenon caused by placement offset has greatly increased the proportion of the entire defect occurrence rate. , Becomes a key factor.

  How to avoid tombstone phenomenon

   1. There is no oxidation on the surface of pads and components.

  2. The pad design is the same, and there is no via hole on the pad.

  3. Try to ensure that the placement accuracy is above 90% during placement.

  4. The reflow soldering furnace must be tested first when soldering, and after finding a suitable temperature profile process, it can be soldered in large quantities.

The above are the causes and countermeasures of the "tombstone" phenomenon in the SMT production process. Ipcb also provides PCB manufacturers and PCB manufacturing methods.