Analysis and countermeasures of the "monument" phenomenon

In the surface mount process of reflow soldering process, chip components will produce due to warping and desoldering defects, people image called "vertical monument" phenomenon (i.e. Manhattan phenomenon).

The "monument" phenomenon occurs in the CHIP components (such as chip capacitors and chip resistors) reflow soldering process, the smaller the size of the component is more likely to occur. The reason for this is that the solder paste on the pads at both ends of the component is not balanced in terms of surface tension between the two soldered ends of the component when it is melted in reflow.

"Vertical monument" specific analysis of the following seven main reasons:
(1) uneven heating uneven temperature distribution in the reflow oven uneven temperature distribution of the board surface
2) Problems of components The shape and size of the weld end varies greatly The weldability of the weld end varies greatly The weight of the components is too light
(3) Material and thickness of the base plate Poor thermal conductivity of the base plate material Poor thickness uniformity of the base plate
(4) Shape and solderability of pads Large difference in heat capacity of pads Large difference in solderability of pads
(5) Solder paste Poor flux uniformity or activity in solder paste Large difference in thickness of solder paste between two pads Too thick solder paste
Poor printing accuracy, serious misalignment
6) Preheating temperature Preheating temperature is too low
(7) Poor mounting accuracy, severe component offset.

The "monument" phenomenon is the result of a mixture of the above factors, the following is a brief analysis of these major factors.

Welding method Incidence
GRM39 (1.6×0.8×0.8mm) GRM40 (2.0×1.25×1.25mm)
Vapor-phase heating 6.6％ 2.0％
Infrared hot air reflow soldering 0.1% 0

Preheating period: Table 1 is the infrared heating and gas-phase heating reflow soldering in the vertical monument phenomenon in the test statistical results, in the test used 1608 and 2125 chip capacitors, the test is infrared and hot air reflow soldering and no preheating of the gas-phase heating reflow soldering, from the table can be clearly seen in the vertical monument phenomenon of the incidence of the latter is much greater than the former, which is due to the gas-phase heating without a preheating zone, so that the temperature rise is very fast, and the results Components at both ends of the paste does not melt at the same time the probability of greatly increased.

Preheating temperature and time is quite important, we were on the preheating time of 1-3 minutes, preheating temperature of 130-160 degrees under the conditions of reflow soldering test statistics, the results can be clearly seen, the higher the preheating temperature, preheating time is longer, "vertical monument" phenomenon of the incidence of the lower.

We test the preheating * high temperature is 170 degrees, slightly higher than the normal production preheating temperature to be a little higher, found that the preheating temperature rose from 140 degrees to 170 degrees, the "monument" phenomenon occurs greatly reduced. This is because the higher the preheating temperature, into the reflow soldering, components at both ends of the temperature off the smaller, the closer the two ends of the solder paste melting time. But the longer the solder paste is at a higher preheating temperature, the more serious the degradation of its flux, the worse the flux, the more likely to produce welding defects.

The test results of the relationship between the pad size pad size and the incidence of vertical monument phenomenon, it is clear that when B and C decreases, the incidence of vertical monument phenomenon decreases, but when C is less than 0.7mm, with the decrease of C, the incidence of defects of component displacement increases significantly.

Tests found that the pad spacing from 2.8mm reduced to 2.0mm, "vertical monument" phenomenon has reduced the incidence of 90%, only one-tenth of the original. This is because the size of the pad is reduced, the amount of solder paste coating is reduced accordingly, the surface tension of the solder paste melting is also followed by a reduction. So in the design, under the premise of ensuring the strength of the solder joints, the pad size should be as small as possible. Solder paste thickness printing template thickness of 20UM when the incidence of vertical monument phenomenon is much greater than the template thickness of 100UM when the situation. This is because 1, reduce the thickness of the mold, is to reduce the amount of paste, paste melting surface tension decreases. 2, reduce the thickness of the mold, so that the paste is thinner, the entire heat capacity of the pad is reduced, the probability of two pads on the paste melt at the same time greatly increased.

Mounting accuracy in general, mounting components generated by the offset, in the reflow process, due to the surface tension of the solder paste melting, pulling the components and automatically corrected, we call it "adaptive".

However, when the offset is serious, pulling will make the component stand up, resulting in the phenomenon of vertical monument. This is because:
1, from the welding end of the component to the solder paste transfer of heat is not equal, the solder paste is less at the end of the first melting when heated.
2, the component ends and the adhesion of the solder paste is not flat.

Substrate material test used three different materials of the substrate, found that the phenomenon of vertical monument in the paper-based epoxy board in the incidence of * high, followed by glass epoxy board, alumina ceramic board * low, this is because of the thermal conductivity and heat capacity of different materials are different.

Solder Paste Due to the different composition, activity and metal content of the flux in the solder paste, the occurrence of the "monument" phenomenon is not the same.

Component weight The smaller the weight of the component, the higher the incidence of defects.

Of course, there are many other influencing factors, such as severe offsets, pads with holes, inconsistent pad design, uneven solder distribution, and so on.

With the mounting precision continues to improve, the smaller 0603 and 0402, 0201 and other components are more and more used, only due to the mounting offset caused by the phenomenon of vertical monument accounted for the proportion of the overall defects occurring rate has increased greatly into a key factor.

How to avoid the occurrence of vertical monument phenomenon
1, no oxidation on the surface of pads and components.
2, the pad design is consistent, no holes above the pad.
3, try to ensure that the placement accuracy of more than 90%.
4, reflow soldering furnace in the welding must be the first test, to find the appropriate temperature curve process before welding in large quantities.