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Printed circuit board (PCB) problems can be a challenge, but not all of them lead to premature obsolescence of your PCB, such as the monolithic effect.The monolithic effect is caused by the uneven surface tension created during the melting of solder paste on opposite sides of a passive component.These unequal forces cause one end of the passive component to lift up and break contact with the circuit, creating a phenomenon very similar to that of a monument in a cemetery (see Figure 1).

Reasons for erecting the monument

A variety of factors can contribute to the monumentation phenomenon. Monumentation is almost always caused by uneven wetting forces at the component end connections. When one end is ‘wetted’ before the other, the unbalanced wetting force of the solder ‘pulls’ on the component and causes it to rotate, causing it to stand upright. We also experience stand-offs when PCB components are not heated evenly. When a printed wiring board (PWB) passes through a reflow oven, the front side of the passive component is usually heated first (see the left side of the passive component in Figure 2). This uneven heating causes the solder paste deposit closest to the heat source (left side of Figure 2) to melt first. When this solder melts, its surface tension causes the passive component to stand upright.

Solder Alloy

One way to reduce monumentation is to use solder alloys that have a large ‘mushy’ or ‘plastic’ range when melted. The plastic range is the temperature range over which the solder melts and solidifies. For eutectic solders (e.g., Sn63/Pb37 solder), there is no plasticity range because the solder is fully cured at slightly below 183°C and fully liquid at slightly above 183°C. SnPbAg alloys (e.g., Sn62 or Indalloy ® 100) are widely used for eliminating stepping in leaded assemblies because of their larger plasticity range. 

Among lead-free solders, SAC3510 (Sn/3.5Ag/1Cu) has a narrower plasticity range, while SAC305 has a wider plasticity range. Therefore, one would think that SAC305 would be better at reducing monumentation - and it is. Figure 3 shows the experimental results for determining the rate of monumentation. Note that SAC3510 has more than six times the monumentation rate of SAC305. Since SAC305 is one of the most common lead-free alloys, the incidence of monumentation has been greatly reduced in the lead-free era.

PCB design

It is extremely important that manufacturing engineers work closely with design engineers to eliminate challenges and defects in PCB manufacturing. A riser defect is a defect that can be eliminated by proper design. If a board is designed with a heat sink (e.g., a copper layer) under or near a passive component on one side of the board and farther away on the other side, the heat sink may affect the thermal balance of the component; the solder paste on the side without the heat sink may first become liquid, which can lead to a standing stone defect.

Stencil Design

Minimising the amount of solder paste printed on the PCB pads will also reduce monumentation.It is particularly useful to reduce the amount of solder printed directly behind the end of the passive component, which will eliminate almost all of the riser effect.Figure 4 shows a typical stencil design that helps achieve this goal.In some experiments, this design completely eliminated the monumentation phenomenon.

Stencil design recommendation 2

PCB standstills are a problem caused by uneven PCB  thermal conductivit Another cause is poor solderability or component oxidation. The opening or thickness of the stencil can also affect standstills. Other factors that affect the quality of solder paste printing include press parameters and component placement.

One way to improve the template printing process is to make it thinner or thicker. the aperture size of the POR template is smaller than that of the BAE template. The two templates are similar in size, but the BAE template is thought to be better at limiting tombstone defects. In general, BAE apertures work better than POR templates for 0.4 mm printing. However, BAE apertures are significantly more effective in limiting tombstone defects when using fresh solder paste.

To avoid the tombstone effect, it is important to ensure that components are placed correctly. A good stencil design ensures even coverage of the copper pads, thus preventing this problem. In addition, the thickness of the soldermask must be appropriate to avoid oxidation and tombstone effects. If the soldermask is too thick, solder beads may result.

The stencil design of the PCB is an essential part of the assembly process. Any error may lead to failure. Therefore, good stencil design is essential to obtain consistent printing results. Although we can print most PCBs without using complex stencils, some cases require more complex stencil designs.

Stencil design also affects solder paste release. Stencil material should be 4 to 8 thousandths of an inch thick to accommodate solder paste. In addition, the stencil should be thick enough to allow five solder particles across the smallest aperture.

Solder Paste Printing

Printing process and transfer efficiency are key factors in many end-of-line defects, including riser defects.If there is more solder paste on one side of a passive component than the other, the component may be placed in a position where it is only exposed to higher deposits.This is likely to result in monumental defects.The use of Solder Paste Inspection (SPI) equipment can help ensure that solder paste deposits are within specification and that one deposit is not higher than the other.Optimising the size and shape of the aperture can also help minimise variations in solder paste volume between pads.

PCB Standing Stones occur because there is a large difference in the wetting power of the solder paste at each end of the device.One end of the device may be well soldered while the other end is poorly soldered.The wetting power of the solder paste at the bad end is lower than the melted solder paste at the other end.As a result, the wetting force of the solder paste at the bad end is not uniform.This imbalance in force is what causes the standoff phenomenon.

Printing offsets are another problem, resulting in inadequate solder paste contact.In addition, the printed board is incomplete.This will result in the tombstone not being fully functional.Fortunately, there are some simple solutions to avoid this problem.The key is to control the solder paste application process and maintain proper thermal balance.

There are two main reasons for the PCB Phenomenon of Monumentalisation. Firstly, pads are poorly designed. As a result, they usually do not cover more than 50% of the component terminals. In addition, the spacing between pads is too wide. When these problems occur, reflow soldering may result in standoffs.

Second, the board surface must be preheated before starting the soldering process. Excessive temperature differences at the end of the board may cause tin beads to form, making the PCB look like a tombstone. A reflow soldering process with a controlled temperature profile will reduce tombstoning.

Another problem associated with the tombstone phenomenon is the wetting force. The wetting force at one end is much greater than the combined force at both ends. The result is that one end of the assembly will rise and the other end will fall. To avoid this problem, choose smaller, lighter assemblies.

chip

Improper mounting pressure and/or Z-height are often the cause of monolithic phenomena.It is important to ensure that the mounting pressure and Z-height are appropriate for the assembly and optimised before production.Parts may also be skewed during placement.Although solder tends to self-align, incorrect placement and misalignment of components can also cause standoff.

reflow soldering

One way to reduce the monumental effect is to reduce the total heat input during reflow by gradually increasing the rate of temperature increase; however, this condition may be difficult to achieve in a reflow oven. Another option is to use a soak-type reflow profile to achieve thermal equilibrium between the two solder paste deposits so that both deposits enter the liquid phase phase line at the same time.

In addition to avoiding solder that does not have a plasticity range, the nitrogen environment in the reflow oven can exacerbate the standoff phenomenon because nitrogen increases the rate of wetting, causing surface tension to appear more quickly. Unless the board has fine featured solder paste deposits or package stacking (PoP) components, there is likely no reason to use nitrogen, as most top modern solder pastes perform well in an air reflow environment.

Reflow soldering2 Uneven fountain power

A PCB monument defect is a PCB defect caused by uneven solder wetting forces. The uneven wetting force is due to a large difference in solder paste temperature between the two ends of the device. This causes the solder paste at one end to melt while the paste at the other end does not. As a result, the wetting force of the standing monument is lower than the other end.

The most common cause of the standing monument phenomenon is an imbalance in the wetting force between the two terminals. The force generated at one end of the component is greater than the combined force at both ends, causing the component to protrude from the solder. This is especially problematic in smaller and lighter components.

Another common cause of tombstoning is uneven thermal conductivity of the PCB. This can be caused by improper solder application or insufficient solder paste thickness. Therefore, solder paste printers should also be calibrated to ensure uniform thickness.

Uneven wetting power of PCB monolithic solder joints is a common problem when reflow soldering components. This uneven wetting force causes the component to stand up at one end and overwhelm the component at the other. This often occurs when the component is small and in a small package. Properly balancing the wetting forces at both ends can prevent this problem.

Uneven wetting forces on the PCB riser can be reduced by adjusting the pad size. Larger pad sizes increase the surface area of the molten solder, which creates a stronger wetting force on the end of the component. This also reduces the likelihood of component separation.IPC standards provide recommendations for pad sizes. However, pad sizes vary from manufacturer to manufacturer.

Manhattanattan borough of New York City

PCB tombstone defects, also known as the Manhattan effect or Stonehenge effect, are very serious in surface mount devices. It is caused by an imbalance in wetting forces and can lead to circuit breakage. This problem usually occurs when the solder paste at one end of the device is much hotter than the other. This can lead to a severe force imbalance and result in tombstoning.

An important factor in preventing tombstoning is component placement. When components are small, they must be placed evenly across the pads. In addition, the width and direction of the alignments must be the same. If they are not equal, the solder will not reach the component. This problem is common in low-cost, low-margin products, but can also occur in higher-margin devices.

Another factor that affects the PCB standoff phenomenon is uneven wetting forces. During PCB reflow soldering, uneven wetting forces can cause chip components to stand up. This condition is known as the Manhattan effect. Improvements in process design, quality control, process modulation and straight-through assembly rates can reduce this condition. Good solderability of the solder paste is also essential. Solder joint size errors can be controlled, but this is more sensitive to the size of the PCB pads.

Imbalance can cause a Manhattan effect in the solder paste on both sides of the PCB. When this happens, the solder paste at one end of the device may melt before the other. As a result, it pulls the device towards the melted side. The Manhattan effect, also known as the drawbridge effect, can lead to a tombstone effect.

reach a verdict

Monumentation can be minimised by following the guidelines outlined in this document. A reliable way to reduce standoffs is to use solder alloys with a large plasticity range, such as Indalloy ® 100 (leaded) or SAC305 (lead-free). It is also important to maintain an open dialogue with the board designer to minimise heat sinks near or under the passive component side. Use reflow soldering temperature profiles to move slowly from solid phase to liquid phase lines, or minimise thermal mismatch by soaking. Incorporate SPI to ensure that solder deposits are of similar height and to specification, and print smaller solder deposits, especially at the ends of passive components. Ensure that placement pressure and Z-height are appropriate and that parts are not placed at an angle. Avoid using nitrogen in the reflow oven; use premium solder paste, which is unnecessary and exacerbates standoffs.