Analysis and Solutions to "Tombstoning" Defects in SMT Soldering Process

Abstract

The  The technical requirements for the assembly of short, small, light and thin discrete chip components and IC (Integrated Circuit) chips on PCB (printed circuit board) are becoming increasingly demanding. Reduce the "erection monuments" defect in SMT (Surface Mount Technology) soldering process, Based on the  years of project design experience, this article provides a basic explanation of PCB pad design, routing and pad connection methods, scientific design of optical positioning mark, screen printing and placement accuracy, furnace temperature curve design, etc, Explore the defects of "erecting monuments" and methods and measures to reduce or eliminate component "erection monuments", in order to improve SMT soldering quality.

0 Introduction

Currently, global electronic products are gradually developing towards multi-functionality, intelligence, integration, and miniaturization. When electronic products have more functions and the space layout of printed circuit boards (PCBs) is limited, surface mount technology (SMT) can combine short, small, light, and thin discrete chip devices with different performances and high-density PCBs, which has become the mainstream of electronic assembly technology.

SMT technology is relatively mature, but it involves many manufacturing processes. In actual production, some common defects may occur. This paper briefly introduces the "tombstoning" defect and provides corresponding solutions to help improve the production quality and efficiency of SMT placement and reduce manufacturing costs.

1 Phenomenon and Hazards of "Tombstoning"

In SMT production, during reflow soldering, the two ends of the component tilt or flip due to unbalanced force, resulting in one end of the discrete chip component "standing upright" or both ends lifting, as shown in Figure 1. This phenomenon, where one end of the component detaches from the pad and lifts while the other end is not fully soldered, is commonly known as "tombstoning" [2]. This defect can cause open or short circuits at the solder joints, leading to failure of electrical performance and subsequent impact on circuit functions.

                   (a) Capacitor "tombstoning"             (b) Resistor "tombstoning"

（Figure 1 "Tombstoning" Phenomenon in Reflow Soldering）

2 Analysis and Solutions to "Tombstoning" Defects in SMT Soldering

2.1 Solving "Tombstoning" through Design

2.1.1 Pad Design

2.1.1.1 Unreasonable Pad Design

Pad design is a key part of PCB design. It not only determines the soldering position of surface-mount devices (SMDs) on the printed board but also plays an important role in the reliability of solder joints, potential "tombstoning" defects during soldering, testability, visual inspection, and maintenance. For small discrete chip components, designing pads of different sizes at the two ends of the component may cause the component to stand upright. When the pad size at one end is larger than that at the other end, it is the main cause of the "tombstoning" effect.

2.1.1.2 Optimized Pad Design

Symmetrical Design: The size and shape of PCB pads must be strictly symmetrical, referring to the IPC-7351 standard (e.g., the recommended pad spacing for 0402 components is 0.5 mm). Adjustment of Pad Spacing: Avoid excessive spacing leading to component floating or insufficient spacing causing solder paste bridging. The design requirements for SMD pads are shown in Figure 2 and Table 1.

                      (a) Pad                              (b) Mounted Component

（Figure 2 SMD Pad Design）

Table 1 Design Elements of SMD Pads

2.1.2 Trace-Pad Connection Design

2.1.2.1 Unreasonable Trace-Pad Connection Design

Different conductor areas of the trace connected to the pad will result in one end conducting heat too quickly during soldering, causing a difference in melting time between the two ends, which may also lead to component "tombstoning". For example, if one end of the pad is connected to the ground or power supply, the end with a large-area ground wire will have an increased heat capacity, resulting in uneven heating of the two ends of the component during soldering. Alternatively, if one end of the pad has a ground via, the via will cause solder loss, thereby reducing surface tension, which also leads to unbalanced force and is prone to "tombstoning".

2.1.2.2 Solution: Thermal Balance Design

Adopt "thermal isolation" design for high thermal conductivity pads (such as reducing trace width or adding thermal isolation slots). The adjusted trace-pad connection methods are shown in Figure 3.

Figure 3 Design Elements of PCB Pads for Trace-Pad Connection

2.1.3 Design of Optical Positioning Mark (Fiducial Mark)

2.1.3.1 Defects in Fiducial Mark Design

Fiducial marks are used for optical positioning during solder paste printing and component placement, providing common measurable points for all steps in the SMT assembly process. Asymmetrical or insufficiently precise placement fiducial marks will cause component misalignment. During mass production, it is necessary to ensure that each equipment used in assembly can accurately locate the circuit pattern.

2.1.3.2 Solution: Correct Fiducial Mark Design

(1) Composition of Fiducial Marks: A complete fiducial mark consists of a positioning point (also called a mark point) and a clear area.

(2) Shape of Fiducial Marks: The preferred shape is a solid circle.

(3) Size of Fiducial Marks: The preferred size is 1.0 mm in diameter, with a maximum diameter of 3.0 mm; it is particularly emphasized that all fiducial marks on the same PCB part number must be of the same size.

(4) Solder Mask Opening for Fiducial Marks: The solder mask shape is a circle concentric with the fiducial mark, with a size twice the diameter of the fiducial mark. A circular copper wire protection ring is required at its edge, with a wire thickness of 0.2 mm.

(5) Position of Fiducial Marks: ① Each assembly unit board must have at least 2 centrally asymmetrical (for foolproofing) fiducial marks, distributed on the longest diagonal or at three corners; for large-size or high-precision placement such as dual flat package (DFP), chip scale package (CSP), and ball grid array package (BGA), local fiducial marks should be separately set at the diagonal or center positions. ② The distance from the edge of the fiducial mark to the PCB board edge is recommended to be more than 5 mm, with a metal ring for protection.

(6) Requirements for the Clear Area Around Fiducial Marks: To ensure the recognition effect of printing equipment and placement equipment, a clear area free of other silk screens, printed wires, pads, or v-groove cuts (V-CUT) must be reserved around the fiducial mark. The radius r of the clear area is ≥ 2R (R is the radius of the fiducial mark), and the machine recognition effect is better when r reaches 3R.

2.2 Solving "Tombstoning" through Process Factors

2.2.1 Screen Printing and Placement Offset

Generally, during the placement process, screen misalignment and different printing forces and directions can cause offset of small discrete chip components. During reflow soldering, the surface tension when the solder paste melts pulls the component to automatically correct its position, which is called "self-alignment". If the offset is excessive, the pulling force will instead cause the component to "stand upright" and produce "tombstoning".

Analysis of Causes: Poor repeatability of the printing machine, placement alignment offset (inaccurate screen/PCB alignment, improper screen opening or printing parameters) leading to solder paste printing outside the pads and uneven distribution of solder paste volume; the end of the device in contact with more solder paste obtains more heat capacity and thus melts first. Mismatch between the screen window size and thickness design leads to excessive solder paste, resulting in different adhesion forces between the two ends of the device and the solder paste during soldering heating.

Solutions: Prepare a thinner screen before printing, and carefully check the screen's font size, line thickness, pattern, position dimensions, presence of broken lines, deformation, or damage. Inspect the contamination, oxidation level, and presence of foreign objects on the pad metal layer of each substrate to ensure no foreign objects exist; do not touch the PCB substrate directly with hands and wear anti-static gloves. Adjust the placement accuracy of the printing machine, correctly place the substrate into the printing machine for operation. According to the printing program compiled by the printing machine, the placement machine can calibrate by manually or automatically finding fiducial marks by the operator to ensure that each equipment used in assembly can accurately locate the circuit pattern. After printing, pay attention to observing the printed quality.

2.2.2 Temperature Profile Control

Out-of-control reflow temperature profile, uneven preheating, or excessively high peak temperature will exacerbate the difference in thermal stress between the two ends. After placement, put the PCB into the reflow oven for reflow soldering. Before soldering, test the preheating zone of the oven temperature with an actual board: control the heating rate at 1~3 ℃/s to reduce thermal shock, and the tested curve should adopt a thermal curve with a very slow heating rate. When producing the first side (bottom), the PCB can be placed on the grid of the reflow oven for soldering. Reflow Stage: The recommended peak temperature is 235~245 ℃ for 40~70 s to ensure synchronous melting of the solder joints at both ends. When producing the second side (top), it must be soldered on the track. When soldering on the track, pay attention to whether the track affects the components on the PCB. Cooling Zone Stage: The forced air cooling rate ≤ 4 ℃/s to avoid stress concentration caused by rapid cooling. After the substrate passes through the reflow oven, conduct a visual inspection of the product. Eliminate the "tombstoning" phenomenon, reduce soldering defects, improve product quality, and achieve cost reduction and efficiency improvement.

3 Conclusion

With the rapid development of artificial intelligence technology, there is an increasingly urgent demand for low-cost and high-reliability electronic assembly products. There are many factors affecting the soldering quality of SMT production, and this paper only lists the common component "tombstoning" defect. To reduce or avoid other soldering defects, in addition to optimizing PCB pad symmetry and thermal balance from the design end, controlling printing, placement, and reflow soldering from the process end, and strictly managing the production process, it is also necessary to improve the ability of process personnel to judge and solve defect problems, improve management measures, and formulate and improve the production process flow. Only in this way can high-quality electronic products be produced.

                                                                              by Liuxiang