What is the Soldering drawbridge effect?

Solder Drawbridge Effect: Analysis of the Monumental Phenomenon in Surface Mount Technology

I. Definition and Phenomenon

Soldering drawbridge effect, also known as Tombstone Effect or Manhattan drawbridge, is a common assembly defect in surface mount technology (SMT). It is characterised by chip passive components (such as resistors, capacitors, inductors) in the reflow soldering process, one end of the PCB pads from the warping, shaped like a tombstone or drawbridge structural abnormalities.

II. Causes and Mechanisms

1. Core causes

Uneven melting of solder paste:

Components at both ends of the solder paste heat melting time is not consistent, resulting in differences in wetting power. One end of the paste melts first, generating surface tension, the other end is not melted or melted slower, resulting in an unbalanced moment.

Solder pad design problems:

Size asymmetry: pads are too large or inconsistent in size at both ends, causing components to slide.

Connection to the ground board: the pad is connected to the ground board or large copper area, resulting in uneven heat dissipation and affecting the melting speed of the solder paste.

Alignment design: Asymmetrical alignment of pad connection, resulting in heat capacity difference.

Component characteristics:

Chip capacitors/inductors: square end face, large height, long wetting force arm, easy to generate torque differences.

Resistors: lower incidence due to smaller height.

Process Parameters:

Reflow temperature profile: too fast temperature rise rate (>2°C/sec) leads to large temperature difference.

Solder Paste Thickness: Too thick to increase the surface tension difference, recommend using a thin stencil (e.g. 0.1mm).

Mounting accuracy: severe component offset, resulting in poor contact with solder paste at one end.

2. Physical mechanism

Wetting force imbalance:

When solder paste melts, the surface tension difference causes one end of the component to be pulled up.

Buoyancy effect:

Components suspended in the liquid solder, uneven force generated by the rotation, the formation of "monument".

III. the impact and harm

Electrical performance: lead to open circuit or poor contact, affecting the function of the circuit board.

Reliability: increase the cost of rework, reduce product qualification rate.

Production efficiency: additional testing and rework, affecting the production beat.

IV. Measures to solve the problem

1. Pad design optimisation

Symmetrical design: Design symmetrical pads according to IPC-7351 standards to avoid oversize or asymmetry.

Alignment planning: The pad connection alignment should be symmetrical to avoid connecting to the ground board or large copper areas.

Hole location: keep the distance between the hole and the edge of the pad ≥ 0.25mm to avoid solder core suction.

2. process control

Temperature profile: control the reflow soldering temperature rate ≤ 2°C / sec, to ensure uniform heating.

Solder paste management:

Use a thin stencil (0.1mm) to control the thickness and reduce surface tension differences.

Avoid solder paste contamination or oxidation, store in 0~10°C environment.

Mounting accuracy: Improve the precision of the mounter to reduce component offset.

3. Component selection

Priority heavy components: choose larger size and heavier components to reduce the impact of buoyancy.

Avoid thin components: such as 0402 the size of the following components, vulnerable to process fluctuations.

4. Other measures

Through-hole filling: Fill the through-holes in the soldering discs to reduce solder core absorption.

Employee training: Enhance the production line staff's understanding of the phenomenon of standing monument and the ability to deal with it.

V. Difference with psychological drawbridge effect

Psychological drawbridge effect:

Refers to people in dangerous or stimulating environments, due to physiological arousal (such as rapid heartbeat) mistakenly attributed to the environmental stimuli to the attraction of the people around them, resulting in romantic feelings.

Welding drawbridge effect:

A physical phenomenon in engineering technology, unrelated to psychological effects, that results in structural defects due only to uneven stresses on components.

VI. the summary

Welding drawbridge effect is a typical defect in SMT chip processing, need to optimise the design, process control and material selection comprehensive solution. Its name comes from the phenomenon resembles a drawbridge, but no direct correlation with the psychological "drawbridge effect", belongs to the engineering field of terminology.