What are dry solder joints and solder beads?

Table of Contents

During the PCB (printed circuit board) assembly process, dry solder joints and solder beads are very common welding defects, which may cause circuit malfunction or even failure. Let's take a deeper look at what dry solder joints and solder beads are and common problem solutions.

1. Dry Solder Joint

1.1 Definition

Dry solder joints refer to the failure to form a reliable metallurgical bond between the solder (tin paste) and the component pins and pads. The surface appears to be welded, but the actual electrical connection is unstable or the mechanical strength is insufficient.

1.2 Main Reasons

• Dry solder joints are mainly caused by the failure of the solder and solder surface to be fully wetting, and the surface is contaminated before welding.

• The surface of the component pins and solder has oxides, grease, flux residues or dust that hinder the diffusion of solder. Or the plating thickness of the pins or pads is not enough or the adhesion is poor, and the plating falls off during welding, resulting in dry solder joints.

• Insufficient soldering temperature. The temperature of the soldering iron tip or reflow soldering does not reach the melting point of the solder. The solder fails to melt completely, which can also cause false soldering.

• Short heating time, and the solder fails to fully diffuse to the metal interface to form an alloy layer (normally it takes 2-3 seconds to form a reliable solder joint).

• Excessive pressure in manual soldering can crush components or cause solder accumulation without internal fusion.

• Unstable solder wave height and too fast heating rate in the preheating stage. Thermal expansion coefficients of some solder joints and components and pads can also cause false soldering.

1.3 Solutions

(1) Clean the surface to remove contaminants. Wipe the component pins and PCB pads with anhydrous alcohol or flux cleaning agent to remove oxides, grease, flux residue or dust.
(2) For severely oxidized copper feet, we can first dip a small amount of solder and flux into the soldering iron for tinning to form a new solder surface.
(3) To control the quality, use Automatic Optical Inspection (AOI) after reflow to identify abnormal solder joints (insufficient solder, irregular contours).
(4) Use X-ray Inspection (AXI) to find solder joints (such as BGA) and observe the connection status between solder balls and pads.
(5) Perform functional tests on PCBs, test working environment vibration and repeatedly switch temperature tests to expose potential dry solder joints.

2. Solder Beading

2.1 Definition

After soldering, the solder is spherical (usually 0.1-0.3mm in diameter) or irregular, with a smooth surface or burrs, which may cause short circuits or contamination. It mainly appears around the pad, the gap between component pins, the gap between PCB solder mask or large copper foil areas. During reflow soldering, tin beads may splash on the surface of the IC package.

2.2 Main Reasons

(1) Excessive solder paste printing, too large stencil opening size, or too thick stencil thickness will lead to excessive accumulation of solder paste. After heating, part of the solder will break away from the solder joint due to gravity or tension to form solder beads.
(2) Moisture in the solder paste causes the solvent inside to volatilize rapidly when heated during soldering, pushing the molten solder to splash and form solder beads.
(3) If the Sn content in lead-free solder paste is too high, the surface tension during melting will be low, making it prone to forming small spherical particles.

2.3 Detection Methods

(1) Visual inspection (VI): Observe the PCB surface through a magnifying glass or microscope to see if there are tiny spherical particles.
(2) Automatic optical inspection (AOI): High-density circuit boards can use image recognition technology to identify tin beads.
(3) Ion contamination test: Through the ionic contamination of the PCB surface and measuring the electrical inversion rate, the risk of tin beads containing metal ions can be indirectly determined.

3. The Difference Between Cold Solder Joint and Solder Bead

• Dry Solder Joint: Roughness and cracks are easy to appear on the surface of the solder joint, mainly because the soldering interface is not fully combined. The main risk is intermittent circuit breakage, which can be solved by optimizing temperature, cleaning surface and controlling soldering amount.

• Solder Bead: It is manifested in independent spherical particles attached to non-welding areas, mainly because of abnormal solder aggregation/splashing. The main risk is short circuit or ion contamination, which can be solved by adjusting steel mesh parameters, optimizing reflow curve and increasing curvature.

4. Summary

In short, dry solder joint and solder bead are defects that need to be avoided in PCB assembly. Dry solder joint is mainly caused by substandard welding process, and solder bead is mainly related to solder control and environmental parameters. By optimizing design (such as increasing pad spacing), strictly implementing process standards (such as temperature and curve monitoring) and introducing automated detection (such as AOI optical detection), the incidence of two types of defects can be significantly reduced and product reliability can be improved.

Solder bead may cause instantaneous short circuit or long-term leakage, get stuck in the gap of components during thermal expansion and contraction, causing mechanical failure and accelerating PCB aging.

To Meet Your Needs, You Need a PCB Company with the Following Qualities:

1. They can customize the process plan according to the needs to reduce the risk of cold soldering.

2. They can ensure that each PCBA meets the factory standards from component selection (recommending cost-effective alternative materials) to AOI + X-ray inspection.

3. Provide transparent quotation details, from bare boards, patches, and detailed quotations for testing, and support small batch trial production.

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