In this article, we will introduce PCBA failure analysis in PCB assembly production and their solutions.
#1 Cold Solder Joints
Definition: Cold solder joints refer to the phenomenon of electrical isolation between the solder terminal/pin and the solder pad after soldering.
(Cold Solder Joints)
Features: The solder and the component pin do not form an alloy layer of sufficient thickness, resulting in poor conductivity, low connection strength, and solder joint failure during use, causing cracks in the solder and the welding surface.
(cold solder picture)
Cause: The main causes of cold soldering are oxidation of the pad/component surface, contamination, or too low a soldering temperature. In fact, the PCB manufacturing process, solder paste, component solder end or surface plating, and oxidation can all cause cold solder joints.
Improvement measures: Strictly control the quality of incoming components and PCBs to ensure good solderability; improve PCB assembly production process conditions.
#2 Tombstoming
Definition:
Tombstoming refers to a defect in which one end of the component is lifted.This defect only occurs on chip resistors and capacitors (with only two solder terminals). The tombstoming is also called a suspension bridge or a standing piece.
(Tombstoming component)
Features:
One end of the component is lifted and separated from the PCBA’s pad.
(lifting up component)
Cause:
Tombstoming is caused by solder on one end of the component melts and wets earlier than the other end, and the surface tension generated pulls the component up. If the two pads of the component are of different sizes, or the amount of printed solder paste is different, or the solderability of the two ends is different, this defect will occur. The smaller the component, the easier it is to occur.
Improvement measures:
Tombstoming defects can be improved from the aspects of PCBA pad design and solder paste printing, try to make the heat capacity of both sides the same to ensure that both sides melt and wet at the same time during reflow.
#3 Solder Bridging
Definition:
Solder bridging is a defect in which adjacent pins or terminals are connected by solder.
(Solder Short picture)
Features: Adjacent pins or terminals are connected by solder.
(Solder Bridging problem picture)
Cause: Solder bridging is caused by component placement offset exceeding the soldering process spacing or excessive solder paste.
Improvement measures:
Solder bridging problems can be solved by reducing the amount of solder paste and adjusting the placement of components.
#4 Open
Definition: Open defect refers to no solder connection between the pin and the pad, and there is a clear gap. It often occurs on multi-pin components such as QFP and connectors.
(Open defect picture)
Features:
There is a gap between the pin and the solder surface of the pad.
Causes:
The reason for the PCBA open defect is that the coplanarity of the component’s pins is poor, or individual pads or pins are severely oxidized.
Improvement measures:
Be especially careful when operating fine-pitch QFPs to avoid pin deformation, and strictly control the coplanarity of the pins; strictly control the solderability of the material.
#5 Solder Balling
Definition: Solder balling refers to tiny solder ball defects distributed around the solder pad.
(Solder Balling defect picture)
Features: The solder balls are very small (most of them are solder powder particles), there are many of them, and they are distributed around the pads.
(solder balls around the pads picture)
Causes: The reasons for the formation of solder balls are that the solder paste contaminates the PCB during solder paste printing; the solder paste oxidizes; the preheating speed is too fast during reflow soldering.
Improvement measures:
Improve process conditions, such as more frequent stencil cleaning, no residual solder paste, control workshop humidity, reduce preheating speed during reflow soldering, reduce steel stencil opening size, etc.
#6 Non Wetting
Definition:
Non-wetting refers to solder pads or solderable ends that have not been wetted by solder.
(Non Wetting components picture)
Feature:
There is no visible solder layer on the exposed surface.
(No visible solder layer picture)
Reasons:
The reasons for Non-Wetting include: poor solderability of solder pads and pins; Insufficient flux activity;
There are grease pollutants on the welding surface; The pads and pins have undergone oxidation.
Improvement measures:
Strictly control the incoming quality of components and PCBs to ensure good solderability; Improve PCBA manufacturing process conditions.
#7 Poor Wetting
Definition:
Poor Wetting refers to the phenomenon where melted solder infiltrates the surface and shrinks, leaving a thin layer of solder covering a portion of the area, resulting in an irregular shape of the weld seam. Also known as semi wetting.
(Poor Wetting picture)
Features:
Irregular areas of solder droplets are formed, leaving a thin solder coating layer between these areas.
(poor wetting)
Causes:
The causes of Poor Wetting include: poor pad solderability or uneven pads; pad solderability degradation; unreasonable reflow soldering temperature curve; oxidation or contamination of component pins or PCB pads.
Improvement measures:
Strictly control the quality of incoming components and PCBs to ensure good solderability; improve pcb assembly process conditions.
#8 Wicking
Definition:
Wicking refers to when molten solder wets the component pins, the solder climbs up the pins from the solder joints, leaving solder joints with less tin or open circuits, also known as rope suction.
(Wicking)
Feature:
Solder bulging occurs on component pins.
Causes:
The reason for wicking is that the heat capacity of the component pins is small. During reflow soldering, the solder paste in contact with the pins melts first and is absorbed into the component pins.
Improvement measures:
Improve the assembly process and design, such as using a slower heating rate.
#9 Leaching
Definition:
The phenomenon of the base metal dissolving into the molten solder during reflow soldering. The direct consequence of this defect is the formation of loose intermetallic compounds or non-wetting.
(Leaching)
Feature:
Non-wetting locally.
Causes:
The formation of leaching is due to the high dissolution rate of base metal into solder; too thin metal layer; high activity of solder resist; high reflow solder temperature and long reflow solder time.
Improvement measures:
Replace the base metal with a metal with a lower solubility rate; add base metal elements to the solder; use low-activity flux; use low reflow soldering temperature and short reflow soldering time.
#10 Cold Joint
Definition:
Cold welding refers to a welding point with a rough surface and an oxide layer between the base metal and the solder due to the welding temperature being too low. It is also called false welding.
(Cold Joint)
Features: The solder joint surface is rough and wrinkled, and the electrical performance is normal after re-soldering.
Cause:
Cold Joint is formed because the welding temperature is too low or the residence time above the liquidus temperature is too short.
Improvement measures:
Start by improving the soldering operating conditions (temperature and time are the most important) so that the solder can absorb enough heat and prevent the solder joint from vibrating when it is molten.
#11 Void
Definition:
Void refers to the presence of voids in solder joints. The presence of voids will affect the mechanical properties of solder joints, deteriorating their strength, ductility, creep and fatigue life, and may also cause local overheating.
(voids in solder joints)
Features:
Voids are distributed within the solder joints.
(Voids within the solder joints )
Causes of formation:
The causes of void formation in different solder joints are not exactly the same. Generally speaking, as the boiling point of the solvent in the solder paste decreases, the void content continues to increase; the higher the peak temperature of reflow soldering, the easier it is to form voids; the metal content in the solder paste and the size of the solder powder particles have an impact on the formation of voids.
Improvement measures:
Reduce gas evolution in solder joints and create exhaust channels.
#12 Cracking
Definition: Cracking refers to the defect of cracks in the component body.
（cracks in the component）
Features:
There are cracks on the surface of the PCBA board’s component body. This defect is more common in ceramic chip components, especially chip capacitors.
(cracks on the surface of the PCBA component)
Causes: Cracking is mostly caused by excessive mounting pressure or force during mounting or improper post-welding operation.
Improvement measures:
Reduce mounting pressure; optimize process parameters; reduce thermal stress; it is strictly forbidden to press the PCB by hand when separating the PCBA.
In summary, in order to avoid PCBA failures, PCB assembly quality management needs to start from multiple aspects such as raw materials, assembly production process and testing to ensure the stable and reliable quality of PCBA boards. If you have any questions or inquiries about PCB and assembly manufacturing, please contact us: service@pcbbee.com, we will reply within 2 hours.
