Poor solder paste wetting is usually an unseen cause of unreliable soldering processes and poor solder joints. Therefore, it does not imply that it is unavoidable just because bad soldering acts as a major problem. You can easily remove and prevent your board from occurring bad solder wetting and make the solder joints last longer with some tips and tricks.
What Is Wetting Soldering?
Soldering involves merging two metals with each other using a solder base or alloy. This type of technique is an old method of joining metals together permanently. At the same time, soldering wetting represents a type of process that bonds the metal with the solder metal over the surface of the PCB. During the process of wetting, the solder turns into a molten liquid, which attaches to the electrical components properly to make an adequate solder connection.
Adequate metal bonding or solder wetting needs a certain type of environment to commence this process properly. However, wetting needs a surface of copper that contains no contaminants and reacts with temperature properly. Here you can learn why adequate solder wetting acts helpful and how you can achieve success in this process.
Why Is Good Wetting Soldering Important?
A good type of solder wetting requires metal joining in an appropriate manner. Without good solder wetting, metal does not attach completely, which also results in not passing the standard of the industry for proper usage. Thus, consider them generally defective. While proper and good solder wetting results in a proper solder connection that withstands the time of the test.
Appropriate solder wetting sees from it looks as it looks smooth and shiny solder which already reaches its max flow. On the contrary, poor or bad solder wetting is also visible. The solder turns porous, dull, and grainy, which shows its poor ability of adhesion. This type of poor solder may not be used in industrial settings as it only wastes time and money and reduces productivity.
What Is Non-Wetting Soldering?
Non-wetting defects are a type of soldering issue that happens when the solder does not fit to make the bond with a PCB metal alloy. However, when the inability of bonding is not able to attach the terminals of electrical components and the pads of the PCB and give a secure connection. The surface of the board material remains exposed. At the same time, the solder itself resembles lusterless or giant. Moreover, non-wetting also causes voiding directly. It’s a process of creating holes within the joints of solder that may lack the materials of solder.
Wetting Soldering Vs. Dewetting
The defect of non-wetting differs from that of dewetting. Dewetting is a type of process that occurs when solder pastes coat the terminals of the components and pads of PCB but then leave some parts. This leaves thin solder surfaces on metal alloys and some thick and irregular clumps. The PCB surface material generally does not stay exposed all the time. However, whenever dewetting comes, it typically affects the joints of solder and filet quality first.
Whereas non-wetting does not always but mostly occurs with solders that have no lead, for example, solder of copper-silver-tin. Some metals act promptly in non-wetting, while others don’t because of the different materialization, as they have distinct spread and wicking features. Bare copper circuits with preservatives of organic solderability are sensitive to the PCB pad’s non-wetting, particularly if they pass through more thermal cycles. Whereas pure tin offers well spread and minimizes non-wetting. Same as silver immersion surface finish work. Gold-and-nickel alloy also solders properly and reduces the non-wetting as long as they do not have any impurities.
Importance To Fix Non-Wetting Defects of Soldering
Non-wetting results in causing severe issues of structure that affect the performance and function of the board. This causes poor solder joints over the PCB surface. However, poor solder joints on the PCB break easily, which results in poor conductance of electricity. Moreover, when solder paste does not attach to the PCB pads, components, and the metal of the base, it may lose integrity and make food joints or may come off either quickly or be subjected to stress. Also, improper components connections affect the functionality of the PCB.
Common Causes Of Non-Wetting Soldering Defects
The defects of non-wetting are caused because of many reasons. Here are some practical reasons for occurring non-wetting over the surface of PCB.
1. Inadequate PCB Finish
The circuit board’s surface finish determines how much wetting causes and how well a solder paste reflow. However, if PCB boards have an inadequate surface finish leaving the board bare, it makes it difficult to reflow the solder and may stick poorly on the PCB.
2. Improper Pin Platting
If PCB copper pins receive improper copper plating before going through lead/ tin plating. However, it may become sensitive and cause non-wetting. This is because a sufficient amount of copper plating plays a vital role in keeping zinc from intervening with lead or tin plating. Thus, keeping the solder paste from attaching properly.
3. Degraded Flux
Degraded or old flux, a cleaning agent with chemicals used after and before soldering, can cause non-wetting. Moreover, flux degrades faster in a container with an opening. Although, even if the solid residue stays intact, it may diminish its performance subsequently. Therefore, a regular type of flux serves as helpful in this case.
4. Wrong Type Of Fluxx
The kind of flux you use for cleaning also depicts whether it causes non-wetting on the surface of the PCB or not. Generally, fluxes with high-activity results in adequate solderability and reduce the risk of non-wetting. On the other hand, fluxes with low residue and low activity remain inadequate to clean the oxides and residue from the PCB surface. Thus, these oxides may interfere in causing wetting.
5. Incorrect Lead/Tin Times
When PCB contains a very thin layer of lead and tin plating, any incorrect thickness may result in non-wetting and solderability. Nevertheless, the thinner plating layer possesses short life and does not survive longer until the process of soldering occurs.
6. Long storage Times
If the PCB stays too long in storage, it makes soldering difficult, which causes non-wetting. Although, solderability typically fits directly to the thickness of the plating. However, during a long stay in storage, it may break the good solderability of the necessary plating. A PCB which remains in storage for over a year or maybe more is at higher risk of non-wetting and poor solderability.
7. Poorly Applied Board Resin
If resin spots on the surface of the PCB, it reacts with soldering, which results in non-wetting. For instance, the resin of the board can spot over the edges of pins, which results in non-wetting only in those specific areas.
8. Problems With The Bath
In some scenarios, non-wetting is produced across the coating of the PCB. For instance, a coating of gold surface finish on the circuit results in non-wetting throughout the board due to the imbalances in the bath of electroless gold.
9. Oxidation
PCB oxidation which receives soldering, results in non-wetting. The intervention of oxides, the base substrate, and the solder prevents adequate adhesion. This results in creating non-wetting all over the PCB surface and makes it oxidize.
10. Insufficient Solder Paste
A solder paste with low volume may constantly result in non-wetting. A good solder paste spread seems essential for creating stable and reliable solder connections. However, when the assembly of PCb uses solder paste in less amount, it may not create a proper adhesion connection between the board’s base substrate and the solder. Therefore, sufficient or an adequate solder paste amount causes wetting of around 100% on the surface of the PCB.
11. Poor Choice Of Solder Paste
Solder paste plays an important role in preventing non-wetting and works better than others. Typically, along with fluxes, a solder paste with high activity affects more than a solder paste with low activity. It minimizes the non-wetting risks and enhances the solderability.
12. Expired Solder Paste
If PCB solder paste surpasses its expiry date, then it may not adhere as strongly as it did before because the flux does not activate sufficiently to eradicate oxides or contaminants from the surface of the board for better wetting and solderability.
13. Insufficient Soldering Temperatures
Too low soldering temperatures tend to encourage non-wetting over the PCB as solder needs a level of the thermal threshold for bonding adequately with the metal substrate. However, if somehow the soldering temperature does not surpass that threshold, then the solder paste does not attach properly to the pads of the PCB or the PCB components. This issue usually arose in lead-free bases, which usually possess high melting points as compared to the alloy of lead or tin.
14. Inconsistent Soldering Temperatures
Fluctuating or Inconsistent soldering temperature may cause non-wetting over the surface of the PCB as they forget the flux activation process and provide poor solder adhesion in some areas of the PCB. However, if specific problem spots over the surface of the PCB do not get the heat that already reaches activation temperatures of the flux, then the solder paste may not adhere to those specific areas as it needs to.
15. Short Soak Time
The time duration a solder paste stays over the circuit directly impacts the ability to make a bond with the metal substrate of the circuit’s surface. Moreover, when there is not sufficient time to attach the solder properly, it leads to non-wetting within the board or certain parts of the board.
16. Long Soak Time
On the other hand, if the soldering paste stays long enough on the circuit during the process of reflow, the excess time may tire out the PCB flux before the process of soldering. However, if PCB flux turns inactive due to the excess soak time, then it rises the non-wetting subsequently.
17. Solder Paste & Plating Material Mismatch
If the material of the metal plating layer and the solder paste type are incompatible, then the soldering paste becomes ineffective for stimulating links between circuit pads and components of the surface.
18. Smaller Chips
Smaller chips comprise thin solder paste layers. In such scenarios, the solder paste layer may not allow sufficiently for whole PCB prop-wetting.
19. Contaminated Flux Or Solder Paste
Solder paste in contaminated flux does not connect components or clean the circuit effectively and also approves sufficient wetting. The contaminants can also leave undesirable residues over the surface of the circuit. Tintervenesvene with the flow of solder and results in non-wetting.
Preventing Poor Solder Wetting During Reflow
Various steps help in improving the wetting performance. However, these steps do not always work. Therefore, choose the method that suits well for your PCB design.
· Higher Activity Solder Paste
This provides better wetting, particularly for soldering tough surface finishes and oxidized surfaces.
Selecting a distinct type of solder paste drastically affects the wetting efficiency. Therefore, we provide a waste range of solder paste that falls perfectly for the unique needs of manufacturing, along with enabling maximum wetting.
· Minimize Oxides
You may also choose less-oxidized PCB surface material. This may seem impossible to work. However, you may also work with nitrogen along with the high-activity paste during the reflow process. This aids in reducing the oxide level that is created during the process of reflow.
· More Solder Paste
An efficient way of fixing poor wetting issues is to alter the stencil. This helps in using the solder paste with increased volume. If additional solder paste doesn’t work for you, then choose larger lead components. This allows reimbursement of low-volume solder paste and enables the solder connections to complete.
· Adjusting The Reflow Profile
Enhancing the soak time helps you in equalizing the temperature of the soldering process and removes the problem areas. This allows a reasonable wetting for both the component leads and the pads. However, enhancing the time beyond liquidus aids in enabling the complete liquid flow of solder in liquid form before it starts freezing.