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How to prevent PCB tombstone and open defects during SMT process?

By Mer-Mar Electronics | Date posted: | Last updated: November 28, 2022
PCB tombstoning

Tombstone is a common defect that occurs when SMT components are lifted from the PCB pad. The lift may be partial or complete. While one end is soldered, the other stands up. Visually this gives the appearance of a tombstone in a cemetery, hence its name. Tombstone defect is also sometimes known as the Manhattan effect, Drawbridge effect, or Stonehenge effect. Tombstoning isn’t just limited to passive devices. It can also occur on surface mount devices and can prove to be a fatal defect;

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SMT tombstone root causes

In fact, incidents of tombstoning are on the increase on account of two major issues:

  1. Increasing miniaturization
  2. The transition to lead free

Essentially the tombstone defect is caused on account of the fact that different wetting speeds result in imbalanced torque. Wetting refers to the solder reaching a fluid state so that it can attach to a component lead or pad. In an ideal situation, the solder attaches to both pads and the wetting process is uniform at both ends. It is unbalanced soldering, however, that causes the issue of tombstoning. However, tombstones can also result on account of a number of other causes during SMT Process, such as:

  • Improperly designed solder pads
  • Different sizes of the pads
  • Inaccurate placement of components
  • Uneven reflow oven temperature
  • Presence of nitrogen
  • Chip is placed parallel to the reflow oven

PCB rules to prevent tombstoning

To ensure that PCB tombstoning does not occur, you need to take care of the following during SMT Process:

1. Pad Dimensions

It is important to ensure that the pad dimensions are correct. Making them too large, too small or having one pad of a different size can all lead to tombstone defect as one side of the pad could complete wetting before the other.

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2. PCB Finish

With small component sizes such as 0402 or 0201 it is advisable to use a different PCB finish as opposed to the traditional Hot Air Solder Leveling. Although HASL is known for its low cost, the fact is that it can also cause uneven surfaces, which in turn leads to the tombstone defect.

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3. Component Placement

Component placement and routing play a major role when it comes to tombstoning, especially when the components are small. It is imperative to check that the traces connecting the pads are equal in width. You also need to ensure that the traces are connected in a similar orientation.

4. Copper Coverage in the Inner Layer

If the inner layer of the board has unequal copper coverage, it can lead to uneven heating process, which in turn can lead to the tombstone defect.

5. Solder Mask Thickness

Although the solder mask plays a big role in guarding against oxidation, it can also restrict the solder movement. It is therefore important to carefully evaluate the solder mask thickness

6. Stencil Design

This in turn has two aspects- aperture design as well as stencil technology. A good stencil design ensures that there is adequate solder paste. If the solder paste is too much it results in greater torque. Also there needs to be appropriate component to paste overlap. If the overlap is too little there will not be adequate adhesion. On the other hand, if the overlap is too much solder beads will show up on the side of components.

The good news is that tombstoning is preventable. Attention to its causes can ensure that incidences of tombstoning are reduced and that there is reduced need for reworking.

Open circuit defects in PCB

Another commonly occurring defect is what is known as the Open Circuit defect. This is known to occur when there is an electrical continuity between at least one terminal of the chip and the solder pad. Open Circuit Defect is caused by many of the same reasons that cause tombstone defect. However, there are some additional factors as below:

  • There is oxidization in components
  • Inaccurate pad sizes
  • Poor flux

In order to correct open circuit defect, the following actions are required:

  • The pad sizes need to be kept to the specifications
  • Gradual soak ramp rate needs to be adhered to
  • The pressure for pick and place nozzles needs to be adjusted

Mer-Mar Electronics has 4 decades of experience in providing high-quality SMT PCB assembly in the USA. We provide all-in-one SMT PCB Assembly Services for leading edge and accurate components mounting. Whether you require SMT PCB assembly, SMT SMD PCB, Surface Mount assembly or any custom SMT PCBs; You can get a quick PCB assembly quote based on your custom requirements.

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Top Reasons to Reuse or Recycle Printed Circuit Board

By Mer-Mar Electronics | Date posted: | Last updated: April 2, 2022
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The use of printed circuit boards is ubiquitous with dozens of industries from consumer electronics to military aircrafts making use of it. With new electronic devices literally flooding the market, the usage of PCB is only slated to grow.

The flip side to all of this is that there is already over 40 million tons of electronic waste being produced annually as per an estimate by the ILO, which is going to only increase.

This makes the issue of recycling electronics and that of recycling PCBs extremely crucial. Simply throwing them in landfills can cause tremendous harm to the environment. Incinerating boards also leads to release of toxic elements since the boards are coated with flame retardant substances. On incineration, they release toxic elements which when inhaled can lead to a whole lot of diseases.

It is therefore crucial that a recycling facility that can handle electronics disposal, be used. This typically involves dismantling the device and separating the toxic elements while recycling them through safe processes. Recycling can go a long way in reducing solid waste, landfill as well as land pollution.

Circuit board recycling also helps yield value material such as gold, silver, platinum and other precious metals, which are affixed to the boards. There is also copper in the board, which can be recycled

Circuit Board Recycling Methods

There are a number of methods that can be used for PCB board recycling. Primary among them include:

Mechanical process

This begins with a pre-treatment where the metals, polymer and ceramics are separated. In the next phase, metals are sent for metallurgical refining where they are subject to communication, classification and separation. The first involves reducing particle size and releasing metals. The classification stage involves separation of particle according to size. The separation technique involves separation of impurities.
In case of PCBs, it is important to remove non-conducting materials such as polymers and ceramics from the metals. This can be done in the following ways:

Pyro metallurgy

This process works on the principle of using high temperature to produce pure metals. The issue associate with the process, however is the emission of toxic compounds as also the fact that it requires high energy.

Hydrometallurgy

This method of separating metal, unlike pyro metallurgy is more energy efficient while also not causing as much pollution.

Electrometallurgy

This process makes use of electrolysis whereby metals can be recovered with high degree of purity.

Bio metallurgy

This involves exposing the metal to microbial action and is time consuming.
 
To sum up, here are the top reasons to recycle PCBs:

  • Recycling ensures that you do not add to the already growing e-waste, which is dumped in landfills and adds to environmental hazards. In fact with more recycling greenhouse gas levels can be reduced.
  • It helps us recover precious metals which can be melted down and re used in new devices. It is estimated that one metric ton of circuit boards can contain 40 to 800 times the amount of gold and 30 to 40 times the amount of copper mined from one metric ton of ore in the U.S.
  • Lately several other wastes have also been recycled. These include printed circuit board edge trim, tin/lead solder dross, wastewater treatment sludge, copper sulfate PTH solution, copper rack stripping solution and tin/lead spent stripping solution.
  • Mining of raw materials leads to air pollution, water pollution and greenhouse gas emissions which can be avoided when we use some of the raw materials that are available in the circuit boards and which can be recycled.
  • It also goes without saying that recycling PCBs and other e wastes leads to more e-waste recycling jobs and helps the industry thrive.

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Simple Tips to Prevent PCBs from Bending or Warping

By Mer-Mar Electronics | Date posted: | Last updated: November 28, 2022
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PCB bending or warping can lead to a whole range of issues. If you are lucky, the impact of warping may be so small that it isn’t noticed. On the other hand, it might be so intense that it could cause immediate electrical issues.

What Is PCB Warping

Warping can often be a result of Reflow soldering, as a process. In severe cases it can even lead to a situation where the components become erect and it leads to empty soldering. Warping can also be the result of a thermal mismatch as different materials experience thermal expansion at different rates.

Reasons of PCB Bending or Warping

While the reasons for warping and bending could be many, primarily it is caused by high stress on the plate. This could be on account of the following:

1. Copper Surface Area

When the area of the copper surface is uneven, it can lead to uneven heating and cooling, leading to deformation.

2. Weight of the board

The weight of the board itself can lead to sagging

3. V-Cut

The depth and connection of the v-cut can lead to deformation

4. Connection Points

The Vias of each layer can limit the expansion and contraction of the board.

Prevent PCBs from Bending

The maximum permissible wrap for surface mounting of PCB is 0.75% as per IPC-6012. It is therefore extremely important to look for effective methods to ensure that warping doesn’t happen. Here are a few:

1. Reduced temperature

A large amount of stress on the plate is caused on account of temperature. It is therefore imperative that the temperature of the furnace is reduced or the rate of increase in temperature and the subsequent cooling is adjusted.

2. High Tg

The transition temperature of the glass, which is the temperature at which the glass changes its state, affects warping. The lower the Tg value of the material, the more serious the deformation is likely to be. Use of sheets with higher Tg can improve ability to withstand stress, even though the material could come at a higher cost.

3. Thickness of the board

The thinness of electronic products required these days, affects the board and often causes deformity after passing through a reflow oven. If it is possible to keep the thickness at 1.6mm, the probability of deformation of the board is significantly reduced.

4. Size and number of boards

Most reflow ovens use chains. Large sized boards, therefore are likely to be deformed in the oven on account of their own weight. It is therefore advisable to put the long side of the board as a board edge.

5. Stove tray

Use of a stove tray can also reduce deformation. In case a single tray cannot reduce deformation, adding a cover can also do the trick.

6. Use of router

Use of a router instead of a v-cut divider can also ensure that the structural strength of the boards isn’t destroyed.

7. Symmetrical arrangement

The semi-solidified sheets should be arranged in a symmetrical manner, otherwise there are chances of warping. For example, in an 8-layer board, the thickness and numbers of prepreg should be the same in 1~2L and 7~8L. Otherwise, there are chances of the board bending. It is also advisable to use products from the same supplier when it comes to the multi layered board as well as the semi-cured sheets.

8. Latitude and longitude of semi-cured tablets

The warp and weft shrinkage rates of semi-cured sheet after lamination are different. It is advisable to roll the semi-solidified sheet in the direction of the longitude and the width in the direction of the latitude.

9. De-stress after lamination

Post the hot and cold pressing, the laminates need to be taken out and laid flat in the oven to release the stress in the plate. Baking the laminate for about 8 hours at 150 degree Celsius can remove excess water and solidify the resin.

10. Straightening of plate

When thin multilayer plates are used for surface and pattern plating, it is important to use special clamping rolls. Once the plates are clamped on the plating line, the clamping rolls need to be stringed so that all the plates are straightened. If this process is missed the plate can bend and the deformity can be hard to correct.

11. Cooling of board after hot air leveling

The printed board has to bear the high temperature of the solder groove. It is important that it should be allowed time for natural cooling by putting it on flat marble or steel plate.

12. Treatment of boards

All PCBs are inspected for flatness. The boards that do not make the cut are put in the oven for between 3-6 hours at a temperature of 150 degree Celsius and under pressure. Sometimes a part of the plate can be saved on account of this process of anti-warping.

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Conformal Coating – Protect PCBs from adverse environment conditions

By Mer-Mar Electronics | Date posted: | Last updated: December 29, 2022
Conformal coating for pcb and environment hazards

The conformal coating has a very important role to play when it comes to protecting circuit boards against unfavorable environmental conditions. In environments that have extremes of temperature, moisture, and dust, circuit boards can often suffer from defects, compromising the shelf life of the product. The conformal coating acts like a protective film that keeps the PCB moisture-f, and dust-proof protects it against temperature variances, keeps it free from corrosion & more. Another common aspect that impacts printed Circuit Boards wide variety of pollutants. They can lead to corrosion and even failure of PCBs. While several pollutants could accrue from the operating environment, oftentimes there are a lot of contaminants on account of residues during the PCB manufacturing process.

Uses of Conformal Coating in different sectors

The application fields of conformal coating include, but are not limited to:

1. Consumer Electronics

Consumer electronics particularly must face harsh external conditions. Be it water and detergent in the case of washing machines and dishwashers or contaminants as in the case of computers, microwaves and others.

2. Automobiles

With break fluids, gasoline vaporization & more, there are enough and more reasons for the use of conformal coatings in automotive.

3. Aerospace

With compression and decompression being the norm, it only stands to reason that the aerospace industry should see a widespread application of conformal coating.

Components that shouldn’t have conformal coating

It is also important to remember that some components cannot receive conformal coating. Some such components include:

  • High power components
  • Adjustable resistor
  • Battery holders
  • DIP Switch and more

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Benefits of Conformal Coating for PCB Protection

While the benefits of using conformal coating are well established, however, to reap the benefit of conformal coating, it is imperative that some preparations be made in the design phase itself.

You need to make sure that there are no contaminants on the board as contaminants can be sealed in and can create issues in the long run. In fact, the boards need to be thoroughly cleaned as well as dried. This will go a long way in ensuring that long-term performance isn’t impacted.

Conformal coating is also not effective in the case of applications that are exposed to water significantly. In such cases, it is advisable to offer protection through the process of encapsulation.

Ideally PCB conformal coating works best for applications which have:

  • Good electrical properties
  • Chemical resistance
  • Are durable mechanically

Also, the conformal coating does not work well over sharp edges as it results in edge crawling where the coating isn’t present on such edges. To prevent edge crawling it is necessary to check the viscosity of the coating and use multiple coats. A specialized coating of 100 microns works best for improved edge cover.

Some of the tests that might be carried out on conformal coated boards include:

  • Damp heat testing
  • Thermal cyclic performance
  • Chemical resistance test
  • Flammability test

Conformal coating is normally applied by the process of dipping or spraying. Typically, the thickness of the coating is 20-50 micron, even though specialist coatings can be applied at up to 100-micron thickness. In the case of PCB Prototypes or small batch production, it is also possible to apply the coating by the process of brushing.

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