Introduction
Ideally, your PCB is perfectly flat, but there is some PCB bow and twist that is common. This warpage of the circuit board can cause significant assembly issues and reliability concerns. Learning why PCB warpage occurs is the way to prevent it. This guide shows you how and why it occurs and how to avoid PCB warpage.
What is Bow and Twist in PCBs?
You can recognize PCB bow as a simple curve along the length or width of the board. This type of circuit board warpage keeps all four corners on the same flat plane.
Twist is another kind of PCB warpage where one corner lifts, breaking that plane. It results in a diagonal deformation across the surface of your board.
The IPC board and twist standard establishes the acceptable PCB flatness tolerance. It allows 0.75% warpage in PCB for SMT boards and 1.5% for other boards to provide a stable assembly.
Why is PCB Flatness Important?
A flat board is important for successful automated assembly. Any circuit board warpage can cause uneven solder paste application, leading to poor connections and defects. Even slight warping can affect solder joint contact during placement and reflow. This can cause misalignment of small components or soldering defects, which can reduce assembly yield.
In addition to component placement, a warped board can physically jam your production line equipment. It could become stuck in transport belts, shutting down the entire production process. Even if the board makes it through assembly, you can still encounter problems fitting it into its final enclosure, requiring difficult and expensive PCB warpage rework.
Causes of Bow and Twist in PCBs
A number of design and manufacturing factors are the leading causes of PCB warpage. They generally involve either thermal stress or unbalanced construction. Knowledge of these underlying causes is the first step in knowing how to prevent PCB warpage. That knowledge will enable you to create a more solid and reliable board.
Uneven Copper Distribution
Uneven copper is a significant cause of PCB warpage. You should balance copper distribution between each layer and throughout the entire stack-up. A large copper plane on one side with light traces on the other will make the board bend when it gets hot. Using copper thieving helps to create a more uniform layout.
Unbalanced Stack Up
A lack of balance or asymmetry in the stack-up can cause warpage of the circuit board. You must make your layer stack-up symmetrical from the center outwards. For instance, having a very thick copper layer opposite a thin one creates internal stress. This stress usually causes PCB bow and twist during thermal cycles.
Pre-Preg Materials Arranged Incorrectly
The grain direction of the material is a major factor in board flatness. Your PCB’s fiberglass weave has an orientation called the warp and weft. You want to alternate grain direction between layers when laminating. Having all layers oriented the same way accumulates internal tension, causing PCB bow and twist.
Mixed Material Types
When you layer materials with different thermal expansion rates (CTE), you are creating stress. This is one of the most common causes of warpage during reflow of PCBs. Although hybrid stack-ups are at times necessary, they are more prone to warping. When you have no choice but to use mixed materials, a slow controlled heating profile is necessary to reduce thermal shock.
Finished Thickness
Thinner boards have less structural rigidity and are more prone to PCB bow and twist. The risk is even higher for larger or longer boards. Standard thicknesses are normally stable, but you have to give your design more attention when using boards thinner than 0.8mm (0.031″).
Moisture
Trapped moisture within your board is one of the major causes of warpage of PCBs. When this moisture vaporizes during reflow, it attempts to escape and is referred to as outgassing. Outgassing results in internal pressure that causes delamination, blistering, and extreme warpage of circuit boards. Baking and storing properly can avoid this.
Rapid Heating During Assembly
Heating a PCB too rapidly produces thermal shock, a leading cause of PCB warpage when reflowing. The laminate and copper will expand at different rates and produce tremendous stress that will warp the board. You can avoid this using a gradual reflow profile. A better-controlled process comes with an oven that has more heating zones.
Assembly Conveyor Setup
Your assembly equipment may also cause PCB bow and twist. If your conveyor rails holding the board are too tight or unbalanced, they will physically warp it. You need to have your conveyor system calibrated along its entire length. This takes external mechanical stress out of the list of reasons for PCB warpage.
Problems Caused by Excessive Bow and Twist
Unreasonable PCB bow and twist can create numerous critical issues. This warpage of the circuit board impacts everything from assembly yields to the reliability of the final product. If you don’t manage it, you’re risking increased costs and delays in production. Knowledge of these problems explains why keeping boards flat is so important.
Reduces Assembly Yield
Excessive PCB bow and twist prevent components from seating correctly on your board. Through-hole leads become misaligned, and small SMT parts do not place properly. These alignments immediately result in assembly defects, decreasing your manufacturing yield. This tends to escalate the requirement for challenging and time-consuming PCB warpage rework.
Affects Coplanarity
PCB warpage causes major issues for components such as BGAs that require a flat surface. If your board is not flat, not all of the pads will be in contact properly during assembly. Some of the solder joints get stressed and some may not make contact at all. This makes connections unreliable. These kinds of defects can cause intermittent faults.
Hampers Reliability
Physical stress due to circuit board warpage damages your product’s reliability over the long run. The constant stress fatigues solder joints and plated through-holes over time. These stressed connections are more prone to failure after the product is put into use. This can result in intermittent faults or outright failure. Thus, ruins your product’s reputation.
Increases Rejection Rate
Your PCBs need to comply with the PCB flatness tolerance specified by the IPC. When the PCB bow and twist are over this limit, the boards will fail inspection. You have to reject these out-of-spec boards, which wastes time and materials. This directly raises your costs of fabrication. It can also cause major production delays.
Limits Panel Utilization
In order to control PCB warpage, you might need to use smaller fabrication panels. This controls warping but cuts down on your overall efficiency. Thus, producing fewer boards per panel.
Interferes With Electrical Testing
PCB warpage prevents your board from establishing good contact with a test fixture. Gaps between the board and the test pins can cause false failures. That means you might discard good boards. You also waste time debugging non-existent problems. In extreme situations, warpage can even hurt the test fixture itself.
Creates Rework Issues
PCB bow and twist frequently lead to in-process test failures. If a warped board fails a test, you have to remove it from the production line. This interruption ruins your flow and results in costly delays. Doing the required pcb warpage rework is frequently challenging. It is also time-consuming and extremely costly.
Restricts Component Choices
If your layout is likely to warp, you will have to restrict your component selection. Heavy or bulky components introduce mechanical stress that worsens circuit board warping. This limits your design flexibility, requiring you to avoid particular connectors or transformers. This compromises your board’s overall performance. It even affects its final performance.
Increases Costs
Finally, PCB warpage greatly adds to your overall project expenses. You will incur extra costs from lower production yields and delayed fabrication. A lot of pcb warpage rework also contributes to the cost. All of these add up to the financial burden. Controlling the PCB bow and twist control is important for your budget.
Quantifying Bow and Twist in PCB Standards
Bow Measurement
In order to measure the bow, we first need to calculate the maximum distance acceptable based on the length and width of the bow. The PCB parameters that will be measured:
- RL = L x B / 100
- RW = W x B / 100
Where RL and RW represent the maximum bow of L and W, the length and width of PCB, and B represents the maximum allowable bow percentage. Given our PCB is 200x300mm (L x W) and we are using SMD components (B=0.75%), then:
- RL = 200 x 0.75 / 100 = 1.5 mm
- RW = 300 x 0.75 / 100 = 2.25 mm
In that case, the distance from the worst point of the bow to the plane where the PCB is to be measured should not exceed 1.5 or 2.25mm along the longest and widest sides, respectively. We can easily compute the percentage of a given bow by rearranging our formula. Assuming 1.4mm is the distance between the bows when measuring the length, we get:
- BL-MEASURED = RL-MEASURED / L * 100 = 1.4 / 200 * 100 = 0.7%
Twist Measurement
Calculating the maximum twist distance is done in the same way as the bow, except that the calculation used is based on the diagonal length of the board.
- R = 2 * D * T / 100
D is the diagonal of your PCB, and T is the (maximum) percentage twist value of your board. The equation also includes a factor of two, which doubles the maximum distance shown because one end is fixed during the process.
With the same board specifications as the bow calculation, you may check the maximum allowed twist:
- R = 2 * sqrt(2002 + 3002) * 0.75 / 100 = 5.4mm
To get your physical measurement back into a percentage, you should be able to use a reverse calculation:
- TMEASURED = RMEASURED * 100 / 2 / D
So for a 4mm measured twist, we get:
- TMEASURED = 4 * 100 / 2 / sqrt(2002 + 3002) = 0.55%
This method of measurement will only be accurate when ¾ corners of the board are in contact the flat reference surface. Should the warpage of the circuit board be too serious in such a case, IPC-TM-650 recommends that the more involved “referee method” should then be used instead.
IPC Standards for Bow and Twist
The IPC bow and twist standard gives the official guidelines you need to use. You can read these specifications in the IPC-A-600 bow and twist document. The acceptable PCB flatness tolerance for your board depends directly on its final thickness, which you can see in the table below. This helps ensure your design is of industry-wide acceptability standards.
| PCB Thickness | Bow | Twist |
| ≤ 1.6 mm | 0.75% | 1.50% |
| 1.6 to 3.2 mm | 0.75% | 1.00% |
| ≥ 3.2 mm | 0.50% | 0.75% |
You might also want to remember these important points from the standard:
- The twist percentage is determined by the length of your PCB’s diagonal.
- Your board’s combined total bow and twist should not be more than a maximum of 2%.
- When you’re using stiffeners or building a metal core PCB, you’ll need to comply with stricter flatness standards.
Guidelines to Minimize Bow and Twist
You can easily minimize PCB bow and twist by following key design and production rules. This is the best method to avoid PCB warpage, as these practices solve its most common cause.
Symmetric Layer Stackup
You have to design a symmetrical stack-up to balance thermal stresses. This is an important first step in avoiding PCB warpage.
Balanced Copper Distribution
Distribute copper uniformly over your board layers. Uneven copper is a primary cause of PCB warpage since it promotes unbalanced thermal expansion.
Thinner Dielectrics
Applying prepregs and bonding sheets less than 100 μm in thickness decreases thermal mismatch between the laminate and copper. This reduces internal stress and avoids PCB bow and twist.
Tailored CTE Dielectrics
Choose dielectrics with a 17–21 ppm/°C CTE to better match copper’s thermal expansion. This selection reduces thermal stress, a major warpage cause of circuit boards.
Low CTE Reinforcements
Inserting glass fabrics with a very low CTE (2-5 ppm/°C) into your prepregs counteracts thermal expansion. This is a good method to avoid PCB warpage in complex designs.
Low Stress Plating
You can minimize plating bath stress and use lower current densities in fabrication. This reduces panel warpage, solving one of the manufacturing causes of warpage of a PCB.
Balanced Panel Cutout
You need to place any panel cutouts symmetrically in order to balance internal stresses. This simple action prevents the distortion causing circuit board warpage.
Perimeter Support Tooling
Use support tooling along the perimeter of the panel during lamination and plating. This reduces unsupported bowing and helps prevent PCB bow and twist.
Bow Reducing Routers
You can use high-precision routers to minimize the mechanical stress caused during depaneling. This is an important final step in avoiding PCB warpage.
Methods of Correcting Bow and Twist
If a board is already warped, there are various rework techniques you can use. These types of PCB warpage rework have the ability to return boards with excessive PCB bow and twist to tolerance.
Panel Symmetrization
Selective addition or subtraction of material allows you to conduct PCB warpage rework. This rebalances the board’s construction to counteract the stresses causing circuit board warpage.
Stress Relieving Anneal
You can do a post-etch bake of more than 180°C to relieve internal stress. This type of pcb warpage rework works effectively to minimize circuit board warpage.
Bow-Twist Correction Fixtures
Special fixtures, which provide localized heat and vacuum, can be used to fix PCB bow and twist. This targeted PCB warpage rework physically flattens the board.
Corner Support During Handling
You should use specialized tooling with corner pins to hold panels in place when handling. This easy measure prevents you from accidentally causing PCB bow and twist.
Panel Splicing
You may cut a big warped panel into smaller components to relieve internal stress. Although this can lead to flatter individual boards, it reduces your panel utilization.
Flattening Pressing
You can use a flattening press to apply high, even pressure onto a warped board. This type of PCB warpage rework physically pushes the board back into a flat state.
Mitigating Bow and Twist Effects During Assembly
No matter how well you fabricate, some circuit board warpage is inevitable. You can follow these steps during assembly to control its effects. This will prevent defects due to PCB warpage during reflow.
Robust Component Securing
You may use adhesive and fasteners to mount heavy components. This keeps them from adding stress that can worsen the PCB bow and twist.
Board Stiffening
You can bond stiffeners, such as metal strips, to the back of a board for additional support. This easy process keeps the board flat and counteracts warpage of the circuit board.
Stress Relieving Anneal
Carrying out a post-assembly bake above 100°C can relieve built-in stress. This is a type of PCB warpage rework that will minimize the final deformation.
Selective Conformal Coating
You may apply conformal coating on certain areas of the board for additional reinforcement. This prevents further bending or twisting in highly stressed areas.
Blanket Foam Layer
Applying a foam sheet to the back of the board will absorb and dampen mechanical stress. This serves to prevent external forces from causing further warping.
Clamping During Soldering
You must clamp your board flat using fixtures while soldering. This is an essential step in handling PCB warpage during reflow and making reliable connections.
Panelized Assembly
You are able to maintain your boards in panel during the course of assembly. This prevents stress due to early depaneling, which is among the PCB warpage causes.
Conclusion
Managing PCB bow and twist is really hard, but you can limit its effect. The best way to guarantee the quality is to be aware that there are numerous causes of circuit board warpage. This understanding guides you in implementing effective solutions and prevention techniques. Working with design and fabrication teams will show you how to avoid PCB warpage successfully.
Contact MV Flex Circuit to receive a quote today and work with expert fabrication team that minimizes the risk of PCB Bow and Twist.
FAQs
At What Point in the PCB Production Flow Are Bow and Twist Generally Introduced?
The majority of circuit board warpage happens during manufacturing because of pressure and heat. But subsequent processes such as assembly of components can worsen the issue.
How Are Very Large Boards More Vulnerable to Bow and Twist Problems?
Large boards expand and contract more unevenly when they are heated. The size and weight of heavy components also adds mechanical stress that adds up to PCB bow and twist.
Is It Possible to Completely Eliminate Bow and Twist?
No, you can’t totally eliminate it. The objective is to manage circuit board warpage so that it remains within the acceptable IPC tolerance for bow and twist.
Are Some PCB Laminate Materials More Prone to Bowing and Twisting Issues?
Yes, certain materials are more prone to warp. Normal FR-4 is more likely, whereas newer laminates have improved dimension stability.
Can Warped PCBs Be Reworked Back to Flat Shape?
Yes, a hot press can be used to flatten a board. This is a potentially risky process that can cause damage to components, so preventing warpage is the better option.
Can Soldering Process Induce or Aggravate Board Warpage?
Yes, the soldering process is a major reason for PCB warpage during reflow. Placing components on the single side causes uneven thermal stress that results in circuit board warpage.
How Do I Measure to Check If My Board Exceeds IPC Bow and Twist Criteria?
The IPC bow and twist standards determine the PCB flatness tolerance. Your board warpage should be 0.75% or less for SMT boards and 1.5% or less for all other board types.
