Understanding Bow and Twist on a PCB

What is Meant by Bow and Twist on a Printed Circuit Board (PCB)?

Bow and twist refer to two types of warpage that can occur in printed circuit boards (PCBs) during the manufacturing process or after assembly.

Bow is when the PCB warps along one axis, causing it to curve up or down. This gives the board a convex or concave shape along its length or width.

Twist is when one corner or side of the PCB lifts up while the opposite corner/side is pushed down. This causes the board to twist about an axis running diagonally across it.

Excessive bowing or twisting in a PCB can lead to various issues such as:

• Components not sitting flush against the board
• Cracks forming in solder joints
• Short circuits developing between traces
• Boards not fitting properly into enclosures
• Failures during automated assembly

So maintaining control over bow and twist is an important consideration when manufacturing PCBs.

What Causes Bowing and Twisting of PCBs?

There are several potential root causes of bowing and twisting in PCBs:

Uneven Stress and Expansion Differences

During the PCB fabrication process, various materials like laminate, copper and soldermask get bonded together. Each material has a different coefficient of thermal expansion (CTE). As temperatures change, some materials expand/contract more than others, creating mechanical stress. These uneven forces can warp the shape of the board over time.

Laminate Direction and Buildup Layers

Most PCB laminates are manufactured with the fiberglass weave running along one direction. This makes the material stronger and more thermally stable along that grain direction. During multi-layer PCB construction, the direction of the laminate core layers may be alternated. These differences can lead to asymmetric distributions of internal stress, causing bowing or twisting motion.

Component and Process Factors

The way components are assembled and attached to the PCB can impact mechanical stress:

• Surface mount parts only soldered on one side lead to asymmetry
• Tall, heavy components act as localized points for leverage against board flatness
• Depositing conformal coatings require heating/cooling that induces strain
• Routing thickness differences create instability

Handling and Environmental Issues

PCBs can warp over time when handled incorrectly or exposed to certain environmental conditions:

• Flexing or bending boards during assembly and transit
• Absorption moisture causes swelling of laminate
• Heating/Cooling cycles generated during rework, test or operation

So there are many sources - from materials to manufacture to use conditions - that may contribute to bowing and twist issues. Let's look at analyzing and solving the problem next.

Analyzing Bow and Twist

The first step is developing an understanding of the amount of warpage through quantifiable measurement of PCB bow and twist:

Tools for Measuring Bow and Twist

Several types of tools can be used, like:

• Dial indicators - Simple mechanical gauges that have a sweeps across the board and indicates vertical deflection. Resolution is generally 0.001".
• Laser micrometers – Optical sensors scan along a line across the surface, generating surface profile data to high resolution.
• 3D scanners – Non-contact digitizers create a 3D point cloud model that can be analyzed.
• Shadow Moiré - An interference pattern technique requiring simple equipment.

Relevant Standards

Acceptable amounts of bow and twist are defined for both rigid and flex PCBs within manufacturing standards:

Any measurements beyond these thresholds indicates excessive warpage requiring corrective action.

Key Characteristics to Capture

When measuring bow and twist, some key quantifiable characteristics should be recorded:

Bow

• Direction/axis relative to board orientation
• Location along length/width showing greatest deflection
• Maximum deviation from a flat plane

Twist

• Extent of opposite corner/side lifting and lowering
• Relative angular twist between diagonal corners or sides
• Axis showing greatest twisting displacement

Capturing this data helps pinpoint where and how the warpage is occurring so that root causes can be investigated.

Now that we can measure bow and twist, let's look at methods for controlling excessive warpage.

Controlling Bow and Twist

A number of design and process strategies are available for minimizing unacceptable PCB warpage:

Symmetric PCB Stack-Up

Creating symmetry between the materials, components and construction on both sides of the PCB splits internal stresses more evenly, reducing distortion tendencies:

Bow-Twist Compensation Shape

Boards can be fabricated with an intentional curve or twist included so that subsequent processing or assembly can help flatten it back out. This requires advanced modeling and simulation to calculate the effect.

Panel Design Strategies

Arranging PCBs symmetrically within panel frames creates a more stable average stress distribution across the full panel. Bow/twist then gets minimized when singulating boards.

Material and Thickness Adjustments

Altering PCB core, prepreg or copper materials/thicknesses shifts thermal/mechanical properties. This helps tune expansion characteristics and rigidity to reduce asymmetry.

Controlled Bake-Out

Application of an even bake-out after fabrication remove some internal moisture and relaxes epoxy cure stresses, helping to flatten boards. The schedule must be developed carefully.

Selective Reinforcement

Addition of stiffeners or metal backing plates under heavy components offsets their localized influence on shape. Discrete reinforcement in high stress regions reduces tendency to distort.

Impact on PCB Assembly Process

Excessive bow and twist can negatively impact production of populated PCBs in several ways:

Pick and Place Machines

SMT equipment use small vacuum nozzles to pick parts off feeders and place them precisely onto warped boards. This becomes much harder beyond certain limits:

Reflow Process

Both convection and vapor phase reflow relies on transferring heat uniformly to solder joints. Board warpage hinders this:

Automated Solder Testing

Many test fixtures use pogo pin contacts that must all connect properly across each board:

• Bowing reduces ability of pins to compensate and engage pads
• Twisting separates two sides making simultaneous contact difficult

So clearly assembly processing is made much more challenging on warped PCBs, risking defects and rework. Component placement may also need adjustment.

Mitigating Impact Through Board Handling

While limiting bow/twist in fabrication is preferable, some accommodation is possible in assembly through board handling considerations:

However these measures do not fully resolve underlying distortion issues. So addressing root causes within manufacturing processes is still needed to completely control bow and twist problems.

Conclusion

Bow and twist represent difficult to control forms of warpage affecting PCBs. But through careful measurement, modeling and mitigation, their impacts on quality and assembly can be minimized. This requires understanding the many process sources involved alongside practical solutions. With tight collaboration between design, fabrication and assembly groups, robust bow and twist prevention strategies can be implemented.

Frequently Asked Questions

Q: At what point in the PCB production flow are bow and twist generally introduced?

A: The majority of bow and twist gets introduced during the PCB fabrication process itself due to the heat and pressures involved in laminating layers together combined with inherent asymmetries. However additional stresses from component assembly, testing and environmental exposure over time can worsen bow/twist.

Q: How are very large boards more vulnerable to bow and twist problems?

A: Larger boards have proportionally larger differences in expansion across their dimensions as temperatures vary. They also tend to have heavier components placed on them creating leverage forces distortion. The larger area also makes evenly distributing internal stresses more problematic.

Q: Is it possible to completely eliminate bow and twist?

A: In principle yes, but in reality boards always exhibit some small degree of dimensional instability and warpage. The goal is to control bow and twist within acceptable tolerances rather than pursuing perfection. With robust engineering it is feasible to restrict warpage to under 0.1%.

Q: Are some PCB laminate materials more prone to bowing and twisting issues?

A: Yes, the choice of base laminate material impacts bow/twist tendency. Standard FR-4 glass reinforced epoxies are most problematic due to their high stiffness and brittleness. Recently introduced laminates with spread glass fibers or added fillers provide much better dimensional stability.

Q: Can warped PCBs be reworked back to flat shape?

A: Yes, rework departments can use hot presses with precision tooling to flatten assembled boards through the application of high uniform pressure. However this risks damage to components, extensive thermal stress and reliability questions. Avoiding bow/twist during fabrication is strongly preferred.