PCB Deformation During Reflow Soldering: Causes, Diagnosis and Prevention
Explore the causes of PCB deformation during reflow soldering, diagnose warpage by process stage, and validate design, support and thermal-profile changes.
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Table of Contents
Table of Contents
Short answer: A PCB deforms in reflow when heat softens the laminate and something pushes it out of shape — uneven copper or stackup, uneven heating, component weight over an unsupported span, or moisture trapped in the board. To fix it, find out when the shape changes (incoming, first pass, second pass) and where (whole board or under one part). That usually points straight at the cause.
Quick Takeaways
- Measure incoming boards first. Many “reflow” warpage problems arrive with the bare board.
- Bow and twist across the whole board is a different problem from local warpage under a BGA.
- Profile and support work as a pair: a good profile cannot save an unsupported 300 mm span.
- A flatter board is only a fix if the solder joints are still good.
1. Know What Kind of Deformation You Have
“The board is warped” is not enough to act on. Start by naming the shape.
Bow, twist and sag
Bow is a cylindrical or spherical curve where all four corners still sit in one plane. Twist lifts one corner out of the plane formed by the other three. Both are defined in IPC-TM-650 2.4.22C. Sag is the board drooping between conveyor rails or supports while it is hot — often it partly recovers on cooling, often it doesn’t.
Photograph the board with its orientation and support points visible. That one photo saves a lot of back-and-forth with suppliers.
Local board warpage vs. package warpage
Under a BGA, QFN or large connector, two things can move: the PCB and the component package itself. Package-on-package research at Auburn University shows how loss of coplanarity between them drives head-in-pillow, opens and bridging. A whole-board flatness number won’t tell you which side moved.
Cold flatness vs. hot shape
A board can look flat on the bench and still be curved at peak temperature. That’s when the joints form. Shadow moiré measurements during simulated soldering show board shape changing considerably through the profile. If defects point to reflow, measure at temperature.
2. What Causes PCB Deformation During Reflow?
Usually several factors stack up. Here are the main ones.
Laminate softening and Tg
As the laminate approaches its glass transition temperature (Tg), its stiffness drops sharply and its Z-axis expansion accelerates. Lead-free peaks of 240–250 °C sit well above the Tg of standard FR-4 (~130–140 °C), so the board is soft through the critical part of the profile. Component weight then bends unsupported areas, and some of that bend stays after cooling — a mechanism examined in Lars Bruno’s board-support study.
High-Tg material helps, but it isn’t a cure-all. Auburn’s work shows that repeated reflow itself shifts laminate Tg, so process history matters as much as the datasheet.
Residual stress from lamination can also release in reflow, especially if the bare board was already marginal. Ask the fabricator for lot records or comparison samples before assigning blame.
Unbalanced stackup and copper distribution
Copper and glass/resin expand at different rates. If one side of the board has a solid plane and the other has sparse signal routing, or the stackup isn’t symmetric about its center, the board curls toward the side that expands less. Bruno’s experiments found copper balance, board thickness, board width, component weight and routing/milling all significant. Fixes here include copper thieving on sparse layers, mirrored stackups and matched prepreg/core on both sides.
Uneven heating and thermal mass
A board full of heavy components on one end and fine-pitch parts on the other won’t heat evenly. The resulting temperature gradient becomes a stress gradient. Pallets and carriers add their own thermal mass — iNEMI’s PCB warpage initiative singles out pallet-induced temperature differences as a key factor. Place thermocouples near the heavy parts and near the deformed area.
Unsupported spans and fixtures
Thin, wide boards on edge rails sag in the middle. Bruno’s tests showed that shortening the unsupported width reduced warpage. Practical options are center support pins, a reflow carrier, or panelizing with rails and breakaway tabs that stiffen the panel. Don’t over-clamp, though: a fixture that blocks thermal expansion can create the warpage it was meant to stop.
Moisture and repeated heat cycles
Moisture absorbed during storage turns to vapor in reflow. The usual result is blistering or delamination rather than a smooth bow, so look closely at the damage pattern. CALCE’s moisture study covers the mechanisms and the downsides of over-baking. Double-sided assembly and rework add more heat cycles, each one a chance for the board to move further.
| Suspected cause | What to check | Typical fix |
|---|---|---|
| Material / fabrication | Laminate type, Tg, lot records | Higher-Tg or low-CTE material; fabricator review |
| Stackup / copper imbalance | Stackup drawing, copper maps per layer | Symmetric stackup, copper thieving |
| Uneven heating | Thermocouple data at hot and cold spots | Profile tuning, carrier redesign |
| Mechanical loading | Span between supports, heavy-part locations | Center support, carrier, panel rails |
| Moisture / extra cycles | Storage, packaging, bake and rework history | Dry storage, controlled bake, fewer cycles |
3. Diagnose Before You Touch the Oven
Changing the profile first is the most common mistake. Build a before-and-after record instead.
Check incoming boards and panels
Record lot, revision, thickness and panel position. Measure with a fixed setup and orientation. If only certain panel positions are bad, look at panelization or lamination, not the oven.
Measure after each heat cycle
Measure the bare board, after first-side reflow, after second-side reflow and after any rework. Note whether each measurement is hot or cold. The first stage that shows a change is where to dig.
Pair the profile with the support setup
Save the actual thermocouple traces together with recipe revision, board direction and carrier setup. A recipe name alone won’t let anyone reproduce the run. Review the data with your SMT assembly process engineer.
Go local when defects cluster
If head-in-pillow or opens cluster under one BGA, measure that area at temperature. A Chang Gung University study showed strain gauges can track board warpage during reflow with results consistent with shadow moiré — a cheaper option when moiré equipment isn’t available.
| What you see | What to compare next | Common wrong conclusion |
|---|---|---|
| Bare board already warped | Lot, panel position, storage, fab records | “The oven did it” |
| Change after first pass | Baseline, profile, support | “It’s only temperature” |
| Change after second pass | First-pass shape, second-side load and support | “First pass was flat, so design is fine” |
| Defects under one package | Local board and package shape at temperature | “Board is flat overall, so it’s coplanar” |
| Varies by carrier or direction | Matched trials | “The carrier has no effect” |
4. Prevent Warpage Through Design and Process
Design: material, stackup and copper
- Keep the stackup symmetric about its center layer.
- Balance copper across layer pairs; add thieving to sparse layers.
- Avoid placing all heavy components at one end or on one side.
- For thin or large boards, consider high-Tg PCB materials with low Z-axis CTE — and have the fabricator review the full construction, not just the Tg label.
Panel and support
Add rails and tabs that stiffen the panel; avoid large cutouts near the center. Use center support or a carrier for thin boards and wide spans. iNEMI’s project plan splits the problem into fabrication, board design and pallet factors — a useful way to divide responsibility between designer, fabricator and assembler.
Profile
Work within the solder paste and component limits for ramp, soak, peak, time above liquidus and cooling. A gentler ramp and a sufficient soak reduce gradients across the board; controlled cooling reduces locked-in stress. But don’t lower the peak just to make the board flatter — cold joints are worse than a slight bow. Check solder quality on every trial.
For each trial, log: the hypothesis, the one variable changed, board lot and history, profile and support setup, measured deformation, and inspection results. Change one thing at a time whenever possible.
Storage and baking
Keep boards in dry packaging and follow the supplier’s handling rules. IPC-1602A covers dry packaging, moisture uptake and baking. Bake when exposure warrants it, using a procedure suited to the board’s finish — excessive baking degrades solderability.
5. Measure Warpage the Right Way
Test method vs. acceptance limit
IPC-TM-650 2.4.22C describes how to measure bow and twist on bare rigid boards and panels; it excludes populated assemblies. Acceptance limits come from the product specification — for bare boards, IPC-6012 is commonly applied, with 0.75% often used for SMT boards. For hot, local coplanarity under BGAs, you need separate agreed criteria.
Report the conditions with the number
A warpage value without its setup is meaningless. Every report should state:
| Item | What to state |
|---|---|
| Object | Bare board, panel, assembly or package |
| Area | Whole board or defined local region |
| Temperature | Room temperature or defined thermal sequence |
| Support | Support points, load, restraint |
| Method | Equipment, reference plane, calculation |
| Requirement | Agreed limit and document revision |
6. What to Do With Boards That Are Already Warped
Quarantine and record. Keep the as-found boards and note whether they cause assembly, inspection or enclosure-fit problems. If you see blistering or layer separation, treat it as delamination, a separate failure.
Decide on rework or reject. Heat-and-press flattening is risky: it can stress solder joints and components, and the board may spring back. Only do it under an approved procedure.
Re-verify. After any corrective step, check electrical function, solder joints and mechanical fit. Looking flat is not the same as being good.
7. FAQs
Does high-Tg FR-4 stop reflow warpage?
It reduces softening during reflow, but it won’t fix an unbalanced stackup, poor support or uneven heating.
Can a board pass incoming inspection and still warp in reflow?
Yes. Incoming checks are done cold. Warpage at peak temperature can be very different.
Will baking fix warpage?
Baking removes moisture. It doesn’t fix structural or support problems, and over-baking can damage finishes and solderability.
Should a carrier clamp the board flat?
Not completely. The carrier should support the board while leaving room for thermal expansion. Validate it with the real assembly.
What should I send my assembler about a warpage problem?
Board and panel drawings, stackup, component placement, lot details, thermocouple traces, labeled photos, measurement conditions and the stage where the change first appears.
References & Sources
- IPC-TM-650 2.4.22C: Bow and Twist (Percentage) — IPC
- IPC-1602A:2024: Standard for Printed Board Handling and Storage — ANSI Webstore
- A Comparison of PWB Warpage Due to Simulated Infrared and Wave Soldering Processes — ORNL / IEEE
- Warpage Measurements of PCB During Reflow Using Strain Gauges — Chang Gung University / IEEE
- Effect of Reflow on Tg of PCB Laminates and PoP Warpage — Auburn University
- Controlling Moisture in Printed Circuit Boards — CALCE
- Board Warpage During Reflow Soldering—Need for Board Support? — Lars Bruno
- Approaches to Minimize PCB Warpage in Board Assembly — iNEMI
- PCB Warpage Characterization & Minimization Project — iNEMI
ORINEWPCB · FABRICATION & ASSEMBLY
Fighting Warpage on a Current Build?
OrinewPCB handles both PCB fabrication and assembly, so stackup, panel design and reflow support can be reviewed together. Send us your stackup, panel drawing, component placement and any reflow data, and tell us where the deformation shows up.
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