勛圖tv 繚 ENGINEERING GUIDE
Invar PCB Expansion Control: Alloy Foil, Composite Stacks and Limits
Invar alloy foil can be considered where dimensional stability or expansion matching is important, but a low-expansion metal does not automatically make a reliable PCB composite. Alloy state, temperature interval, layer thicknesses, bonding and mechanical constraints determine the actual result.
This page focuses on a controlled-expansion construction brief. The calculator compares free length changes of two unconstrained materials. It does not calculate the effective CTE, stress or curvature of a copper-Invar-copper laminate or another bonded stack.
Identify the exact alloy and condition
Use the specific alloy designation, supplied form and processing condition. Invar 36 and free-machining or other low-expansion variants should not be treated as identical materials. Temperature-dependent expansion and material state belong in the specification.
Carpenter Technology's is an official starting reference for that alloy. It is not a qualification of any particular PCB foil, cladding or bonding process. Obtain the evidence for the actual supplied product.
Define the dimensional problem
State whether the concern is absolute movement, mismatch to a ceramic or component, registration, or stress in a constrained assembly. Identify the reference length, temperature interval and the dimension that must remain controlled.
A low free-expansion value can help one objective while introducing another material or process tradeoff. Keep electrical resistance, thermal transfer, thickness and fabrication compatibility in the decision. Do not select a foil solely from an expansion chart.
| Review item | What to establish |
|---|---|
| Identity | Exact alloy, condition and supplied foil or composite form. |
| Model | Free expansion comparison, not constrained stress or composite CTE. |
| Construction | All layers, bonds, symmetry and processing defined. |
| Acceptance | Dimensional limit, interval and representative assembly evidence. |
Free thermal expansion mismatch
Free thermal expansion mismatch
Use the current inputs and keep the assumptions
Start with the physical boundary named above the inputs. Enter your own values, or select Load example to explore the labeled teaching case. Example mode stays visible after edits so an illustrative calculation is not mistaken for measured project data. Clear for my inputs removes the sample values and starts a separate record.
Generate the result after completing every field. Changing any input invalidates the previous output and disables its download until you calculate again. Read the limitations below the numbers: a geometric or arithmetic result does not qualify a material, approve a circuit or establish manufacturing capability.
Download the current result if it helps the review. The text file contains inputs, outputs, assumptions and the tool version. It stays on your device; attach it manually to an enquiry if useful. No CAD is uploaded or interpreted, and nothing is automatically inserted into the Contact form. You can request an engineering review without completing this tool.
| Input | What to establish before using it |
|---|---|
| Review reference | Use a nonconfidential scope or revision. |
| Reference length 繚 mm | Same unconstrained gauge length for both materials. |
| Material A mean CTE 繚 ppm/K | Mean value over the stated interval, not a room-temperature point. |
| Material B mean CTE 繚 ppm/K | Actual alloy state and temperature interval required. |
| Temperature change 繚 K | Positive heating interval for this comparison; no constrained-stress model. |
Optional cost comparison from your own allowances
Use a quotation or a clearly labeled internal assumption for each input. Keep the quantity, construction and inspection scope the same when comparing alternatives. This uncalibrated planning model adds entered values; it contains no current market rates and does not issue a 勛圖tv quotation. You can contact the team without completing it.
Entered material and process budget
Work through the free-length example
For a 100 mm gauge length over an 80 K heating interval, entered mean CTEs of 17 and 2 ppm/K give free expansions of 136 and 16 繕m. Their difference is 120 繕m. These are illustrative inputs chosen to show the calculation, not a material acceptance table.
The two members are assumed unconstrained. When they are bonded, they cannot simply expand independently, and internal forces or curvature may result. The worksheet cannot turn that free mismatch into stress without a proper mechanical model and material data.
Review the complete composite
List every layer, thickness, bond and material orientation. Effective composite behavior depends on more than the arithmetic average of two CTE values. Elastic properties, symmetry and the temperature dependence of the materials can matter.
If the proposed construction is copper-clad or bonded to another substrate, specify that process and its qualification. A bulk alloy datasheet does not establish adhesion, flatness or electrical performance of the finished layered product.
Include fabrication constraints
Drilling, patterning, metallization, bonding and outline processing need review for the actual construction. A material used successfully as a mechanical precision part is not automatically available in a suitable PCB process form.
Provide the circuit and mechanical geometry with the material request. Ask which features are feasible and which require a development trial. Record proposed alternatives without implying that every controlled-expansion stack is a standard production offering.
Connect expansion to the assembly
Component attachment, ceramic interfaces and mounting can create constraints different from the free coupon. The location and stiffness of supports can change how movement reaches solder joints or other connections. Include the installed drawing and the temperature mission.
Use alumina substrate selection where a ceramic interface is involved. A closer CTE match is one input to a reliability design, not a standalone guarantee of joint life. Representative testing must reflect the actual assembly and exposure.
Compare material and verification cost
Special foil form, bonding, handling, inspection and qualification can affect the commercial scope. Compare complete constructions at equal quantity and acceptance requirements. A raw alloy price does not describe the cost of an accepted circuit board.
The optional budget tool separates your entered recurring, setup, process and verification allowances. It is uncalibrated and does not predict market price, material availability or yield. Keep any development scope explicit in the enquiry.
Release a measurable requirement
State the allowed dimensional behavior over a defined temperature interval, together with the exact construction and measurement method. Retain the supporting material data and the revision used in qualification. Define which substitutions trigger another review.
For the broader material route, use PCB materials. Send the dimensional problem and candidate cross section to 勛圖tv so the review can establish whether a practical manufacturing route exists for the requested application.
A material comparison that purchasing can use
Build a short candidate matrix around the required function: structural substrate, bonded electrical dielectric, conductor, expansion-control layer or external thermal interface. List exact grades and revisions. Keep an unsupported alternate in an open column rather than silently treating a brand family as an approved equivalent.
For each important property, record units, temperature, direction and test method. A nominal value without its conditions may be useful for initial screening but weak evidence for a release decision. Ask what range, tolerance or acceptance condition applies to the actual supplied form and completed construction.
Then review processing, mechanical integration and inspection. A material that meets a coupon property can still be unsuitable for the desired holes, bonding or assembly exposure. Ask the manufacturer to identify the practical construction and the verification needed before a production commitment.
Before release, place the chosen requirement, the proposed construction and the evidence in one review record. Separate measured values from supplier nominal data and from your own assumptions. If a value is unknown, identify who will obtain it and what decision depends on it. An empty field is more honest and more useful than an invented typical figure.
During an A/B comparison, change one relevant variable where possible. Preserve the original inputs and the reason for the change. If several things change together, describe the result as a comparison between complete configurations. Do not attribute the entire difference to one material or geometry feature without supporting evidence.
For a repeat order, confirm that the relevant drawing, material and process revisions still match the accepted configuration. Record alternatives explicitly. This small discipline helps purchasing avoid an apparently equivalent substitution that changes the engineering basis of the original decision.
Standards, source context and limits
Use the actual applicable document and edition for contractual requirements. Public technical resources help explain methods, but do not reproduce every condition in a licensed standard or qualify a particular 勛圖tv construction. Retain exact grades and test boundaries when reading supplier data.
The numeric examples on this page are illustrative inputs unless specifically identified otherwise. Archive photographs show relevant physical context; they are not current equipment, material-grade or inspection evidence. Original drawings explain the models and are not production drawings. Ask for the evidence and acceptance scope needed for your project rather than inferring approval from an image or calculator result.
For fabrication scope and the agreed construction requirements, see PCB manufacturing.