Keep CTI Material Evidence Separate from Board Spacing Decisions

A laminate description containing a CTI value can help an insulation review, but it does not complete that review. The useful question is whether the value belongs to the exact material being purchased and how the responsible product standard uses that evidence. Treating CTI as a universal board-voltage rating skips both steps.

Identify what the test describes

UL Solutions describes comparative tracking index testing as a way to evaluate the relative resistance of insulating materials to electrical tracking under the specified test conditions. The result characterizes a material response. It is not a stand-alone approval of the completed PCB layout.

Start with an evidence record containing manufacturer, full material designation, report or recognition reference, test method, classification and relevant construction restrictions. Request clarification if a distributor provides only a broad FR-4 description. Different grades within one commercial family should not automatically inherit a single reported value.

Connect purchasing language to engineering evidence

Check whether the drawing and purchase specification identify the material closely enough to preserve the reviewed property. An alternate that meets thickness and glass-transition requirements may still need a separate tracking-data review. Include that property in the substitution decision when it influenced insulation design.

Keep the evidence reference attached to the approved construction rather than copying a number into an isolated spreadsheet. If the source changes, the reference allows a reviewer to see exactly which statement was originally accepted. Record the document revision and any restrictions supplied with the value.

Review the actual insulation path

Provide the product’s insulation reviewer with conductor geometry, exposed surfaces, environmental assumptions and the applicable design requirements. Ask which material classification is relevant and which additional factors determine spacing. Do not select a clearance or creepage distance solely by searching for a CTI number that produces the desired answer.

Solder mask, coatings, slots and enclosure barriers also need their own assessment when the design relies on them. A laminate result does not automatically establish the performance of these added features. Identify the feature being credited and the verification required before counting it in the design.

Handle uncertainty visibly

If the exact material record is unavailable, mark that dependency as unresolved rather than borrowing a value from a similar product. The team can then obtain the correct evidence, choose a documented material or revisit the design assumptions.

The final review should name the material, the accepted evidence and the design decision it informed. That concise chain is more useful than a claim that the board is safe because its substrate has a high CTI. It also makes later supplier changes easier to assess without repeating the entire investigation.