Halogen Free Laminate Guide

A halogen free laminate avoids the brominated flame retardant that has been the standard in glass reinforced epoxy for decades. The change was made for environmental and regulatory reasons, and it is not a cosmetic substitution: a different chemistry is used to achieve the same flame performance, and that chemistry changes the thermal and the mechanical behaviour of the material in ways that affect the fabrication process.

Why the Change Was Made

Brominated compounds are effective flame retardants, and the concern about them arises from the substances that can be formed when a board containing them is burned or recycled. The regulatory pressure has moved from voluntary schemes to customer requirements and, in some markets, to formal restrictions.

A halogen free material achieves the flame rating with a phosphorus or a nitrogen based system, sometimes combined with a metal hydroxide that releases water when it is heated. Each of those mechanisms works differently from bromine.

The requirement is usually described by the content of the halogens rather than by the flame rating alone. A material that meets the flame test but contains a small amount of a halogenated compound does not meet a specification that limits the total halogen content.

What Changes in the Material

The replacement chemistry usually raises the filler content, which changes the mechanical properties. The material is often more brittle, it is more sensitive to the drilling parameters and it absorbs moisture differently.

The decomposition temperature is the figure to watch. A halogen free material can have a lower decomposition temperature than the brominated material it replaces, which narrows the process window at the top of the reflow profile.

The glass transition temperature is usually comparable or slightly higher, so the two materials can look similar on a summary datasheet while behaving differently during a lead free assembly.

Process Consequences

The drilling parameters have to be reviewed. A more highly filled and more brittle material produces a rougher hole wall and more tool wear, and a drilling program that was tuned for a brominated material may produce a barrel with a defect at the knee.

The desmear and the plating are affected by the same filler. The chemistry that removes the smear from one material may be too aggressive or not aggressive enough for the other, and the process window should be re established rather than assumed.

The lamination cycle may need adjustment because the resin flow differs. A material that flows less may leave voids in a thick stack, while one that flows more may produce a thin dielectric in a high pressure area.

Comparing With Standard FR4

The electrical properties are usually close enough that a digital design does not need to change. For a high frequency design, the loss tangent and the dielectric constant should be compared rather than assumed to be the same.

The thermal properties are where the difference is most likely to matter. The decomposition temperature and the time at temperature are the figures that decide whether the material survives a lead free profile with rework.

The mechanical properties determine the handling. A more brittle material is more sensitive to a panel that is dropped, to a board that is flexed during assembly and to a depanel that is done by hand rather than by a router.

Compliance and Documentation

A halogen free claim should be supported by a test report that states the method and the limit. The common methods measure the content of chlorine and bromine, and the limit is usually expressed as a total for the two.

The declaration should cover the whole board and not only the laminate. A solder mask, a marking ink or a surface finish that contains a halogenated compound will place the finished board outside a specification that limits the total content.

The documentation should be traceable to the material batch. A customer who is auditing a claim will ask for the report that corresponds to the material used, and a general statement from the supplier is not the same as a batch record.

Where It Is Required

The requirement appears in consumer electronics, in products that carry an environmental label, in some automotive and medical specifications and in any product that has to meet a customer’s restricted substance list.

It is less common in an industrial board where the customer is the end user and the specification is written around the function rather than around a materials policy. In that case the decision is made on the process and the cost.

The decision should be made at the start of a project because the material affects the stackup, the process and the qualification. Changing the laminate late invalidates the process window that was developed for the original material.

Cost and Availability

A halogen free material usually costs more than a standard FR4 of the same class, and it is available in a narrower range of thicknesses and glass styles. The narrower range can force a stackup change that has its own cost.

The lead time can be longer for a non standard construction, and the minimum order quantity may be higher. On a small production run those factors can outweigh the material price difference, and the storage life of a non standard material should also be checked before a large quantity is ordered.

The cost comparison should include the process. A material that needs a longer lamination cycle, a different desmear or a slower drill consumes capacity, and the effect should be included in the comparison rather than treated as a technical detail.

Qualification

The qualification should follow the same path as any material change: a coupon that represents the real stack, a thermal cycling test and a reflow simulation with the number of passes that the product will see.

The process window should be established by a designed experiment rather than by a single successful run. The variables that matter are the drilling parameters, the desmear, the lamination cycle and the reflow profile, and each of them should be confirmed.

The results should be recorded with the material batch and with the process settings, so that a later change of material or of supplier can be compared with the qualified state.

Practical Rules

Confirm the halogen content with a report that states the method and the limit, and compare the decomposition temperature and the process window with the material being replaced. Re establish the drilling, the desmear and the lamination.

Record the material and the process data with the build records and the laminate properties, and review the high Tg selection and the environmental compliance when the material policy is set.

FAQ

Is a halogen free laminate electrically different? Usually not enough to change a digital design. For a high frequency design the dielectric constant and the loss tangent should be compared rather than assumed.

What property is most likely to change the process? The decomposition temperature and the filler content. A lower decomposition temperature narrows the reflow window, and more filler changes the drilling and the desmear.

Does the laminate alone decide compliance? No. The solder mask, the marking ink and the finish all count, so the declaration has to cover the finished board and be traceable to the material batch.