Sticking Point Print E-mail
Written by Mark Verbrugge   
Friday, 03 May 2013 16:55

There are no poor choices, but in certain cases some adhesives do outperform others.

Often when designing a flex we take certain things for granted, adhesives being one. We accept what material manufacturers give without a lot of thought as to why. Given a marked increase in client questions regarding adhesive types and their use, let’s look at commonly available adhesives and why we choose them.

The most common types of adhesives used in flex circuits are modified epoxy, acrylic and “adhesiveless.” Less common, but not unheard of, are polyester and PTFE.
Let’s review each one with an eye toward gaining the highest benefit from each specific material for the lowest cost.

One of the main reasons a flex manufacturer may select a particular adhesive over another has to do with via drilling and desmear operations necessary for good via formation and plating. With this in mind, we find acrylic adhesives are most common within North American manufacturing circles, whereas modified epoxy-based adhesive films are more common in Asia and Europe. North American shops use a plasma etch process to prepare hole walls for plating, typically using a gas mixture of CF4/O2 (should not be used for PTFE/Teflon). Epoxy adhesive films are prepared for plating by treating chemically with a potassium permanganate bath. It is critical to note that while these aspects do not impact the end-user or product, they may impact the final cost of a product when a buyer selects a material that requires non-standard processing from their chosen manufacturing partner.

Let’s start with the most commonly used adhesives and work our way down. Epoxy, or modified epoxy films, is an excellent all-purpose adhesive/bond ply. Epoxy films are used to bond FR-4 or polyimide stiffeners, or with a dielectric cover layer, most commonly polyimide film. With an MOT (maximum operating temperature) of 115˚C, they can handle a slightly harsher working environment than other common adhesives. Their increased chemical resistance is ideal for some tougher solvent exposures (my first choice for printing equipment applications!). Glass transition temperature (Tg) comes in at 60˚C. Bond strength is also slightly improved over similar low-cost adhesives.

Acrylic adhesive is another alternative that can be used in many of the same applications as epoxy. Acrylic exhibits excellent bond strength and good chemical resistance. Its downside is a somewhat low Tg of 40˚C and its relatively high CTE (coefficient of thermal expansion). CTE is not an issue in low layer-count boards, but as layer-count rises, special attention must be paid to the overall percentage of adhesive in the stack-up. Some flame-retardant versions are somewhat cloudy to opaque, making inspection of internal conductors a little difficult. Most acrylic adhesive manufacturers rate them at a maximum MOT of 105˚C. Acrylic has a big advantage to the manufacturer, as it has an extraordinary shelf life and excellent processing characteristics, a very cost-effective material.

All polyimide constructions (sometimes referred to as “adhesiveless”) are another option. Typically this option pertains to the base or core of a flexible inner-layer and still employs a standard adhesive cover system. An adhesiveless base uses a “sputtering” or “direct cast” process to bond itself to a copper layer. Their distinct advantage is a more flexible/thinner construction (often 1 mil or less) with significant increases in MOT.

There are, however, adhesiveless cover systems available, though the term “adhesiveless” is a bit of a misnomer when used to describe a cover-coat. These systems do in fact use a “B-stage” polyimide to act as an adhesive. The use of an all polyimide construction can result in a circuit with a high MOT. It should be noted that while all-polyimide cover systems are available, they typically have poor fill properties, making them suitable for only the thinnest foils (9µm or less). Cost is considerably more than other adhesive systems, and a smaller number of suppliers offer this system.

Polyester comes in as our last and lowest-cost alternative. Typically polyester adhesives are only used on polyester substrates. With low-temperature processing, they are easy to use and very inexpensive, but this comes at a cost in performance. They can be a bit stiffer than other available materials. Good chemical resistance properties and the cost make them worthy of consideration for a variety of applications. The chosen material for FFCs (flat flexible cables), they come in a large number of stock configurations.

PTFE/Teflon adhesive systems are relatively new in the flex arena. They require special processing not all manufacturers are capable of. One big difference is via preparation for plating must be performed with a special plasma process, typically using nitrogen/hydrogen or helium mixtures. Not surprisingly, bond strength is somewhat reduced compared to more common materials. By far the biggest advantage here is in controlled impedance applications. Its low Dk (dielectric constant) of 2.2 - 2.4 permits significantly thinner cores where higher impedance values are needed. The downsides are a less forgiving process window and increased cost.

Realize what goes into the material stack-up and select the best materials for your end-application. None of the adhesives discussed are “poor” choices; all have positive and negative aspects. There’s not a single one I would not recommend for any standard flex application. Only a thorough review of both material properties and end-use requirements can ensure delivery of a flex circuit that will perform well in the field and have a long service life.

[Ed.: To enlarge the figure, right-click on it, then click View Image, then left-click on the figure.]

Mark Verbrugge is a field applications engineer at PICA Manufacturing Solutions (picasales.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it . He and co-“Flexpert” Mark Finstad ( This e-mail address is being protected from spambots. You need JavaScript enabled to view it ) welcome your questions.

Last Updated on Monday, 06 May 2013 17:23
 

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