A variety of flex PCB producers have produced unusual multilayer flex PCB frameworks, resulting in high performance interconnection frameworks. In many cases, it is not the versatility of the circuit that is looked for, but the amenability to specific types of processing and resistance to certain prospective defects. The adhering to are a few instances:

PRE-DRILLED STANDARD GRID SUBSTRATES

One plan for creating high thickness multilayer structures, stimulated on by the development of area array packaging, was based on using a basic grid pre-drilled on a flexible substrate. This generated the notion that a typical grid could be made use of to promote building of basic base grid products.
Flex circuit leader Sheldahl (now Multek) aimed to provide high density substrates based upon their advanced microvia innovation in the mid 1990s. They developed a flexible base material item that was pre-perforated with tens of hundreds of openings (approximately 400 openings each square centimeter or 2500 openings each square inch) positioned on a selected grid that they wished could be standard. Their intention was to deliver a flex PCB product with the through-holes already plated and the panels pre-coated with photo-imageable resist based upon a cost-effective hole formation innovation with all openings generated on an usual grid. The item was designed to address some of the major issues in the search of greater thickness affiliation frameworks, including the provision of tiny holes to enhance wire transmitting capability on substrates. However, the technique did not take hold for a variety of logistical and company reasons. Nevertheless, the idea is still of passion as well as could be of value in the future if the principle of typical grids and also their benefits ever sinks in and also takes hold.

Z-AXIS INTERCONNECTION TECHNOLOGIES

Z-axis affiliations for building of multilayer circuits have actually likewise been explored by several business for use in high thickness flex PCB circuits. The Z-axis interconnection modern technology contrasts substantially with consecutive accumulation technologies utilized in stiff multilayer board manufacture. The intent of Z-axis interconnection technology was to decrease the cost of high density multilayer by making simple, high producing single- or double-sided substrates and consequently joining and adjoining them in a high yielding lamination process. The concept seems to be appropriate to flex PCB circuits interconnection. Stiff board building and constructions making use of the same fundamental ideas have been shown in Japan. The innovation is divided into 2 basic courses, anisotropic as well as isotropic.

ANISOTROPIC INTERCONNECTION

Anisotropic adhesives have actually remained in usage for a number of years. Nonetheless, it is just lately that they have obtained prevalent focus. The technology, a combination of an adhesive and also a diffusion of conductive fragments, produces interconnections in the Z (upright) route just, while maintaining side circuit isolation.
Consequently, any point of intersection in between 2 breeding circuit fifty percents will certainly be electrically connected. The idea is particularly ideal for adjoining flex since using a coverlayer will certainly safeguard the circuits from shorting in aspects other than those open to shorting by design. Anisotropic affiliation is additionally beneficial for lapped affiliation in between a flex circuit as well as a rigid flex PCB and also is progressively made use of for that purpose. Present driver circuits for use with all type of different display innovations from earlier fluid crystal displays (LCD) and also light emitting diode (LED) displays to advanced organic LED (OLED) and also energetic matrix (AMOLED) innovations are archetypes.

PROGRAMMABLE INTERCONNECTIONS

Alternatives to anisotropic affiliation consist of programmable interconnection structures wherein the affiliation points of the laminate structure are pre-designed and ready. One such variation was a concept developed at Tessera. The innovation enables the formation of interconnections at the same time that the multilayer flex PCB framework is being formed.

The most noticable benefit of programmable interconnections is their capability to generate extremely brief affiliation courses between components. Short course, grid-based routing is often referred to as Manhattan transmitting as a result of its conceptual similarity to the mixed grid-like framework of New York City’s streets. Using standard grid products and also interconnection methods may well end up being commonplace in future digital packing.

Various other approaches have actually been checked out as well as created for producing multilayer flex PCB circuits utilizing co-lamination handling to produce upright interconnections within the framework. In one type of up and down integrated flex PCB structure, anisotropic conductive bonding films are utilized to make various brief Z-axis interconnections instead of standard (as well as longer) layered through-holes. The interconnections are made throughout lamination, providing n perhaps easier procedure Other frameworks utilize dielectric bondplys with configured signing up with factors of metal or conductive adhesive.

These type structures are playing a progressively vital part in electronic affiliation for high density applications such as hand held electronics. Up and down incorporated flex frameworks could change rigid flex PCB in some applications as a result of their capability to prevent redistribution circuitry, specifically for area selection affiliations. This makes them optimal prospects for high density frameworks that use high pin matter ball grid arrays (BGA) and also chip scale plans (CSP).

Multilayer flex PCB circuits are much less common than their inflexible laminate counterparts. There are a variety of reasons why this has actually remained the instance for years, including price of materials, dealing with worries and a basic absence of seasoned vendors. However, as the line in between flex as well as stiff remains to obscure, the thinner enhanced core materials becoming much more usual and more recent composite products being introduced, it appears that the void between both brother or sister technologies is gradually being linked.

While it is not yet common, multilayer flex PCB circuits constructed utilizing flexible circuit laminate materials have some innate attributes that provide convincing reasons for their increased use. As an example, flexible base products are largely unreinforced, as well as are thus pure polymer substrates as opposed to composites. This function leads to a product that corresponds in electric buildings and makes it a more effective selection for highest performance applications. Furthermore, the material is superior compared to glass fiber strengthened materials, where the dielectric consistent and also loss tangent of the product could differ on a localized basis as the signal is sent with the material. Thus, with enhanced composite materials the control of signal features at higher regularities (the domain where skin results start) is more difficult compared to with the even more uniform flex products.

Flex PCB product homogeneity is a significant advantage when drilling or punching openings in close proximity to each other to save room or increase performance. The advantage is that the structure is immune to the supposed conductive anodic filament (CAF) phenomenon where ionic movement along glass fibers creates shorting in between surrounding layered through-holes. Problems concerning CAF are expanding as proof of it in finer function PCBs is on the surge.

A variety of methods for making multilayer flex circuits have been explained for many years. A lot of summaries follow typical techniques made use of in the manufacture of rigid flex PCB. Various other methods, however, have actually departed from the mainstream to check out options. Dyconix, for instance, was amongst the first to use flex PCB circuits materials to create high integrity multilayer flex circuits with microvias using their cutting-edge plasma handling methods. Various other companies have actually also suggested flexible circuit options for multilayer flex PCB. Tessera Technologies, for instance, has actually defined a number of different methods to produce such affiliation structures. Tessera’s initial principle was a structure wherein basic two-metal layer flex PCB circuits were bound and also adjoined throughout the lamination process making use of special interposer material.

While the initial electrical affiliation and joining medium was a silver-filled conductive resin in a flex PCB bondply, later on ideas consist of making use of deformable plated metal attributes that flawed, mated as well as signed up with the interconnection points during the lamination process.

During growth, the driving objective was to create high thickness multilayer structures using high generating double-sided circuits with reduced facet ratio layered through-holes. These layers would then be joined and also adjoined making use of a high yielding lamination process. Examination information from very early experiments were motivating however commercialization has actually been sluggish to take place.

Other business likewise have modern technologies developed to provide similar solutions where interconnection between layers is made during the lamination process. For example Toshiba’s hidden bump affiliation innovation (B2it) technology,
Matsushita’s any-layer inner via hole (ALIVH) technology, as well as a much more current offering, likewise from Japan, the NeoManhattan bump process from North Corporation (the technology is currently owned by Tessera). The former two techniques have actually been relatively strong while the last is complying with the various other strategies right into the market.

Some flex PCB manufacturers in Japan remain in production with B2it technology and also have developed design rules for prospective users. They have adjusted the modern technology specifically to be used with liquid crystal polymers (LCP) for high thermal and nicer electrical performance. While every one of the modern technologies defined are potentially capable of being utilized in the building and construction multilayer flex PCB circuits, crossbreed buildings could discover some application too. Making use of stiff and also flex PCB products with each other in a common building and construction can provide some one-of-a-kind possibilities for high performance applications.

Multilayer flexible circuits have three or more layers of metal conductors. Over the years, they have become much more preferred as a packing plan in some applications in spite of their higher family member cost. They are really complex, engineering-intensive and also requiring, both in design layout as well as manufacture. It is commonly feasible to lay out a design on paper, which, though it has obvious reasoning and order to it, could not be effortlessly produced or manufactured at all.

Regardless of these obstacles, the fact that multilayer flex circuits structures permit the designer to develop unusual high density and high performance digital package deals keeps them on the checklist of choices for many developers. As a matter of fact, lots of product packaging engineers feel that multilayer flex circuits are an anticipate the many benefits they bring, specifically in complex wiring situations.

Handling of these extremely intricate interconnection frameworks is incredibly requiring and calls for an awesome command of manufacturing procedures. Unlike double-sided as well as single-sided flex circuits, which can be processed in a reel-to-reel style, multilayer circuits are normally only fabricated in panel kind. (A multiple-conductor layer construction is possible using PTF in an internet style, yet its efficiency is
restricted currently, so it is not considered an usual multilayer flexible circuits strategy.).

In addition to inflexible flex circuits, multilayer flexible circuits are among the most pricey types of digital affiliation structures. Nevertheless they could likewise be among one of the most costeffective. Creating as well as outlining a multilayer flex circuit is best performed in close coordination with the flex circuit manufacturer. The majority of manufacturers are eager to give help in order to help with access of the design into manufacture and also yield far better outcomes. The complying with are summaries of a few of the techniques of multilayer flexible circuits building and constructions that have actually been discovered to date.

Hardly any about multilayer flex circuit processing is genuinely traditional. Each brand-new design brings with it a host of new challenges. Multilayer circuits have the tendency to be engineering-intensive as well as call for well thought-out, proficient preparation. Due to the fact that each design is distinct, it is hard to define a normal handling series; nevertheless, there are some usual styles and the complying with summaries are offered to give a feeling of the intricacies involved in the manufacture of multilayer flexible circuits.

STANDARD MULTILAYER FLEX PCB MANUFACTURING

There are many similarities between multilayer flexible circuits and conventional rigid PCBs, but the processing is typically much more tough. In flex PCB MLB (multi-layer board) processing, flexible laminates that are to be the flexible circuits layers are provided with tooling holes. Often the tooling openings are put first, yet there are variations on this means that involve the use of a post-coverlayer application lamination punch that can properly punch tooling holes in a semiautomated manner.

– Holes might be drilled in the flexible appendages or break-outs prior to imaging.
– Layers are imaged, etched as well as perhaps plated via depending upon the design needs.
– Coverlayers are typically laminated to the etched patterns while supplying access to interconnect factors on tentacle ends via openings in the coverlayer.
– Covercoated flexible circuits layers are laminated with each other making use of flexible bond plies, leaving the tentacles unbonded where the design calls for.
– Outermost layers are generally still coated fully with copper foil.
– Panel is drilled using the exact same tooling system that was utilized for picture transfer to ensure that interior lands will be in register with the pierced openings.
– Holes are cleaned up using a plasma procedure or other ideal method.
– Holes are metallized by electroless copper deposition or another appropriate method.
– Holes are electroplated with extra copper to fulfill the needs of full plating or to just lock the electroless plating on.

For a subtractive process, the complying with additional steps are needed:.

– Negative picture of the external flex PCB pattern is applied to the copper aluminum foil.
– Open area or circuit pattern is plated with an etch-resistant steel such as tin or tin-lead.
– Resist image is removed.
– Pattern is engraved from the history foil.
– Metal engrave stand up to is stripped from the flex PCB.
– Surfaces are cleansed to give a bare copper surface area.
– Outer coverlayer is laminated flooring to the flexible circuits, offering openings to the interconnection features of interest.
– Solder or a solderable covering procedure may be suggested.
– If not soldered, the circuit is punched, routed or reduced from the laminate.
– Tentacle ends are freed for part and also adapter installation, if called for.

This very simplified technique to multilayer flex PCB fabrication supplied below and offers only the least in Compared to singleand double-sided flex circuit handling, it is easy to see the possible difficulties of multilayer building and constructions and why they can be so expensive. The balance of this area offers short descriptions of some alternative techniques and also techniques for generating multilayer flexible circuits.

While single-metal layer flex PCB circuits are broadly made use of, 2 conductor layer (additionally described as two-metal as well as double-sided) flex PCB are coming to be much more preferred as packing densities raise. Two-metal layer flex PCB building and constructions are very popular in a variety of applications from video cameras as well as mobile phones to clinical devices as well as IC packing. Like singlesided flex PCB, there are a variety of different approaches that can be made use of to develop double-sided flex circuits. Two-metal layer flex circuits are not as conveniently adapted to roll-to-roll processing as single-sided flex PCB. Nevertheless double-sided flex PCB circuits can be produced in a roll-to-roll fashion. The complying with are descriptions of means for creating double-sided flex PCB that have been made use of in the past, are currently being used or are under research for future usage.

PANEL PLATE PROCESS

Panel plating of flex PCB circuits is a long-lasting method as well as, like other handling approaches, it has benefits and also drawbacks relying on a number of different factors. From a processing viewpoint, it is less complicated to electroplate a non-patterned panel. Nevertheless, the additional density works against the root cause of flexibility and makes fine attribute interpretation more difficult. A common manufacturing series for this procedure is as complies with:

– Metal dressed laminate is pierced with the desired opening pattern.
– Hole wall surfaces are made conductive using an electroless copper process or an appropriate option, such as a graphite covering provided by OMG.
– Laminate is electroplated with copper to develop the copper plating density in the hole to meet requirements.
– Dry film resist is put on the laminate on both sides.
– Circuit image is formed into the steel clad and plated-through laminate, making certain that, after growth, the through-holes are tented dependably to safeguard versus the etching process.
– Resist-patterned attired laminates are etched to create flex circuit patterns on both sides.
– Resist is stripped and coverlayers are laminated to the top and lower as required.

Besides the plated through-hole part of the procedure, the treatment is nearly similar to the procedure used for the manufacture of single-sided flex PCB. The flow chart supplies greater detail regarding the circuit handling actions.

PATTERN PLATE PROCESS

Pattern plating is a common variation utilized to create twometal flex PCB circuits with plated through-holes.

– Negative picture of the flex PCB circuit pattern is created, which reveals through-holes for plating in addition to the conductors.
– Copper circuit is overplated with an etch chemistryresistant metal such as tin, solder, gold or others. These steels will safeguard the circuit pattern and through-holes throughout the etching process relying on the type of chemistry made use of.
– Plating resist is removed.
– Circuit is etched utilizing an etchant that does not strike protective steel down payments. Sulfuric or ammonia-based acid-peroxide etchants are both typically utilized as they preferentially strike copper.
– Protective metal is removed from the flex PCB circuit leaving a bare copper circuit identical to the one generated by panel plating.

The best benefit of the pattern plating process is recognized when functions are great, hardly any annular ring exists in the danger and a design is high of the withstand being undercut and engraving out the copper from plated through-holes. Because a smaller quantity of copper needs to be engraved though to create the flex PCB circuit picture, this method is additionally of value when better circuit functions are called for. The result is a reduction in undercutting as well as a more precise last circuit pattern.

Interconnection of the two sides of a double-sided flexible circuits can also be completed mechanically. For many years, a number of methods have actually been developed. While all of the means have demonstrated functionality, the individual means are limited to applications where specs will permit their use. The following are some of the many techniques that

The different approaches for interconnecting two-metal layer flex PCB illustrated are not typical, yet stand for prospective solutions for factor to consider when tested with a special interconnection issue. can be utilized for many years to develop electromechanical connections.

Z WIRE INTERCONNECTION

The Z cord is an early forerunner of the plated through-hole.
It is simply a wire bent in the form of a Z as well as carefully soldered to interconnect both sides of the circuit. This strategy is typically used only for vias because various other through-hole links are achieved by the component leads themselves.

EYELETTING

Eyelets are one more early choice to the plated throughhole. Eyelets are the matching of small rivets with holes at their centers, like those discovered around shoe lace openings. Like Z wires, eyelets could make interconnection feasible in between sides of a two-sided circuit.

COOL WELDING

Cold welding, created by inventor Robert Lomerson, is an intriguing method for interconnecting both sides of a double-sided flex PCB. The technique calls for the fabrication of special devices and also manufacture of the flexible circuits according to a prescribed fashion. Long-term interconnection is acquired by pushing copper into copper through openings in the flexible circuits laminate, creating a metallurgical joint by chilly welding using the special etched or machined tools.

The other branch of singulation methods entails punching. Punching is a tried and true approach as well as is achieved by utilizing either sorts of tools: two-part punch and also pass away collections as well as single-part formed blade cutters. The strike as well as die approach has been utilized to punch forms from flat products for hundreds of years. Modern Class A punch and pass away tools are amongst one of the most exact readily available and could last for hundreds of thousands to countless hits if effectively maintained as well as cared for. They are lead as well as relatively expensive times could be very long.

A choice for Class A tool punch is the patterned blade cutter or steel policy die. Below, a sharp blade of springtime steel is placed into a network cut into a base of plywood or other product. A footwear pass away press or remote control press is then made use of to press down on the flex PCB circuit pressing it right into the blade and via right into a cutting board. A foam rubber, a little thicker compared to the blade, aids push the flexible circuits back up from the blade cavity after punching. These tools can be very accurate depending on the means used to produce networks. The price of product made use of for blade assistance as well as turn-around time are generally rather low. With appropriate care as well as usage of appropriate backing materials when punching, they can be made use of for thousands of strikes.

The etched blade pattern method has been made use of for many years to manufacture roll cutters made use of for adhesive label cutting and also has currently been adjusted as well as offered as an alternative to the basic regulation die. The attributes are produced photolithographically and also therefore could precisely recreate the synopsis pattern. The dimensional stability is exceptional. A rare earth magnet is made use of to hold the cutting pattern in position and also supply mechanical assistance. This approach has actually obtained some support with the flex PCB manufacturing and also user community.

There are a number of possible means for singulate flexible circuits from the panel and all apply to all kinds of flexible circuits, especially double-sided flex PCB. The means option is asserted on a variety of various elements including resistance, quantity and time. The right technique will vary with the needs of the individual product evaluated versus price, volume, necessity and also intricacy.

There are a variety of various techniques readily available for singulating (removing as well as cutting) specific flex PCB from a refined panel, and all of these techniques apply to
all kinds of flex PCB. The methods for singulating flex PCB can be branched along 2 main strategies. The first is vibrant vector-based cutting approaches, represented by routers, lasers, water jet cutters as well as numerically-controlled (NC) blade cutters. The second branch entails numerous punching approaches making use of Class A devices, steel rule dies as well as a new classification of etched reducing dies. Ease, tolerance requirements, and also product volume have to be factored right into the equation when selecting the appropriate innovation.

VIBRANT VECTOR-BASED CUTTING

Dynamic vector-based cutting makes use of a computer program to move the flex PCB panel about the methods of reducing by moving the table with a stationary cutter or by relocating the reducing tool itself while the panel is kept in place. The previous technique is the one most generally used in circuit manufacture and utilizes an NC router to cut out rigid printed circuits. Because the majority of flexible circuits PCB producers have internal routers to make inflexible parts such as stiffeners, it is a convenient device. One benefit of this approach is that a number of panels can be routed at once because routers often have more than one head as well as the circuits themselves can be stacked to improve performance. it gives an example of an existing drill and also router equipment.

There is no wait for tooling since the router is soft-tooled by CAD data. On the drawback, if the flexible circuits overview is intricate, routers can be slower compared to punching and also edge top quality might experience if good processing control is not exercised. While routers have been distinguished as an instance, water jets as well as lasers could likewise be used to cut flexible circuits from their panels.

With all approaches, stacking is generally feasible; however the outcomes can differ quite a bit. Water jets are tidy, can cut through a selection of various products and also their cutting rates are rather good. The approach has actually been coupled with laser technology by some device developers. Water jets were taken into consideration exotic at once, however are discovering more uses in a vast array of industries. Lasers are seeing raised usage in flex PCB manufacturing, particularly in exploration microvias. With lasers one need to beware in the option and also usage. As an example, under a lot of scenarios, CO2 laser energy is mirrored by copper. Therefore, if copper is to be cut, a prospective issue exists. There are methods to conquer the trouble, but they need some non-standard prep work actions. On the other hand, CO2 lasers can attain extremely accurate cuts about the artwork when the copper is made use of as a conformal mask. Devices providers could offer referrals on the use of their items.

The following area of interest is in the stack-up of flex PCB products made use of in the lamination procedure. Many different products have been utilized in the lamination process over the years. As a whole, these have actually been included the following:

– Thermal lagging material to regulate or regulate the price of temperature level increase in the laminate stack. flex PCB Materials that have actually been utilized for this purpose include construction/ craft paper as well as silicone rubber pads.
– Conformal material utilized to make certain that the coverlayer adhesive consistently fills the rooms in between the circuits and also preclude the capacity for air entrapment. It likewise lowers extreme “squeeze-out” of glue right into pad areas as well as efficiently obstructs adhesive or material circulation right into other aspects such as solder lands. flex PCB Materials that have been used for this function include fluoropolymer sheet, poly ethylene movie, silicone rubber pads and also fluid crystal polymers. This collection includes particularly created materials. the products do not bond together completely. flex PCB Materials that have been made use of include fluoropolymer movies such as fluorinated ethylene propylene (FEP) or polytetrafluoroethylene (PTFE). DuPont’s TEDLAR ® product is a commonly made use of material.

There are likewise a variety of interesting specialty materials on the marketplace that supply the residential properties of the lamination materials described into a single product. Some of these products are developed to be utilized when as process consumables. One flex PCB product in broad use was developed by Pacothane. It is a solitary layer hydraulic conformable film roughly 125µm thick with launch attributes from both sides.

A lot more lately, flex PCB materials have actually been designed for several usages. One interesting, and relatively new material of this class includes a main core of glass product covered with a relatively thick coating of heat silicone rubber with a fluoropolymer film bonded to both outer surface areas.

The launch surface facing the flex PCB circuit comes to be altered as it complies with the circuit in lamination. Nonetheless, in the following lamination, the material is adored to ensure that the altered side is facing the flat separator plate where it is gone back to flatness while the opposing side is misshaped. According to the flex PCB manufacturer, this enables the material to be made use of hundreds of times, therefore removing a lot of the waste connected with the lamination process. This is an eye-catching attribute as we transfer to develop even more environmentally-friendly flex PCB manufacturing techniques. While there are some restrictions on this product, it is possible, relying on the products made use of and also their order in the lay-up process, to stack as well as laminate flooring a number of flex PCB circuits between carrier plates by utilizing separator plates between the panels. In some cases, the separator layered may even be omitted.

Coverlayers and covercoats are important topics as well as, because of their many ranges as well as facets, demanding of additional discussion. Flexible circuits usually call for a coating of some style over the finished circuits. At the same time, openings are nearly constantly needed to supply access to flex PCB features for interconnection to parts or ports. The finish is described as being a covercoat, coverlayer or cover film. In all cases, a typical feature of the finished protective layer is that it is flexible.

The varying terms, utilized to define products that serve a typical feature, gives away the fact that the items are likely to be different not just in name but additionally in process, which is exactly the instance. What is not clear from their names is that these items normally offer various functions in regards to the application. Some description of the modern technologies and their rationales should aid make this a little bit clearer.

COVERCOATS

Covercoats are typically liquid finishings, usually applied by screen printing. Nonetheless, various other techniques could likewise be made use of. Depending upon the sort of covercoat utilized, the display printing process can include a pattern that leaves the Flexible Circuits features open if it is a UV or thermal cure product, or it could completely cover all functions for later direct exposure as well as growth if it is a photo-imageable product. The process can be really cost-efficient with the former instance, and is best-suited to make use of with coarser features.

These products are frequently made use of in applications where the assemblies do not see much repeat flexing; nevertheless, depending upon the polymer’s specific buildings based on its formula, some covercoats can additionally serve well in some vibrant applications.

COVERLAYERS

The term coverlayer has traditionally been reserved for polymer films that have a sticky layer on one side that is made use of to bond to the flex PCB. In process, the coverlayer is generally machined, most often by boring, routing or punching, to give openings for the circuit functions. After offering the openings, the coverlayer is straightened as well as laminated to the flex PCB making use of warm and stress.

While it is a lot more lengthy to create and also much more pricey than display printing, laminated coverlayers have actually been the main option for flex PCB circuits calling for high cycle life as well as vibrant flexing. This is because the buildings of the base movie and also its thickness can be made identical by placing the flexible circuits in the desired neutral axis of the bend span.

COVER FILMS

Cover movies are most much like dry movie solder mask in kind as well as use because they are laminated, subjected and created to access flex PCB features. Cover films have actually appreciated raising popularity for their capacity to define opening and accessibility very great circuit attributes. While cover films are not ideal for all vibrant applications, they can be rather functional in numerous. it offers a short summary of the various processes.

There are, obviously, variants on processing that can extend one technology over into other aspects. For example, lasers can be used with coverlayers to gain access to penalty attributes as well as chemical or plasma etching of polymer movies can be utilized to accessibility attributes when integrated with certain various other processing techniques such as using indispensable steel masks.