Wire Bonding
One of microelectronic / IC production is Wire bonding. Wire bonding is a method to make electrical interconnection utilizing small size wire and with several parameter combinations such as pressure, heat, and additionally ultrasonic wave. This process is categorized as welding process with solid phase, where two materials (pad surface and wire) are brought into close connection. When the surfaces and wire are in close connection, interdiffusion or electron sharing happens, generating the wirebond connection. In wirebonding step, bonding force may prompt deformation on utilized material, making rough surface become more smooth, and separating defilement layer, which can be upgraded by ultrasonic wave usage. Bond shaping and interatomic diffusion can be accelerated by heat factor too.
More than 4 Tera wires are bonded in single year and most of them are utilized in around 50 billion ICs (incorporated circuits) created based on current literature. Different kinds of bonding techniques like Flip-Chip or Tape Automated Bonding (TAB) or Controlled Collapsed Chip Connection have been utilized along with former wire bonding technique. Wire bonding keeps on being famous and predominant in the electronic business especially in bonding technology.
Previously, a huge proportion of semiconductor-system field disappointments were brought along with the quantity of known disappointment instrument and wire bonds were limited. By the time pass, bonding techniques improves and wire bond quality is increasing, , knowing of failure cases, however they keep on plagueing new assembling lines.
Here are two common wire bonding types. They are Ball and Wedge bonds. Ongoing research shows that about 10% are created with wedge bonds, 90% of electronic bundle and congregations are delivered utilizing ball bonds. People hope and believe that an increment in ball bonding utilization with semiconductor components will improve its functionality or adding features and diminishing in size causing bond pads more modest and make space pad for bond become closer.
Wirebonding methods starts by joining the bottom of chip with a chip carrier utilizing natural conductive glue or solder. The wires at that point are welded utilizing certain bonding instrument (wedge of capillary). Table appeared beneath show the attributes of most used bonding methods.
| Wirebonding | Temperature (oC) | Pressure | Wire | Ultrasonic energy |
| Thermocompression | 300 – 500 | High | Au | No |
| Ultrasonic | 25 | Low | Au, Al | Yes |
| Thermosonic | 100 – 150 | Low | Au | Yes |
Thermocompression Bonding Method
Semiconductor chips should be connected electrically to the other components or system to work. This electrical connection is accomplished utilizing small metal wires that are actually bonded to chip created from silicon toward one side and the other side is connected to electrical connector. Creating bond of a semiconductor chip surface and wire is not a simple work. The wire should make a decent electrical interconnection that won’t decay with time. The most recent methods for bonding wires to economically delivered silicon semiconductor system included small soldering balls that generally bound the chip surface to the wire. The solder bonds demonstrated hard to make reliably. In some case the joint was poor in mechanical bond point of view. On the other case the solder bonds would create a diode intersection with materials, semiconductor for specific, and this condition decrease the semiconductor performance.
Thermocompression bonding, the development viable, includes heating process on the semiconductor surface until around 200 – 300◦C and afterward basically pressing the wire and resulting bond between the semiconductor chip surface and wire at the suitable spot with a pressing force around 5,000 – 10,000 pound/square in. In no time magnificent physical bond and electrical properties is generated. Thermocompression bonder is machine that can be used for this process.
Thermocompression bonding was created by three researchers from Bell Laboratories: H. Christensen, O. L. Anderson, and P. Andreatch. The great achievement of this group was happened in the 1955–1958. The group published their reseach and the need was extraordinary to the point that numerous semiconductor producers constructed their own
thermocompression bonders around 1957 – 1959.
The first commercial bonder based on thermocompression was bided by Soffa and Kulicke in the end of 1959. This product got high demand on it introduction year, net deals surpassed $l million. K&S monitored thermocompression bonding in production line that was working at Western Electric semiconductor factory or production plan on 1958. K&S engineers made small mechanical adjustments toward Western Electric thermocompression bonder prior to commercializing it.
Ultrasonic Bonding Method
Ultrasonic bonding of metals includes polishing together between two metals to be bonded with huge energy that surface pollutant on both bonded metal surfaces are cleaned away and the fundamental metal molecules brought to sufficiently close to create a well bond. On ultrasonic bonding method, the energy for polishing both metals is given by an instrument that vibrates at certain frequency which ultrasonic work.
The benefit of utilizing ultrasonic bonding is without glues or consumable item – like thread or solder. The bond or weld is created rapidly – generally in range 2 and 4 seconds – without giving heat treatment on the surface. This method produces super solid, repeatably precise bonds and welds. Ultrasonic welding is harmless to the environment, affordable to utilize, and creates no waste. car, Attire, gadgets, appliance, filtration, home decorations, packaging, and clinical device are example industries that may use ultrasonic welding.
Ultrasonic bonding as common bonding method was found during 1950s. Its first utilization to bond semiconductor chips and wires happened in the middle l960s, as per interviewees at Sonobond Company, Pennsylvania, a provider of ultrasonic welding machine. Who was in reality first to build up a semiconductor ultrasonic bonder for fabricating usage isn’t clear. It shows up that Sonobond at first provided ultrasonic transducers —ultrasonic energy generators which is required for welding process — to Motorola and Fairchild who at that point made design the former ultrasonic bonding machine in-house.
Ultrasonic bonding machine for making bond between semiconductor chips and wires was introduced during the 1960s by Sonobond. The company at first only gave the ultrasonic energy source to semiconductor makers yet later sold a whole ultrasonic bonding machine for semiconductor chips and wires.
Thermosonic Bonding Method
Thermosonic bonding is the common wire bonding process that is utilized to interface silicon IC electrically. The founder of Thermosonic Bonding, Alexander Coucoulas , introduced and made it during 1966. Because of the Thermosonic bond dependabillity, it is broadly used to bond all key connection in Central Processing Unit (CPU) that are embodied IC which became pillar and “cerebrums” of the PC.
Thermosonic Bonding is performed utilizing mechanical forece, ultrasonic power and heat to making bond between either an alumunium (Al) or gold (Au) surface on a substrate and a gold (Au) wire. Heat is used by spotting the package above special stage which is heated. Some bonders likewise have heated part, that can be used to improve the package located on heated stage. Force is utilized by squeezing bonding tool to wire push it and contact directly with the surface. Ultrasonic energy is utilized by pulsating the bonding wool while wire and bonding wool are physically contacting. Thermosonic method is ordinarily utilized for gold strip/wire.
Prior wire bonding techniques were Thermocompression bonding, which utilized mechanical force and heat and then ultrasonic bonding, which utilized pressure and vibration energy. Thermosonic bonding enhanced the dependability of the previous methods by giving preheat treatment to the lead wire then metallized chip in advance of ultrasonic cycle. Impact of using this method, makers can annihilate expensive silicon chip usage and reduce the occurrences of cracking. The enhancement resulted as an effect of pre heat treatment to the lead-wire make it more mild which encouraged its deformation in generating the necessary contact surface during ultrasonic bonding cycle. With this conditions, the beginning of hot working or recrystallization of deforming wire will in general happen while it is shaping the necessary contact area. Because of hot working while the holding cycle is applied, the bond wire might be in a stable and moderately delicate state. In case the wire was deformed by ultrasonic in room temperature environment, it tend to cold working (strain hardened) and thusly will give mechanical stress load that may damage the silicon chip. At first, Hot Work Ultrasonic Bonding or Thermosonic bonding, was used on making aluminum and copper wires bonding to palladium and tantalum film on glass and aluminum oxide substance which imitated the metallized chip.
Wire bond forms
There are two common wire bond types: wedge bond and ball bond, the relating to bonding method, bonding materials and equipment are shown in table below. Nowadays, thermosonic gold ball holding becomes the most broadly utilized bonding method, basically it is caused by this method is quicker than ultrasonic aluminum bonding. When the ball bond is created on the component, the wire perhaps moved toward any path without bring mechanical stress on wire, which significantly encourages wire bonding automatically, as the motion just need x and y axis movement.
Types of Bonding based on used material
There are several materials that is commonly used for bonding application such as gold, copper and alumunium. We will describe in more detail about each material on description below.
Gold Wire Bonding
Gold wire bonding is bonding process that gold wire is connected to two nodes in one assembly to make an electrical conductive way or connection. Ultrasonics, mechanical force and heat are completely utilized to build the connection for gold wire.
The process toward making the connection point starts with the a gold ball arrangement on the capillary, the wire bond equipment tip. This ball is pushed on the assembly surface which is heated while applying ultrasonic movement with frequency around 60kHz – 152kHz with the equipment and application-explicit mechanical force.
When the first bond has created, the wire will be controlled in a firmly controlled way to make the proper circle shape for the assembly shape. After that the subsequent bond, frequently known as the stitch, is shaped on the other area by pushing down the wire and utilizing a clip to rip the wire located on the bond.
Moreover, Gold wire bonding have interconnection technique inside packages which is profoundly electrical conductive, almost a significant degree more prominent than certain solders. Furthermore, gold wires have unique characteristics like having high oxidation resilience in contrast with other materials and more mild among the other, that is important for delicate susceptible surfaces.
The cycle can likewise differ and it is depend on the assembly requirements. With susceptible materials, utilized gold ball shall be set on the outer or second bonding region to make a “milder” attachment and more solid bond to avoid harm on the component surface. With restricted area, a gold ball can be utilized as a beginning node for two bonds, shaping a “V” bond. If a stronger wire bond is needed, a gold ball can be put on top line to shape a security bond, enhancing the wire strength and stability.
Copper Wire Bonding
| Basically, copper wire bonding method is very much like gold wire bonding because it fundamentally utilizes a similar wire bonder tool with minor equipment and software. Rather than gold wire, it is supplanted by copper wire, however not restricted; the range of wire diameter is normally from 15µm until 50µm relying on package device application. The Copper wire bonding has more benefits prior to gold – unrivaled item in heat conductivity and electrical terms; better dependability because of more slow intermetallic enhancement that may cause voids; and higher break mechanical load on wire pull testing. One of contra reason utilizing copper wire for wire bonding method is oxidation issue that may affect the integrity and dependability of the component inside the electronic enclosure. As we know, oxidation postpone deformed ball welding into bond pad, and it stitch to the substrate or lead frame. Today, this issue has been override because of the immense improvement in technology of wire bonding and various materials processing (for example lead frame, copper wire, or substrate, component metallization, and so on… ) to supplement the copper wire utilization. The use of creating gas (a combination of 5% Hydrogen and 95% Nitrogen)- for an oxidation free cycle during the development of copper FAB or free-air-ball.All wire bonders utilized for copper wire bonding cycle are furnished with copper pack, including electronic fire off unit or EFO with arrangement to guarantee optimum progression of forming gas.Coated Copper Palladium material (Pd covered Cu) wire is option to decision retard oxidation.Software upgrades integrated in copper wire bonders new generation to increase ball bondability performance with small aluminum sprinkle out and programmable sectioned stitch attribute.Special kind of capillary finishing surface with granulated projection for better grasping and to decrease short tail corelated stitch bondability issues. Copper is quickly getting a traction as an connection material for semiconductor packaging process due to its benefits over gold. These benefits include: less intermetallic developments.cost decrease of up to 90%.more mechanical stability. more noteworthy dependability of the bond at raised temperaturesunrivaled heat and electrical conductivity Copper is fundamentally 3 to multiple times less expensive than gold, so changing gold wires into copper wires can extremely reduce enormous yearly expense for company who focused on semiconductor packaging. Copper wire, that has 0.017 micro ohm per m electrical resistivity on room temperature, is more conductive for around 25%-30% than gold wire, which has 0.022 micro ohm per m electrical resistivity on room temperature. Copper material which has lower resistance brings about better performance on electrical usage. Specifically, copper wire is a favored bonding wire material high-power or for high-current aims, since it can convey more current for the same wire dimension. Copper likewise has thermal conductivity about 25% higher than gold (314-318 W/m K for Au and 385-401 W / m K for Cu). Along these lines, copper wires disperse heat inside the enclosure quicker and more effective than gold wire, narrowing thermal load to which uncovered area. Exaggerate heat on wire can advance grain development, which brings down the wires strength. HAZ or Heat Affected Zone generated on wire during development of free air ball likewise have a tendency to be lower in copper wires on due to their better conductivity to thermal. The lower HAZ in copper wires offer better wire looping capacity over gold, a necessary point of die stacking application. Another benefit of copper than gold material is its lower probability to shape intermetallic compounds between aluminum. Gold wire particles have a high probability to interdiffuse with the aluminum bonding pad and generate IMC or intermetallic compounds with them. High interdiffusivity among aluminum and gold may make voids at the bond. Void presence attenuate the bond and can prompt to bond lifting just as other wirebond unwavering quality issues. Beside void development, a few intermetallic compounds shaped by Al with Au are fragile and are along these lines likely to break by stress cracking or fatigue within the thermo-mechanical load presence. |
| Aluminum Wire BondingResemble to gold wire bonding, alumunium wire bonding makes conductive connection but with the fundamental differentiation where the surface area does not need heat treatment until 150oC or more than room temperature. Ultrasonics and mechanical force play important roles in aluminum bond shaping. Aluminum wire bond creation process is equivalent to gold wire bond, that comprises of two connection and a particular loop shape. During the wire connecting process, the fundamental elements required for the bond are ultrasonics and mechanical force, and no heat treatment needed on the pad / surface area. |
Similar to gold wire, Aluminum wire have a good electrical way for making connections among the other parts however has several important benefits. Aluminum bonding admits connections to be constructed on temperature-susceptive condition which the materials will not hold back the temperatures commonly needed for gold wire bonding. Aluminum wire is likewise very much wanted over gold wire material on Aluminum surfaces in airtight sealed enclosure as the temperatures required for airtight sealing can affect the gold integrity on aluminum bonds.
Aluminum wire is generally utilized on wedge bonders, and utilized along the gold wire material depicts the two the most utilized wire bonding materials. The other material types, including several composites / alloys have been utilized in some particular bonding applications for their typical characteristics. The capability to have optimal parameters like temperature, frequency, and mechanical force for utilized material is a fundamental aspect to make well wire bonding.
Types of Bonding Based on Shape
Common bonding shape that is used nowadays are ball, wedge bonding and flip chip bonding. In recent times, people prefer using one shape to another. Then, Why the designer prefer to one method than another ? For comparing all methods, we will explain more and compare all bonding shapes in this articles.
Ball Bonding
Ball holding is the cycle where pads are bond into a silicon die and substrate or leadframe utilizing wire which has fine diameter. The essential process of ball bonding method involve the first bonding shaping (commonly above the chip), the second bond shaping (commonly on the substrate) and the wire loop.
On the start of the wire process, the bonding equipment move down until it reach first bond area. The first bond is accomplished by making bond between a pad and a spherical ball utilizing ultrasonic energy and heat treatment. The first bond is likewise alluded to ball bond. Looping movements are planned in program to meet the requirement of package for loop shape and height.
The second bond includes stitch bond which bonds the tail bond and the opposite end. The tail bond is required to create a tail of wire for the following ball shaping process. After the bonding equipment moves up to release the wire tail, after that the tail is off and the bonding equipment ascends to the ball shaping height. The ball shaping process is accomplished by ionization air gap in a cycle called EFO or electronic fire off. The ball resulted on this process is called a FAB or free air ball.
Keeping ball size is important for this cycle as it sets the general size bond and is relying on dependable bonding process to guarantee a conformable length of wire tail before each ball shaping. In case each bond is not shaped appropriately, there will be huge bump sizing variations.
Mechanical force fine application is important in the bonding cycle as it also set bump’s height and form. At last, shearing step with precise determination to split up the the ball and wire is important to make planar and conformable height bond.
Wedge Bonding
The bottom side of the capillary is used to squeezed a wire stub toward the bond pad, using ultrasonic energy to shape the bond among bond pad and wire, In wedge bonding method. After that, the capillary move toward the second bond area and the cycle is processed with repeating those process. When the second bond is finished, the wire is clipped and snapped over the second bond.
The important process in the wire bonding cycle comprise accomplishing dependable bond (including first, second, and tail bond), keeping up wanted loop, and placing the bonds precisely. Throughput is a necessary point too, as it influences the device production expense. Accomplishing wanted first bond and second bonds generally needs bonding parameters optimization. DOE or design of experiment should be used to exercise parameters optimization, that comprise effect ultrasonic energy levels, mechanical force, bonding power. An appropriate free air ball dimension regularly is resolved prior to starting the DOE first-bond. Looping directions are chosen by the application necessities. There are two common loop types and those are reverse and forward. Forward looping type firstly puts a ball bond over the die, and after that puts a stitch bond on lead frame. In the other hand, the first step in a reverse bonding type, nonetheless, is putting a bump die. Right after the bump is shaped, a ball bond is put over the substrate, then shaping the stich bond over the. Low-profile looping prerequisites have pushed the developing reverse ball bonding utilization, that is a less quick method than forward bonding method.
Application of Fine-pitch. fine-pitch wire bonding competence has been exhibited in lab at 35 µm pitch. Generally 15 µm wire is utilized with 35-µm pitch ball bonding and a bonded ball with diameter around 27 µm. Fine-pitch usage needs a higher wire bonder aptitude, such as better control of ultrasonic energy level, the bonding force, and also fine wires looping ability, that is more disposed to loop influence and weaker. A wire bonder which fulfills the fine pitch needs ought to likewise having precise movement and submicron accuracy on vision system.
Applications of Stacked Die. Stacked pass on implementations are one of the quickest developing patterns in the semiconductor business. The urge for lighter, more intelligent, and smaller gadgets encourage this 3-D packaging research and development. Stacked die usage present variance of wire bonding difficulties, including multi level and low loop wire bonding loop free space needed, loop resistance from wire clear during molding process , and bonding to hang unsupported die verge.
Most wire bonding implementation utilize the ordinary forward bonding method, since it is quicker and more adequate for finer pitch compare to reverse bonding method. Even though, forward ball bonding method has a restriction of loop height because of the neck region over the ball. Exaggerated bending over the ball can cause crack on the neck area, which brings about dependability issues. Reverse bonding can accomplish loop height smaller than 75 µm.
Flip-Chip Bonding
Flip chip bonding is a important innovation for cutting edge microelectronic circuits packaging. It permits connection of bare chip to a substrate for packaging in a face-down arrangement, with electrical associations among the substrate and chip through conductive bumps. Flip chip assembly has numerous benefits. A primer benefit is enhancing electrical performance. flip chip little bumps connection give short electrical ways, that yield incredible electrical properties which have low resistance ,capacitance, and inductance. This brings about extraordinarily improved performance in high frequency working when compared with other bonding techniques, for example, chip wire bond over substrate.
Another key benefit of flip chip assembly method is compactness of package which decrease weight and size contrasted with traditional wire bond method. The electrical interconnection between substrate surface and pads over chip can be spread out as a zone array, as opposed to around the chip that is a particular design for wire bond arrangement. This two dimensional structure can reduce chip footprint over substrate and reduce chip space. The small physical area and low profile of flip chip construction permit small electronic package size to be fabricated. Nowadays, you can find flip chip components in number hand held gadgets, PDAs, electronic coordinators, electronic watches, cameras, and any other products.
How to Choose Proper Bonding Shape for Certain Aim?
When we reach application step, we often deal with question which method that appropriate with our need: wedge bond, flip-chip bond , and ball bond? For which reason would an engineer pick a ball bonder and not a wedge bonder or the other way around? This question come to most the engineers, as a rule, electrical attributes of the package are influenced by the wire bonding technique. Even, there are situations where particular packages have physical restriction like temperature restriction (no heat or low heat usage), avoid gold material and choose aluminum, prefer using ribbon type to wire type and fine pitch usage. This is the case where the appropriate selection of wire bond method becomes play an important role.
Commonly, ball bonding implementation are related to thermosonic and thermocompression bonding techniques. Thermocompression uses temperature from around 150oC and mechanical pressure to make intermetallic bonding. And thermosonic adds energy from ultrasonic from the past step. With the two techniques, in any case, sparkle from an EFO or electronic fire off under the capillary create a free air ball prior to bonding shaping. This free air ball at that point deform when the capillary directly contact with the bond pad surface and applies ultrasonics and mechanical force for several time to change ball shape. Hence the interdiffusion between the bond pad and the wire metallization happens, that creates the intermetallic bond.
Until this day, over 90% of all wire bonds method in electrical packaging use gold ball bonding technique. It is caused by quick process to make ball bonding rather than wedge bonding technique. Ball bonding needs just three movement axis (X Y Z), in the other hand wedge bonding needs four movement axis (X Y Z θ).Just gold or Au wire might be utilized in the ball bonding technique in contrast aluminum (Al) and gold wires are utilized ordinarily in wedge bonding technique. This happen because aluminum wire will oxidize throughout the EFO or electronic fire off process to shape the ball. High-volume Cu or copper wire ball bonding technique is still on research phase. To prevent copper wire being oxidized throughout the ball shaping, the EFO process is implemented into the inert gas. Table below show brief comparison between wedge bonding and ball bonding.
| Applications | Wedge Bonding | Ball Bonding |
| Bonding Techniques | Thermosonic, Ultrasonic | Thermocompression, Thermosonic |
| Temperature | Au wire: T/S 120o – 200oCAl wire : U/S room temperature | T/S : 120o – 200oCT/C : 300o C |
| Wire size | Any size ribbon or wire | Small or < 75 µm |
| Pad size | Smaller size ofpad than ball bond. High performance in microwave usage. Pad size is around 2 or 3 times wire diamieter | Around 3 or 5 times of wire diameter |
| Pad material | Al, Au | Al, Au |
| Wire material | Al, Au | Au |
| Speed | 4 wires/sec | Up to or more than 12 wires/sec |
In spite of the fact that wedge bonding technique need more time than ball holding application, wedge bonding has the other numerous benefits, for instance, fine pitch, short and low loops, and profound access. That is the reason wedge bonding is being utilized widely in optoelectronics and microwave implementation.
Typically, ball bonding method is quicker for around 5 until more than 12 wires each second. Sorts of wire material utilized for this method such as coated palladium, copper, and gold wires. Common application and package for this technique are QFP, BGA, SOP, wafer level bumping, and hybrid MCM. The ball bonding technique is proper for fine pitch implementation on 40 micrometer or less.
The ball lacking on the primary bond gives wedge holding a benefit for a lot better pitch utilizations of 40 micrometer or less than it. Wire from aluminum is the most popular wire utilized for this cycle, trailed by gold wire. Run of the mill bundles and applications incorporate high power, optoelectronic bundling, RF microwave, BGA, QFP, SOP, MCM half and halves and temperature-touchy implementation. Wedge bonding speeds normally ranging from 3 until 6 wires each second.
Bonding process step by step
- Cleaning is the important process required before doing wire bond process. The metallization should be exempt from inorganic and organic pollutant. For instance, residual oil on the bonding surface area will decrease the dependability of the connection. There are two well known cleaning techniques, the one is bright or Ultraviolet ozone cleaning and another is plasma cleaning and. Ultraviolet ozone cleaning produces a lot of radiation (having wavelength in 2537A and 1848A) to eliminate organic contaminants. Plasma cleaning is powerful for eliminating epoxy bleed out, that is created from outgassing.
- Setting the appropriate temperature for ultrasonic, thermocompression, and thermosonic methods are important to assure wire bonding become conformable. Thermo sonic bonding process should be done on temperature ranging from 100oC to 150oC. Ultrasonic bonding might be done on surrounding temperature or around 25oC or. In the other hand, Thermocompression bonding should be set around 300oC and 500oC.
- Setting the appropriate mechanical force for the ultrasonic, thermocompression, and thermosonic methods and gives the appropriate amount of pressure required to make solid wire bonds. Thermosonic holding needs between 0.5 and 2.5g force for each wirebond. And similar to thermosonic, ultrasonic bonding need 0.5- 2.5g mechanical force for each wire bond. Lastly, 15-25g mechanical force needed by thermocompression bonding for each wire bond.
- Setting the appropriate mechanical force is important for the ultrasonic and thermosonic bonding techniques. This is needed to guarantee bond quality, increment the force setting without over-stressing or applying on the wire. You shall know over-stressing is occurring when the mechanical pull testing tool shows a low break.
- Ensure the unit is appropriately cinched inside the work holder, because it is important that ensure no movement might happens. You can check this by prodding the item using tweezers. In case movement is happening, the unit should be safe while high speed bonding process.
- Ensure the capillary is in good condition and works fine. Several factors, for example, bonding pad pitch, bond size, harness type, metallization, and wire diameter can affect bonding characteristics. The appropriate instrument selection is important for creating conformable wire bonding.
Wire bond Design Tips
Elude chip-to-chip interconnection – Unless performance need it, wire bonding straightforwardly between Integrated Circuit ought to be eluded. Making stitch bond will spread mechanical energy into pad surface, that may prompt cracking under, or in, the pad metallization. Crack refer to potential dependability issue; thus, intermediate bonding pads ought to be designed on the substrate.
Try not to cross the wires – Bond wires ought not crossing over between wire, bond pads, or other die. In condition that mechanical stress from external source is applied, the wire bond unsupported loop could hang and meet / touch a wire straightforwardly under it, prompting a short circuit that may damage whole system.
Keep in mind: bond pads is important – Bond pad ought to be arranged to make the the most concise possible wire bond. The wire bond length specifies connection capacitance, inductance, and total impedance of the. Long wire bonds might be bad to the package performance. Utilizing aluminum or gold wedge bonding with flash gold with is a special case. Even, it will be good to check the implementation notes for the integrated circuit package, since some integrated circuit producers do not use wedge bonding because of the mechanical force utilized to die on the production process. At least 0.005 mm is needed between a via and the verge of a bond pad. Created bond that is located near with substrate discontinuity may prompt to harming material caused by spreading mechanical energy to the substrate via bonding. If multilayer substrates wire bonding, used pad ought to be at least 10 mm from the conductor edge to enable wire bonding tolerance, registration, and printing.
How to Select Wire for Bonding
The wire diameter selection upon maximum current limit, cost, and the wire bond pitch. Gold wire with 1-mil diameter is a typically used by designer, and has 1.17 mω electrical resistance per mil, and a maximum current limit around 0.7 A, relying upon heatsinking, wire length, and so forth Common inductance value for 1-mil wire bond is around 25 pico H per mil, however it fluctuates relying on height of bond wire. Aluminum wire is utilized exclusively in wedge-bonding implementation, as the high affinity of Aluminum to oxidize needs ball bonding with Aluminum wires to be applied in an inert environment.
Placement of Bonding
In the design process of IC package, it is critical to determine bond arrangement relative with different parts / components — a determination that will make smaller package dimension and increase layout densities. If wire bonds place the high components / parts, holding device need free space (X) and resilience for both die arrangement and bond precision should be thought of. On a stitch bond, the real bond surface is shift from centerline of capillary. Subsequently, clearance should incorporate extra tolerance, equivalent to a half of tip diameter of capillary to guarantee appropriate clearance. Therefore, Y ought to be 0.005 in. > X for tip capillary with 0.01-in. dimension. Another configuration is to utilize wedge bonding, where the bonding equipment has a vertical face. Tragically, wedge holding is more slow—and consequently more expensive—than ball-line holding.
Spacing and Pad Sizing
Table beneath gives rule of thumb to design substrate pad and determining size of die pad. The dimension relate to wedge and ball bonding and to ceramic substrate and PCB. Exemptions are noted as relevant.
| Description of Spacing | Typical values :0.0007-in wire (in) | Typical values :0.001-in wire (in) |
| Minimum wire length | 0.03 | 0.040 |
| Maximum wire length | 0.075 | 0.100 |
| Loop height clearance over die | 0.015 | 0.015 |
| Minimum available ball on die pad | 0.003 | 0.003 |
| Maximum available wedge on die pad | 0.002 | 0.002 |
| 1 bond substrate pad | 0.010 x 0.008 | 0.010 x 0.008 |
| 2 bonds substrate pad | 0.010 x 0.010 | 0.010 x 0.010 |
| Die | 0.003 | 0.003 |
Evaluating the Wirebond
Wire-bonded parts acceptability and wire bond strength might be assessed utilizing either a DPT / Destructive Pull Test or NDPT / Non-Destructive Pull Test. The most used standard are MIL-STD-883, specifically on Method 2011.7 about Bond Strength Method 2023.5. These standard portray sizes of sample for each test type and acceptance standards for various bonding and wire types. A few assessment test that is recorded in this standard such as:
• Internal visual
• Nondestructive bond pull test
• Destructive bond pull test
• Mechanical shock
• Ball bonding shear test
• Stabilization bake
• Constant acceleration
• Moisture resistance
• Random vibration
