The whole process of developing a mobile phone (Part 2)

Fixed structure of lower shell camera

The fixing structure of the lower shell camera is similar to that of the upper shell camera. Both use PFC or connectors to connect to the mainboard. Both need ribs for positioning and foam and lenses for sealing. The difference is that the positioning of the lower shell camera is assisted by an antenna bracket. The antenna bracket surrounds the four sides and bottom of the lower shell camera to fix the camera. In order to ensure that the center of the lower shell camera and the center of the shooting hole of the lower shell do not deviate, the lower shell positioning rib is still required. The lower shell camera lens is also blown clean of dust after the whole machine is installed, and then installed last.

Structural design of lower shell decorative parts

The lower shell decoration parts can be plastic, IML parts, hardware pieces, etc.

The surface of plastic decorative parts can be electroformed, which has the advantages of strong metal feel, high grade and good wear resistance. Plastic decorative parts are directly hot-stamped or buckled on the shell, with a thickness of 0.9~1.0mm. IML parts are to put a FILM with a silk-screen pattern in a plastic mold for injection molding. This FILM can generally be divided into three layers: substrate (usually PET), INK (ink), and wear-resistant material. After the injection molding is completed, the FILM and plastic are integrated, and the wear-resistant material is on the outermost layer. The injection molding materials are mostly PC and PMMA, which have the effects of wear resistance and scratch resistance, and the surface hardness can reach 3H. IML parts are directly hot-stamped or buckled on the shell, with a thickness of at least 1.2mm.

Nickel sheet is the most commonly used hardware decoration, with beautiful appearance, ultra-thin tweezers with a thickness of 0.1~0.15mm, which can be made without mold, and the mesh diameter can be as small as 0.3mm. If there are steps and bends, molds must be made, and the overall thickness cannot exceed 3mm, and the side draft of the step must be greater than 10 degrees.

The thickness of aluminum sheet decoration is 0.4~0.5mm. Because of its soft texture, the surface is brushed into straight lines, random lines, ripples, spiral lines, etc. Anodic treatment is also called corrosion treatment. It is an oxide film formed on the metal surface by the action of electric current. It has rich colors, beautiful colors, good electrical insulation, hardness, wear resistance, and high corrosion resistance. Sandblasting is to obtain a surface with film light decoration or a fine reflective surface to meet special design needs such as soft gloss. High light processing is post-processing, milling a beveled bright edge on the edge of the aluminum sheet, the shape is like a guide C angle.

Stainless steel decorative parts are 0.2~0.3mm thick and have a higher hardness than aluminum decorative parts. They used to have a single color, but with the development of technology, the colors have gradually become richer. Other surface treatment effects of stainless steel decorative parts mainly include brushed, high gloss (mechanical polishing), pockmarked (sandblasting), matte, etc.

Structural design of battery box

The battery is where the mobile phone stores energy. It is required that the battery be easy to load, have good contact, not fall out when turned over, and be easy to take out. The structure of the battery box must meet these requirements.

The basic material thickness of the battery box is 1.0mm. The bottom of the battery box should be as closed as possible. If there are components, avoid them. If there are no components, cover them with plastic shells as much as possible to avoid exposing large pieces of PCB.

The clearance is 0.2mm for the single side in the pick-and-place direction, with a draft of 3~5 degrees, and 0.1mm for the single side in the non-pick-and-place direction, with a draft of 1~2 degrees. The clearance between the bottom of the battery box and the battery is 0.1mm. The clearance between the bottom of the battery box and the SMD component is 0.2mm, and the clearance between the bottom of the battery box and the SIM card connector is 0.5mm.

The two ends of the battery removal direction need to be buckled to fit the battery to prevent it from loosening. The amount of buckling can be evaluated based on the rotation when simulating the battery removal. A pit slightly wider than the fingernail should be left at the fingertips to facilitate the fingers to remove the battery.

The battery box should protect the battery connector, that is, after the battery is placed in the battery box, it can only touch the shrapnel of the battery connector, but not the body of the battery connector, so as to avoid impact on the battery connector and damage during the drop test.

Motor structure design

The motor is an electronic component on the phone that produces weak mechanical vibrations. When the user sets the phone to mute, the vibration of the motor can make the user sense the incoming call. The motor shape is divided into columnar and flat. The connection methods mainly include welding wire type, spring type, SMT type and plug-in type.

Welding wire type, shrapnel type and plug-in type cylindrical motors usually have a rubber sleeve on the outside of the body. We only need to build a rib on the shell to cover the rubber sleeve. The gap between the rib and the rubber sleeve is zero. The top of the rib is reserved with a guide angle to facilitate the installation of the motor. Note that sufficient rotation space must be reserved for the eccentric wheel. The shell should avoid the rotation range of the eccentric wheel by at least 0.5mm.

The flat motor does not have a rubber sleeve. One side is pressed on the board, and the other side is directly glued to the shell with double-sided tape. A 2/3 circle of surrounding bone is enough. You may ask, why should there be a 1/3 gap in the surrounding bone? It turns out that when designing the structure, not only reliable assembly should be considered, but also easy disassembly should be considered. With a 1/3 gap, you can just pry with tweezers when disassembling the motor. By the way, the positioning surrounding bone of the induction magnet on the flip phone should leave a 1/3 gap. The same principle applies. The flat motor can also be fixed on the antenna bracket. One side is glued to the antenna bracket with double-sided tape. A 2/3 circle of surrounding bone is built around the antenna bracket. The other side is pressed with the long rubber bone of the shell. Note that the rubber bone cannot press the motor hard. There is a 0.3 gap between the rubber bone and the motor, and a layer of foam is added for buffering to protect the motor.

Structural design of the stylus

The stylus is only used on mobile phones with touch screens. It is usually fixed on the lower shell and pulled out when in use. The stylus is divided into a pen cap, a pen tube and a pen tip. The pen cap should be made into a pen hanger, and the pen hanger should be made into a dial with a grip to facilitate the pen to be pulled out. The pen tube is a hollow stainless steel tube or copper tube, which can be made into one or more sections as needed. The pen tip has a circle of grooves, and is positioned by the groove after being inserted into the lower shell.

Structural design of battery cover

The battery, SIM card and TF card should be designed to be replaceable, and they should be hidden in the battery box of the lower shell, and then a battery cover that can be easily opened and closed should be designed to protect them. The material of the battery cover can be plastic, and the surface can be attached with decorative parts such as lenses or hardware pieces, etc., or it can be formed by bending stainless steel sheets. The structure of the battery cover includes a grip, a buckle, a side buckle, and a pull point. The grip is where the force is applied when removing the battery cover. The buckle and side buckle are the structures that bite when the battery cover is inserted into the lower shell. The pull point is the locking structure that prevents the battery cover from being withdrawn after being inserted into the lower shell.

Structural design of the rope threading hole

The lanyard hole is a structure on the mobile phone used to fix the lanyard. A lightweight mobile phone can be hung on the chest with a lanyard. However, with the increase of hardware decorations on the surface of the mobile phone and the increase in battery volume caused by ultra-long standby, the weight of the mobile phone itself has gradually increased, and some customers no longer require lanyard holes. Anyway, I don’t recommend that you hang your mobile phone on your chest. In addition to the security environment here not allowing it, the insecurity of mobile phone batteries is gradually being worried by more and more people. It’s better to put it in your pocket!

The rope threading hole is usually made in the lower shell, and the pin is inserted in the gap of the antenna bracket. The position is indispensable, and the bone position of the hanging rope must be strong enough.

Data collation of quotation chart

At this point, the structural design of the mobile phone has come to an end temporarily. Let’s do one more important thing first – provide customers with a plastic mold quotation. It takes about 18 days to make a plastic mold, which is the top priority of the entire mobile phone project. Before that, it takes a few days for customers to select a mold factory. Giving the preliminary completed structural drawings to customers to negotiate with suppliers can shorten the process of the entire project. During the few days when customers negotiate with suppliers, we can optimize, review and modify the product. When customers select suppliers, our structural design is also completed, and we just enter the mold review stage with the mold supplier.

It should be noted that the plastic mold quotation drawing includes the 3D drawing of the plastic part, BOM and ID process marking. For the safety of the data, no plastic parts that do not need to be quoted can be given, and no plastic parts that need to be quoted can be missing. It is best to convert it into STP format, which is only convenient for quotation and not for other purposes. The BOM table is also only given for the plastic part to avoid wrong price. The above information is only sent to the customer, and the customer will negotiate with the supplier. Usually the design company does not intervene in the customer's business.

Structural design optimization

Well, now we can calm down and optimize our structural design. If the design plan is to become a product, there are many problems that we need to think about and prevent, such as whether the surface hardware needs to be grounded; how the motherboard and the shell match; how to increase the strength of the shell; how to improve the shell to facilitate mold making, etc.

Structural review

Every design made by a structural engineer always has problems that he cannot find, and needs to be checked by other engineers. Different positions and observation angles will lead to different judgments. If there are mistakes, they will be corrected. If there are doubts, they will be discussed with everyone, which can make the drawing level reach a higher level. During the review, another structural design engineer will first conduct a preliminary review of the drawings. The preliminary review takes about half a day to a day. The drawings need to be comprehensively checked one by one from the appearance, scheme evaluation, upper shell components, lower shell components, surface technology, mold feasibility, to production assembly, and the problem points will be recorded. Then enter the second stage---collective review. The collective review is a meeting attended by all members of the structural department and related ID designers. The problems of the preliminary review are listed, and everyone expresses their opinions and unifies their understanding. This not only ensures that the issued drawings can represent the highest level of the company, but also provides an opportunity for structural design engineers to learn and improve. In Shenzhen, the larger the design company, the more attention is paid to the role of collective review.

Verification of structural prototype

No matter how well the previous structure is done, it is still just on paper. The structural design needs to be verified with the help of the real object. The structural prototype plays the role of verifying the structure. The data for making the structural prototype includes ID process marking, BOM and 3D drawings (of course, it still needs to be converted into STP format). It takes about 4 days to make a structural prototype. The appearance and size are required to be consistent with the drawings, so that it is closer to the final prototype. When you get the structural prototype with the motherboard and battery, it is a real mobile phone. Don't be too happy too soon. Carefully check the size of the parts to see if there is any problem with the assembly of the whole machine, and whether there is any improvement that can make production more convenient and faster. Be careful now so that the mold will be smoother in the future.

Mould Review

Doesn't it take 4 days to make a structural prototype? During this time, we can send the ID process marking, BOM and 3D drawing materials to the customer. The solution company and the selected mold factory will review it. This review usually takes 1-2 days. If the customer has his own technical strength, he can check it again. The solution company can evaluate the possible problems of antenna signal reception. The mold factory will put forward some improvement suggestions for the problems that may occur in mold making, and feedback the improvement suggestions to the design company. If necessary, the customer will hold a meeting with the design company and the mold factory to discuss the mold factory's improvement suggestions. Any issues that need to be paid attention to in mold opening can be raised. The customer will make the final decision on any controversial issues between the design company and the mold factory. The discussion results will be handed over to the customer in triplicate, and the design company and the mold factory will keep them separately. The design company updates the drawings based on the discussion results and issues a formal mold opening drawing. The mold opening drawing includes the final ID process marking, BOM and 3D drawings.

Project follow-up during the casting period

It takes about 18 days to make a mold for plastic parts. Does that mean we have nothing to do during this period? Of course not. In addition to plastic parts, mobile phone suppliers also provide buttons, hardware, lenses, nickel sheets, batteries, stylus pens, auxiliary materials, etc. As long as this phone is used, nothing can be missing. Because the development cycle of these parts is shorter than that of plastic parts, we can use this period of time to make quotations, proofs and molds for plastic parts. Since the mold time for buttons and hardware is also about 14 days, some companies also have the practice of carrying out the project progress of plastic parts, buttons and hardware simultaneously. Auxiliary materials are relatively cheap and can be contracted to the mold factory for purchase. The mold factory sticks the auxiliary materials (such as foam and double-sided tape) on the designated position on the shell before shipping, which greatly simplifies the subsequent assembly process of the whole machine.

Mold trial and mold modification

Let's take plastic parts as an example. Mold trial and mold modification are the trial production of plastic parts and mold trimming after the supplier completes mold manufacturing to meet the design requirements. Before the mold trial, the customer will collect samples of other accessories. During the mold trial, the customer will go to the mold factory with the structural design engineer. The structural design engineer will conduct single product inspection and actual installation inspection on all accessories. Single product inspection includes appearance defect inspection and size inspection. The actual installation inspection is to install all accessories according to the actual assembly sequence of production to find out the problems. All problems are listed in order, and the mold factory is coordinated to determine the improvement plan, improvement time and person responsible for improvement. The customer makes the final decision on any controversial issues between the design company and the mold factory. The discussion results are signed and copied in triplicate and handed over to the customer. The design company and the mold factory keep them separately. The design company updates the mold modification drawings based on the discussion results and issues formal mold modification drawings. The mold modification drawings include detailed text descriptions of the mold modification parts and 3D drawings of the parts that need to be modified. The parts that require mold modification on the 3D drawings must be marked in red. No matter how big or small the mold modification problems are, they must not be missed.

Trial production

After the improvement, the sample needs to repeat the mold trial procedure for inspection. However, this time, the upper and lower shells and other appearance accessories of the mold can be treated with surface treatment (such as the ID process diagram indicates surface spraying, UV, oxidation, brushing, silk screen printing, electroplating, laser engraving) before installation. If the installation is confirmed to be correct, trial production can be arranged. Trial production can find problems that cannot be found when a small number of machines are installed. The number of trial production is generally 50~100 units, and the production line is assembled according to the actual production layout. During the trial production, the customer will go to the assembly plant with the structural design engineer. The structural design engineer will explain the assembly sequence and precautions. The PE engineer of the assembly plant will arrange the production line and guide the operators one by one on the correct operating methods and judgment standards. Before mass production, the PE engineer needs to complete the operation instructions for each assembly station. The assembly at the assembly plant strives to be simple. The auxiliary materials have been attached to the shell by the mold factory, and the hardware sheets have been hot-melted to the shell by the mold factory. The assembly plant only needs to install the buttons, motherboard, shell, screw, install the lens, and finally test and package.

The structural design engineer will record the problems found during production. If the mold needs to be modified, the structural design engineer will produce a mold modification drawing. The mold modification drawing includes a detailed text description of the mold modification part and a 3D drawing of the part that needs to be modified. The part that needs to be modified on the 3D drawing should be marked in red. The relevant suppliers will be notified to make improvements and follow up on the progress and results of the mold modification. After the mold modification is completed, the mass production stage can be entered.

Mass production

After many demonstrations, modifications, inspections, and revisions, the hard work of the structural design engineer is about to come out. All problems have been solved before mass production. If there are no problems, the mass production process does not require the participation of the structural design engineer. According to the current market situation, if the sales volume of a mobile phone priced at around 500 yuan exceeds 10,000, the customer can recover the cost, and if there are more orders, the customer will make a profit. After the product is launched, according to the market response, the customer may propose some modification suggestions, and the structural design engineer can respond accordingly.