Recently, a new mobile phone bought by a friend has been used for less than 2 months and has frequent crashes, black screen and other problems, because it is still under warranty and sent to the mobile phone store to confirm what the problem is.
After disassembling and confirming, it is caused by motherboard problems. So how can the mobile phone motherboard cause frequent crashes and black screens? Today I will take you to a preliminary understanding of some PCB related knowledge: how is the motherboard of the PCB conductive (metal can be conducted, non-superconducting material materials, and no electrolyte)?
With the rapid development of science and technology, more and more electronic products have entered our lives, such as smart phones, laptops, TVs, new energy vehicles and, more recently, artificial intelligence robots. In every electronic product, its operation and processing center is connected to a motherboard, we call it PCB, English full name printed circuit board, that is, printed circuit board. When we use electronic products, the temperature of the general use environment is -20 ° C to 50 ° C, and over time, the use of products will appear such as: lag, slow transmission, crash, black screen and other problems. The field introduced to you today is caused by problems with non-electronic components and chips, which mainly brings quality hidden dangers to the quality of PCBs in use.
As shown in the figure below, the situation after micro-sectioning for a mobile phone motherboard: each layer is connected by metal copper as conduction, so that the signal is transmitted in the middle, the upper two pictures are the schematic diagram, and the lower two pictures are the actual slices.
However, once there is a problem with metal copper in the conduction connection, it will affect the transmission speed and may appear in the use of electronic products and other problems, as shown in the figure below is an abnormal slice in the metal copper conduction connection: we can clearly see that there is a clear crack between the upper layer of metal copper and the lower layer of metal copper, which is the problem.
How to effectively use copper metal to connect each layer of the PCB motherboard, the main process flow is drilling, removing rubber slag, and metallized copper plating three main steps.
1.1 Before introducing the content of removing rubber slag, we need to understand the situation of PCB during and after drilling: drilling is divided into mechanical drilling and laser drilling. At present, the minimum hole diameter of mechanical drilling is 0.15mm, the maximum hole diameter is 6.3mm, and reaming drilling will generally be used after exceeding 6.3mm. The figure below is a schematic diagram of general mechanical drilling:
The so-called mechanical drilling, as the name implies: the use of mechanical force (high-speed rotation of the machine) to drive the spindle, the drilling needle through the layers of the PCB motherboard, so that each layer to produce a connected channel (but at this time only the channel is generated, because it is not metallized, so it cannot be conducted). In the process of high-speed rotation and cutting of the drill needle, the instantaneous friction temperature between the drill needle and the motherboard exceeds 250 °C (as shown in the middle figure below), and the resin in the non-conductive material in the motherboard will soften and coat the hole wall (metal layer) to form a glue paste, which will hinder the quality of the electrical interconnection.
1.2. In addition to mechanical drilling, there is another drilling method in the PCB process: Laser micro-via. Laser blind hole method from ancient times (more than 20 years ago) to open large window, to conformal mask and now LDD (laser direct drill). This type of drilling method is mostly used on smartphones, iPAD, and notebook computers. As shown in the following figure:
In the figure, before making the laser, the Laser aperture is etched out using the outer window through pattern transfer, and then the laser is carried out. Advantages: suitable for thick copper products (general LDD can be applied to face copper ≤12um), and the aperture can generally be made as large as 0.15mm ~ 0.25mm, the size of the aperture is related to the AP aperture and the number of shots selected by the Laser machine.
Disadvantages: high cost (increased window opening, etching process), poor quality (1. After the edge position of the opening window and the filling position of the hole are missing from the bottom copper, it is easy to separate in the subsequent reliability; 2. Poor alignment with the bottom PAD, but it can be solved by burning target alignment). All in all, the disadvantages outweigh the benefits, so the LDD process has been created over time.
1.3. The generation of LDD (laser direct drill) greatly solves the problem of laser cost and quality, and the alignment is also significantly improved. However, after direct laser, rubber residue will still form on the hole wall and bottom pad, which needs to be removed. As shown in the following schematic diagram:
Generally, LDD can be applied to face copper ≤12um, laser aperture between 75um-150um, and the thickness of the PP dielectric layer according to the aspect ratio (Aspect Ratio) in the case of ensuring blind hole reliability: not more than 0.7:1. In addition, laser blind hole products will be subjected to plasma removal (Plasma) before chemical degumming.
1.4. Whether it is mechanical drilling or laser drilling, it will produce rubber residue to affect electrical conduction, so it must be completely removed before metallization to ensure the quality of the interconnection.
From the stacking in the above four figures, it can be seen that the hole pattern after CO2 laser (the rightmost group of figures), the copper surface is smaller than the mesolayer position of the window below, which is called over hang. The following is the slide plot after the actual LDD (red arrow position):
The hanging copper is too long (generally controlled within 12.5um) or the aspect ratio is too high, and when filling holes, it is easy to produce abnormal core abnormalities due to the exchange of medicine (of course, it is also related to blind hole aspect ratio and hole filling parameters). As shown in the figure below, the actual measurement of suspended copper exceeds 18um, and the resulting core abnormality:
Therefore, before blind hole Desmear (chemical degumming), its pretreatment process is generally: LDD laser→ Plasma→ debrowning or deblackening (also called copper reduction, mainly over hang) → Laser AOI (check the quality of blind holes, according to the actual situation of the factory or process). After completing the above process process, blind hole products can enter the formal degumming process together with through-hole products.
2.1 For the degumming process (Desmear), the process flow is consistent regardless of whether the gantry line or the horizontal line is used in the industry, but the equipment is different. Before all kinds of PCB boards enter the degumming, they need to be bulked. The schematic diagram is as follows:
Bulking process: each through hole and blind hole in the expansion tank through 75 °C above the tank liquid, after 1-10min soaking treatment, which is soluble in water organic solvent (leavening liquid main agent) to strengthen the alkali, forcing all kinds of rubber residue to expand and relax. The main components of the leavening agent are NaOH and the main agent of leavening liquid (its composition is the trade secret of each pharmaceutical supplier), but it can be inferred that the reaction principle is: under the action of NaOH strong base, the leavening liquid cracks the molecular structure of the epoxy resin long chain. The expansion status under each time period is shown below:
An unusual problem that can occur in the bulking process: Pocket void. Possible causes: 1) excessive deflection in the drilling process; 2) excessive penetration of leavening agent; 3) Poor coupling treatment of glass fiber cloth will cause leavening agent to penetrate into the plate. As shown in the following figure:
2.2 PCB enters the deglue removal tank after the bulking process, and the tank liquid is mainly NaOH, KMnO4 or NaMnO4. The chemical reaction principle is that the Mn ion with +7 valence can cut off the local polymer in the epoxy resin under the action of strong alkali environment and high temperature (85 °C), and the reaction to generate CO2 and H2O can be dissolved. The following figure shows the reaction principle and reaction schematic:
The hole walls (through-holes and blind vias) of the PCB after the degluing tank will expose the clean glass fiber and honeycomb resin surface. However, due to the different Tg of each plate (Normal Tg, Middle Tg, High Tg), the appearance of the hole wall is different during SEM analysis. Generally, high Tg products, its chemical resistance is strong and not easy to bite. The degumming status in each time period is shown in the figure below:
2.3 Principle of degumming reaction: In the epoxy resin chain theme structure, the local chain polar part of the non-benzene ring part is most likely to be decomposed by oxidation of high temperature Mn+7, so as to achieve the purpose of degumming. The principle of its chemical reaction is as follows:
When the rubber residue in the pore completes oxidative dissolution under the action of Mn+7, the various decomposed substances in the pore still need to be further reduced and cleaned. The specific reaction is shown in the following figure:
In the Desmearing oxidation reaction, the purple Mn+7 itself is reduced to the green invalid Mn+6, but the current equipment process can re-oxidize it back to the effective Mn+7. The anode extremes Mn+6→Mn+7, the specific reaction is as follows:
2.4 Confirmation of the degumming effect of various plates: Due to the different plate Tg of PCB products, the surface morphology of the resin after removing the glue residue is also quite different. Tg150°C and below have a large weight loss and are prone to honeycomb (under SEM); The high Tg (> 150 °C) resin has strong chemical resistance and is brittle and not easy to bite out of the honeycomb, but this does not mean that the adhesion must be very poor, which is also related to the internal stress of the copper layer itself. The following is the comparison chart of different Tg materials after degumming:
Traditional FR4 Tg below 150°C plate, the resin after Mn+7 bite is easier to appear honeycomb, which has become the judgment standard for good adhesion of hole copper. However, LF (Lead Free Lead)/HF (Halogen Free Halogen Free) no longer meets this criterion.
The main reasons are: SiO2 (volume 25% ratio) is added to lead-free material resin, and Al(OH)3 (volume 25% ratio) is added to halogenated material resin, resulting in difficulties in each process. Its slice is as follows:
The general resin after mechanical drilling is much different from the substrate filled with Filler, and the appearance before and after removing the rubber residue is large, and the high Tg is not easy to appear honeycomb. The following is a comparison of SEM before and after removing rubber residue:
After the laser is directly formed into a hole, after confirming the appearance of the copper foil plate surface and the slice hole, the hole wall after removing the rubber residue also shows a honeycomb shape, and the specific slice diagram is as follows:
The other half of the additive process (SAP) carrier plate, the additive plate often uses ABF film, under the CO2 laser or UV laser burning blind holes, the appearance after removing the rubber residue is also very different.
This is a microscopic comparison of semi-added ABF sheet Desmear at different magnifications before and after:
For the semi-additive process (Semi-Additive Process), because it is the process of carrier board products (mainly 25um and below line width products), it is briefly introduced as shown in the following flow chart:
The following is the comparison of three types of PCB products after removing the slag: general hard board, FPC PI, and soft and hard board
At this point, the process before the metallization of PCB through-holes (mechanical drilling, laser drilling) has been completed, and the next stage officially enters the metallization process. #Looking for Digital Reviewers#
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