プリント基板の短絡解析と防止方法

Overview: A circuit board short circuit is something that all engineers do not want to see, and once it happens, it may bring a devastating blow. However, circuit boards are complex and fragile electronic components, and no matter how skilled engineers are, they may expose to a variety of potential risks that could compromise the integrity of each board. In this article, FS Tech will focus on this most troublesome and destructive common PCB issue, and provide you with several methods to detect and fix PCB shorts. Keeping these things in mind will help you make changes that will hopefully save your work.

短絡によるプリント基板焼損
Damaged PCBA board due to short circuit

Reasons and solutions for circuit board short circuit

Circuit shorts occur anywhere on the board frame and can be caused by PCB shorts, misalignment when installing components, secondary damage caused by rework, and it can also be caused by a minute particle that becomes lodged between the pins of an IC chip. The construction and installation of PCBs are vulnerable to the danger posed by short circuits, which can cause the circuitry to overheat and even destroy PCB components. Either a solder bridge connecting neighbouring pins or vias is to blame, or a badly built PCB board is to blame for the issue. For finding the cause of the short circuit, we need to conduct a PCBAの故障解析 具体的な状況に応じて Below we list some reasons for PCB functional short circuit and give corresponding solutions:

  • Soldering short circuit
  • Component short circuit
  • ESD breakdown
  • Electrochemical short circuit (chemical residue, electro-migration)
  • Short circuit caused by other reasons

Soldering short circuit

Soldering short circuit is usually caused by too dense IC pins or too small spacing between components, usually for the following reasons:

Excessive solder paste​

The electric board short circuit due to excess solder paste for the following reasons:

  • The thickness of the stencil used in solder paste printing and the size of the holes are not set properly;
  • There is a problem with the distribution of dripping solder paste due to uneven support or tilted stencil during printing;
  • Problems with プリント基板製造 causing its surface to be uneven;
  • The steel mesh tension does not meet the standard;
  • The stencil is not cleaned in time or not cleaned after use.
 

Optimization:

  • Carefully review the customer’s component list, and select steel meshes of different thicknesses according to the density of component distribution and whether there are small components;
  • Adjust the printing speed, pressure and other parameters of the SMT equipment according to the CPK value of the solder paste thickness;
  • Regularly check and clean the stencil.

PCB position offset

FS Technology strictly requires every process in the assembly plant. When the printing offset exceeds 1/4 pad, we judge it as printing offset. PCB position deviation during printing causes solder paste to drop to the wrong position, resulting in short circuit of the finished board. The reasons are as follows:

  • PCB fixture failure
  • Poor precision of loading machine
 

Optimization

  • Adjust or replace fixtures
  • Manual calibration of printing accuracy
  • Enhanced printer identification

Edge collapse

If you find edge collapse during printing, you need to analyze the cause in time and solve it immediately, otherwise the circuit board will be short-circuited. Causes of edge collapse include the following:

  • Solder paste exposed to the air for a long time may be polluted or oxidized, resulting in low viscosity and poor thixotropy, and it is prone to flow and sag after printing;
  • Due to the roughness of the steel mesh, the dripping solder paste is sharpened, resulting in edge collapse;
  • When the placement machine mounts SOP, QFP, QFN, CSP and other PCB部品, due to excessive pressure, the solder paste is deformed and causes edge collapse;
  • When the reflow soldering machine is preheated, the temperature rises too fast, causing the solution in the solder paste to volatilize too quickly, causing the solder particles to be squeezed out of the soldering area and collapse.
 

Optimization:

  • Prepare the solder paste before solder paste printing, and ensure that the viscosity of the solder paste is moderate;
  • Use more advanced laser cutting machines to manufacture stencils and regularly check the quality of stencils;
  • Adjust the pressure and placement height of the placement machine, and conduct a small batch test before high volume PCB assembly to ensure correctness;
  • Set the temperature of the reflow soldering machine according to the Profile parameters provided by the solder paste supplier.

PCB component short circuit

Component short circuit is a form of short circuit that users feel most deeply. Some ordinary users (without electrical knowledge) attribute the short circuit phenomenon of all electronic products to short circuit of electronic components. Electronic components are usually soldered to the circuit board by PCB assembly companies through ディップアッセンブリー, SMTアセンブリ or mixed assembly technology. Due to the fragility of this electronic device, there are many reasons for its short circuit, including component failure, assembly failure, cleaning problems, etc. The most common electrical failures of all components are: the two ends of the electrical appliances in the circuit are directly connected by wires; the power supply is directly connected by wires without any electrical appliances.

Ohm’s law: I = U / R, the supply voltage V is too large and the resistance R is too small, the circuit through the components of the current will become large, if the appliance is running for a long time, the components can not carry the flow of electricity will lead to line overheating or even burned.

ESD breakdown

Electrical Static Discharge (ESD), contact between human body or components, friction, induction between electrical appliances may produce this natural phenomenon. If the PCB is not designed to prevent ESD, the electronic products using the PCBA board will run unstable or burn out. The following are the measures provided by FS Technology to prevent ESD PCBA short circuit:

  • Using Avalanche Diodes to Prevent Electrostatic Shorting of PCBs
  • Use High Voltage Capacitors to Protect Circuits
  • Using Ferrite Beads to Reduce ESD Current and Suppress Radiation
  • Eliminate high-frequency energy by reducing the area of the loop through decoupling capacitors
  • Use CMOS devices or TTL devices to avoid circuit short circuits
  • Improve the circuit structure and use multi-layer PCB to provide protection for electronic products
  • Adding an LC filter reduces the risk of high-frequency static electricity entering the circuit
  • Spark gap method (The microstrip line layer consisting of copper skins is formed using triangular copper skins with their tips aligned with each other, with one end of the triangular copper skins connected to the signal line and the other triangular copper skins connected to ground.)

Electrochemical short circuit

Electrochemical short circuit means that after the PCB assembly is completed, due to the influence of chemical (ion) residues, the PCBA is corroded and undergoes electrochemical migration, which eventually reduces the life of electronic components or presents a difficult PCB short circuit problem. Due to the complexity of PCB assembly and manufacturing processes, there are many reasons for chemical (ionic) residues, the following are the two most common:

Highly Active Compound Residues

Due to the poor wettability of lead-free solder, the assembly factory has an excessively high failure rate during PCBアセンブリ, so the assembly factory solves this problem by increasing the activity of PCB flux. Then there are gains and losses. Improper soldering process coordination when using these highly reactive compounds will result in active material residues and short circuits in the assembled PCB as a result.

Poor cleaning process

Factories use no-clean or simple cleaning processes to save costs. Although there is no problem with these PCBs and PCBAs in the early stage of production, after a period of storage or after the customer receives the goods and puts them into finished product production, the surface of the circuit board will rot or electromigrate.

Optimization:

  • Carefully look for PCBA suppliers, check qualifications or clarify processes
  • Use solder as needed

How to Test a Circuit Board for Shorts

The results of a short circuit might range from immediately observable damage to mistakes that will not become apparent until the product is put through further testing. On the other hand, sometimes more subtle shorts might develop between the input pins or other components, and the harm caused by these shorts will not become apparent until you run the starting tests. There are many ways to test for this コモンPCB問題. The way most people use is to inspect the board by eye, but there are a lot of tricks beyond this step (requires the use of PCB shorts).

Power-on Detection Method

The easiest way to tell if a PCB is shorted is by applying power to it. Electronic products almost always have a pungent smell or smoke when a short circuit occurs. This is the same as the circuit board. The reason for this phenomenon is mainly due to the burning of components. This type of detection cannot determine which component is short-circuited and requires analysis of the schematic.

Least Resistance Verification

Electronic engineers use PCB short-circuit detection tools to detect the resistance of GND points at different positions on the board, and find the position with the smallest resistance, which is the accurate short-circuit PCB position. Although this method can accurately determine the location, it is extremely demanding and requires precise detection tools and manual review.

Fish dead net breaking method (not recommended)

The short-circuit area is determined by continuously supplying voltage to the circuit board to find the hot location. In actual operation, because the power-on time is too short, the PCB may not change in temperature difference, so it needs to be powered on until it reaches the tolerance limit of the chip. Because this method will cause damage to the board, it is not recommended to use it, unless faced with some special circumstances, that is, the test circuit board displays normal when the power is not turned on, but fails when the power is turned on. This situation is usually caused by insufficient power, indicating that the current parameter during design is smaller than the actual load.

Visual inspection

プリント基板の短絡を手動で検出
Visual inspection method to detect whether the PCB is short circuit

プリント基板上の短絡を見つけるには、まずプリント基板の表面をよく観察する必要があります。回路基板を調べる際には、拡大鏡や顕微鏡を利用できる場合は、適度な倍率で利用するとよいでしょう。電源を起点に回路をたどると、破損箇所を把握しやすくなります。はんだ接合部やパッドの周囲にヒゲがある場合は、気をつけないとショートを起こす可能性があるので、注意してください。過去にショートを起こしたことがある場合、焼け跡や焦げのような形でその痕跡が残っていることがあります。これらは、焦げ跡の色である茶色をしているため、基板の緑色の背景の中で目立ちます。

サーマル/赤外線カメラによる検査

赤外線カメラで撮影したプリント基板
Thermal Inspection of PCB Short Circuit

赤外線カメラを使ってプリント基板の短絡箇所を特定することは、すべての企業にとって手頃な方法ではありませんが、紛れもなく効果的です。回路がショートすると大量の熱が発生するため、サーモグラフィを使えば、基板のどこに問題があるのかを正確に特定することができます。電源を入れ、他の部品よりも熱くなっている部品やはんだ接合部を探せば、それが短絡している箇所である。電流を可能な限り低く設定する。短絡が早く発見されればされるほど、被害は少なくなります。

Digital Multimeter - the most common PCB short circuit finder

Detecting PCB Short Circuits Using a Digital Multimeter
Short Circuit Testing with Digital Multimeter

回路内のさまざまなポイント間の抵抗をチェックすることは、回路基板上の短絡があるかどうかを判断する1つの方法です。目視で短絡の位置や原因を特定できない場合は、マルチメータで問題が発生しているプリント基板上の物理的な位置を特定する必要があります。マルチメータを使用する戦略は、ほとんどの電気関連コミュニティで様々な評価を得ています。しかし、短絡テストの場所を追跡することで、システムの問題ではないものを特定できる可能性があります。

異なる回路にあるはずの2つの導体間で極端に低い抵抗値が検出された場合、内部または外部のブリッジの可能性があります。

グランドに対する短絡のテスト

特に、接地経路やグランドプレーンを含むショートの存在は重要です。内部グランドプレーンを含む多層プリント回路基板(PCB)には、部品近くのビアを通るリターン経路があります。このリターン経路は、基板の表層にある他のすべてのビアやパッドをチェックするのに便利な場所です。一方のプローブをグランド接続部に置き、もう一方のプローブを基板上で動かして、さまざまな導体に接触させます。

破壊試験

短絡検出のためのプリント基板の破壊
Destructive Testing of PCB

上記のテストで結果が得られない場合は、基板を物理的に分解する究極のテストを実施する必要があります。各コンポーネントを1つずつ取り外し、露出したパッドに対してマルチメーターテストを行うことで、これらが問題であるかどうかを判断することができるようになります。これは極端な方法ですので、他の方法がすべて使い果たされたときのために取っておくべきです。やがて、基板のどの部品が欠けていて、ショートを起こしているのかが分かるようになります。マルチメーターを使ったテストでは、部品のすべての問題を明らかにすることはできませんが、そうすることで、問題のある部品を取り除き、後で交換することが可能になります。

イライラさせられるかもしれませんが、回路がショートすることはよくあることです。診断ツールの助けを借りて、これらの問題をすばやく簡単に特定し、解決できるようになることを期待します。

How to prevent PCB short circuit

短絡を探し出すのに苦労したくないのであれば、ミスや設計上の欠陥、製造公差がないか、回路内テストルールを徹底的に確認するようにしましょう。PCBの短絡を避けるには、その原因を徹底的に調査する必要があります。電気的な短絡は、製造現場や工場で発生することがあります。 PCB組立工程.製造工程の最後にほんのわずかでも銅が残っていると、ヘアラインショートが発生します。リフローはんだ付け工程でショートが発生する可能性があります。 

プリント基板ではなく、部品が故障している場合もある ボード が原因です。修理するには、壊れた部品を取り出せばいい。

PCBAの場合、設計を書き換えたり、必要に応じて部品を変更したりすることが日常業務となります。FSテックのターンキーで PCBA サービスでは、PCBAのテスト、組み立て、生産に長けたパートナーと協力することで、時間とコストの両方を節約することが可能です。

短絡は最も一般的なプリント基板の故障の一つであり、唯一の故障ではありません。設計、製造、使用の各プロセスにおいて、あらゆるリンクに注意を払う必要があるのです。ここでは、不具合に関する記事をご紹介します。

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医療用PCB製造・組立

医療用PCB組立・製造 PCBAは、機械の進歩を可能にし、現在の私たちの世界になくてはならない存在となっています。でも

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SMD部品とは何ですか?

SMD部品とは プリント基板(PCB)の表面に直接部品を貼り付けたり、取り付けたりした電子機器を「SMD」といいます。

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リジッドフレックスPCBAとは

リジッドフレックスPCBAとは リジッドフレックス基板は、半剛体で半屈曲の回路構造なので、プロジェクトのコンポーネントを両部品で接続することができます。リジッドフレックス

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