The surface finish of HASL and HASL(Lead Free)


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HASL and HASL(Lead free) is one of the major surface finish in PCB fabrication industry, and it’s a kind of cheap pcb manufacturing technology compared with other surface finishes. The process consist of emerging bare printed circuit board into tin lead alloy melting pot, and then use the “air knife” to blow hot air across the PCB board surface to remove excess solder.
Hot air solder leveling has an unexpected benefit that the process will make PCB exposure to the high temperature up to 265 ℃. This means that the process can well identify any potential delamiantion problem before assembling any expensive components to printed circuit board.

Basically the advantages and disadvantages for hot air solder leveling as below,

Advantages include:


• Low cost,
• Can be widely used
• Reworkable
• long shelf life


Disadvantages include:


• Uneven surface
• Not suitable for PCBS with fine pitch
• Contain Lead (HASL)
• Thermal shock
• Solder Bridge
• Plugged or Reduced plated through holes

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Drill holes in PCB fabrication


PCB drill hole is one of the most important part for multilayer PCB, Normally drilling cost account for 30% to 40% of the total cost in PCB fabrication.

In terms of pcb design, the hole is mainly consist of two parts, one is the drill hole in the center area, the other is the pad around the drill hole. The size of these two parts determine the hole size. Obviously, in the design of a printed circuit board with high speed and high density, the designers always hope that the holes the smaller the better, so that it can leave more space for circuit layout. But the hole size decrease will bring the increase of the pcb fabrication cost, moreover, the hole size is impossible to decrease unlimitedly, because it’s limited by the process of drilling and plating. The smaller hole size, the longer drill time required, and the easier off center position. When the depth of hole is more than 6 times of hole diameter, the copper plating process won’t guarantee that the hole wall could plate copper smoothly. For example, a standard 6 layer PCB with thickness (depth of the hole) about 50 mil, so the circuit board manufacturer can provide the minimum hole diameter to 8 mil.
In terms of technology in PCB manufacturing, the holes are generally divided into three categories, which are blind vias, buried vias and through holes. Blind via located at the top layer and bottom layer surface of the printed circuit board, which used for the connection between surface layer and inner layer, the depth of the hole is usually not exceed a certain ratio. Buried hole refers to connection hole that is located in inner layer of the printed circuit board, buried hole does not extend to the surface of the circuit board. Through holes are drilled through the whole printed circuit board, which can be used for internal electrical interconnection or used as a positioning hole to assemble components. Because the through holes are easier to achieve in the pcb manufacturing process with lower cost, so most of printed circuit boards choose to use it rather than the other two holes (blind via and buried via).  
In terms of function, the holes can be divided into two categories, one is used for electrical connection between each layers; The other is used to fix or position the components.  

Introduction about Hard Gold Plating PCB


Surface treatment is one of  the most important process for printed circuit board manufacturer, It forms a critical interface between component and PCB board. Basically the surface treatment has two essential functions. One is to protect the exposed copper circuitry on printed circuit board, the other is to provide a solderable surface for PCB assembly process.

Hard gold plating (Hard electrolytic gold) consist of a layer of gold plated over a barrier coat of nickel. Hard gold is extremely durable, and it’s most usually used for high-wear areas such as gold fingers and keypads.

Hard gold plating pcb is different with immersion gold pcb, its thickness can vary by controlling the duration of the gold plating cycle, although the typical minimum values for gold fingers are 30u” over 100u” nickel for IPC Class 1 and Class2, 50u” over 100u” nickel for IPC Class3. Hard gold is not generally applied to solderable areas, because of it’s high cost and it’s relatively poor solderability. The maximum thickness that IPC considers to be solderable is 17.8u”, so if this type of gold must be used for soldering, the recommended nominal thickness sould be about 5-10u”.

The advantages to use hard gold plating is: Hard and durable surface, without Pb, Long shelf life.

The disadvantages to use hard gold plating is: very expensive cost, extra processing in PCB production, with more difficulty than other surface finishes, not solderable above 17u”.

The advantages of metal core PCB


Metal core PCB have been used frequently nowadays in various industry, especially for LED products. Here we would like to discuss why Aluminum PCB or LED PCB is so popular and what advantages they have.

1.  Thermal conductivity.

Currently, for many double-sided PCB or Multilayer PCB with high density and power, it’s a challenge for heat emitted. The normal PCB raw material such as FR4 or CEM3 have poor thermal conductivity and inter-layer insulation. The heat is difficult to go out and high temperature will cause the failure of electronic components. Aluminium PCB have excellent thermal conductivity and they could be a quite good cool solution.

2.  Thermal expansibility

Expand with heat and contract with cold is a feature of object, The CTE(coefficient of thermal expansion) is different with different objects. The Z-axis of CTE vary greatly between hole wall with plated copper and connected insulated wall for normal FR4 PCB, the metal hole maybe crack if the heat couldn’t be removed in time., so that it will cause the issue of reliable. MCPCB can effectively deal with the heat and the problems of thermal expansion and contraction, improve the durability and reliability of equipments.
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3.  Scale stable

Compare with FR4 PCB, the scale of metal core pcb is much more stable. Aluminum PCB, heated from 30 to 140~150, the scale changes only from 2.5 – 3%. So the metal PCB is more suitable to work in the condition of high temperature.

 

What is High Tg PCB (Printed circuit board)


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Normally high Tg refers to high heat resistance in PCB raw material, the standard Tg for copper clad laminate is between 130 – 140, High Tg is generally greater than 170℃, and middle Tg is generally greater than 150℃. Basically the printed circuit board with Tg≥170℃, we call high Tg PCB. As the rapid development of electric industry, especially for the computer as the representative of electronic products, developing toward the high performance, high multilayer requires PCB substrate material with higher heat resistance to ensure high reliability. On the other hand, as a result of development of SMT, CMT with high density assembly technology, the PCB manufacturing with small hole size, fine lines and thin thickness are more and more inseparable from the support of high heat resistance.

 

If the Tg of PCB substrate is increased, the heat resistance, moisture resistance, chemical resistance and stability of printed circuit boards will be improved as well. The high Tg applicates more in the lead free pcb manufacturing process.

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