PCBA Tech - SMT chip processing plant material inspection

From the very beginning, SMT only had the simplest devices, and now it is possible to mount various package resistors and capacitors, various sizes of BGAs, chips, and integrated modules. Of course, high-end SMT technology is gradually refined with the integration of materials and packaging changes. Therefore, the maturity of SMT technology is established on the basis of high-quality materials, and the two are complementary. In summary, material inspection is very important to SMT welding technology and an indispensable process in production. Let me talk about material inspection from several aspects.

One, PCB

PCB is the base material of the product as a whole, and the quality of PCB directly affects the welding quality and the scope of application and life of the product. The following

We adopt visual inspection and coordination for the five aspects of PCB warpage and distortion, solderability, appearance, gold fingers and special materials.

Tools such as calipers introduce the pre-welding inspection of PCB. It is necessary to emphasize a premise, as long as it is related to SMT materials.

You need to wear anti-static gloves and a wrist strap to avoid damaging the device or contaminating the PCB.

1. Warpage and distortion

There are many reasons for the warpage and distortion of the PCB. The reasons other than the design may be due to the humidity of the storage environment or the placement position that cannot meet the horizontal requirements. According to the regulations, the acceptable range should be controlled at 0.5% of the diagonal length of the PCB board. the following,

Of course, there should be room for fluctuations in this range for the complexity of the board. For example, the number of large BGAs on the PCB is large, and the integration is high, and the warpage of the board should be controlled more strictly. Similarly, if there are only some small chips and resistors on the PCB If the integration is low, the range can be appropriately relaxed.

2. Solderability

PCB pads are easily oxidized when exposed to the air for a long time. If the pads are oxidized and continue to be soldered, a series of problems such as poor pad wetting and virtual soldering will result. Therefore, the solderability of the PCB must be tested before soldering. The inspection method generally adopts visual inspection. For suspected PCB, the edge immersion test is carried out [Note 1]. Visual inspection can directly pay attention to the brightness of the pad,

Generally, tinned pads or gold-plated pads appear darker after being oxidized; if you are in doubt, you can wipe a certain part with an eraser. Compared with the previous one, it is also a simple method to detect PCB oxidation.

3. Appearance

The appearance of the PCB is very important to customers who directly sell finished PCBs. Of course, it may also directly affect the function of the product. Therefore, the damage of the appearance can be considered in the following two situations by simple visual inspection:

1), only affect the appearance without affecting the use of the board

1. Knock, 2. Halo

3. Friction 4. Scratches (no damage to the solder mask and no obvious depth)

5. Exposed copper (no obvious depth, to ensure that there is no damage to the wire, you can repair the solder mask)

In the above five cases, if the customer does not sell the light board or has special requirements for appearance, it can continue to use, without affecting the performance of the product itself, but if the customer directly sells the finished product, the above situation is unacceptable.

2) The change in appearance may cause damage to the PCB as a whole

1. Bubbling/delamination (will affect the internal circuit of PCB)

2. Scratches (broken ring solder mask, there is a certain depth, which may cause cutting to the PCB circuit)

When encountering these two irreparable problems, you can directly request to replace the PCB, because the consequences of these two situations are unknown and cannot be used blindly.

4. Gold finger

Golden finger is also special for PCB. It is the electrical pin that connects PCB to other devices such as motherboard and chassis. Therefore, its quality is very important for the entire product, so the incoming inspection must be relatively strict. General inspection needs Pay attention to several aspects:

1) There are scratches or pits in the middle 3/5 area of the gold fingers, which are mainly manifested in the deep wound, resulting in copper leakage, the recessed area exceeds 6 mil, or more than 30% of the pressure on the entire row of gold fingers. 2) Gold Finger oxidation, mainly reflected in the darkening or reddening of the color;

3) The plating layer is peeled off, and the plating layer shall be peeled or lifted after a tear test;

4) Gold finger pollution, gold finger

Tin, paint, glue or other contaminants;

2. IC parts

The SMT industry generally classifies the types of ICs in terms of their packaging. Traditional ICs include SOP, SOJ, QFP, and PLCC.

Wait, the newer ICs now include BGA, CSP, QFN, FLIP CHIP, etc. These types of parts are due to their

The size of PIN (part pin) and the distance between PIN and PIN are different, and there are various

shape. Here we no longer distinguish the shape names of ICs, we just briefly introduce the materials

In order to facilitate the description, we divide the IC into two categories according to the type of PIN (solder ball, and pin).

1. Tin ball IC--BGA

The integration of BGA is very high. As PCB design becomes more and more complex, the number of BGA used is also increasing, but because

After BGA soldering, you can’t directly observe the soldering quality like a chip with pins, and repairing is more complicated, so before soldering

It is also necessary to ensure the quality of materials and strive for a one-time pass.

1) Appearance

Inspection of BGA First observe from the appearance to see if the base material and all solder balls are present and intact. Most of the base material of BGA is the same as that of PCB, so there will be deformation and delamination. The requirements of BGA base material are much higher than that of PCB. Once deformation or delamination corners are found, according to With its high integration level, materials must be replaced.

2) Solderability

Secondly, we pay attention to the solderability of BGA. If the BGA solder ball is oxidized or contaminated, it will directly lead to soldering accidents.

After the BGA is oxidized, the gloss of the solder ball is significantly reduced. We can observe the color change with the naked eye. In addition, we can also use a magnifying glass to carefully observe a single BGA solder ball. You can use white paper to rub lightly on the surface. If the BGA is oxidized, a black oxide layer will be left on the white paper. In order to ensure the welding quality, the material must be replaced in this case.

2. Pin type-chip

The pin spacing of "legged" chips is getting smaller and smaller, so the "legs" are getting thinner and thinner, and the difficulty of soldering is getting bigger and bigger.

What they have in common with BGA is that rework is more complicated, so it also requires a very high soldering pass rate, so the inspection of the chip is particularly important.

Three, resistance container parts

Resistor parts are the most common materials in the SMT industry, because sometimes thousands of pieces are needed for a product. The resistance parts mentioned here mainly include the following types: resistance (R), exclusion (RA or RN), inductance ( L), ceramic capacitors (C), row capacitors (CP), tantalum capacitors (C), diodes (D), transistors (Q), according to their size, we can be divided into 1206, 805, 0603, 0402 according to the metric system. Common package.

The upsurge of SMT development continues, and the technology is becoming more and more sophisticated, and the requirements for SMT will continue to increase in the products developed in the future. The high pass rate is the guarantee for the practicability, life and reliability of the product. Therefore, the development of SMT is not only limited to the increase in the integration of materials, the increase in the number of PCB layers, but also the strengthening of the welding pass rate. Higher pass rate It is also inseparable from the solderability and reliability of the materials. The materials are the cornerstone of the product. If there is no good cornerstone, how can we talk about the quality of the product. Therefore, the future material inspection must be more and more professional, so as to provide an effective guarantee for good welding quality.