Poor assembly analysis

- Jan 11, 2020-

Poor assembly analysis

1) Poor assembly (not melted)

Summary

Introduce the unmelted phenomenon in solder assembly.

About the cause

The causes of unmelted surface assembly can be roughly divided into "insufficient temperature" and "inappropriate temperature curve".

 

Insufficient temperature

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Unmelted refers to the phenomenon that flux flows very little and the surface of the solder is matte.

In the central part of large BGAs and aluminum electrolytic capacitors, etc., it is necessary to pay attention to accurate temperature measurement in places where the temperature is difficult to rise, and appropriate temperature curve settings are necessary.

 

The temperature curve of reflow soldering is not suitable

Unmelted due to preheating discomfort

 

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Unmelted due to preheating discomfort

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    ·Temperature curve

 

imageBecause the solder paste has a wide range of deterioration (excessive heating), the tiny pad is prone to unmelting.

 

2) Poor assembly (solder ball)

Summary

There are several reasons for the solder balls left around the Pad. The cause is estimated based on the number and size of occurrences.

Here are the reasons for the occurrence of tiny solder balls in the flux.

Classification of solder balls

According to the number and size of solder balls, it can be divided into 4 modes.

In the case of a single solder powder, the radius is 10 to 40 μm. If the size is more than 50 μm, it is considered that multiple solder powders are fused.

 

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The cause of occurrence

The solder paste collapses during heating

The solder paste collapsed during heating, but instead of completely connecting the two pads (see 0.1mm position), a thin tin bead was formed in the green oil bridge (see 0.2mm position).

 

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Flux flowing out

With the flux flowing out during melting, the late-dissolved solder powder flows out.

Solder powder flows out

 

image Other

Oxidation of solder powder, reflow soldering temperature curve, print exudation, wiping off, etc.

 

3) Poor assembly (the component stands up)

Summary

This chapter will introduce the causes of the rising phenomenon of components called "tombstone phenomenon" and "Manhattan phenomenon".

The reasons for the rise of the component: three main reasons

The component rise occurs when the following factors ① ~ ③ are available.

 

imageReasons why the solder is not attached

It is speculated that the following ① ~ ⑤ are the main causes of solder failure.

The following ① ~ ⑤ are the presumed causes for the solder not adhering.

 

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pad design and parts

T1 + T2 <T3, the pad design and components are shown in this formula.

The pad is prolonged, and the component rises easily when the electrode width is short.

 

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T3 represents the maximum value

RA type parts with good tin adhesion have a lot of components standing up. It is presumed that the electrode wets quickly and the T3 moment is large.

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4) Poor assembly (causes of substrates)

Summary

This chapter describes the phenomenon of poor surface assembly due to substrates.

Electroless plating Ni / Au

The formation of excess P concentration layer

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Corrosion of Ni layer

 

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Other

Formation of micro voids, thickness of Au layer, surface contamination, etc.

Cu (water-soluble pre-flux treatment)

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Spraying tin

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