Preface

 

 

 

 

The pillow effect (Head-in-Pillow, HIP), recently some people have begun to call it HoP (Head-of-Pillow). Both HIP and HoP refer to the defective phenomenon of BGA solder joints, which is similar to a person holding his head. Named after the shape it rests on the pillow.

 

 

 

 

 

 

 

The pillow effect (Head-in-Pillow, HIP) is mainly used to describe the high-temperature process of reflow (Reflow) of the BGA parts of the circuit board. The BGA carrier board or circuit board cannot withstand the high temperature and the board bends or bends. Warpage or other deformation causes the BGA's solder ball to separate from the solder paste printed on the circuit board. When the circuit board passes through the high-temperature "reflow zone", the temperature gradually drops and cools down. At this time, the deformation of the IC carrier board and circuit board gradually returns to the state before deformation (in some cases, the deformation will not go back), but the temperature at this time is already lower than the melting temperature of the solder ball and solder paste, and That is to say, the solder balls and solder paste have already solidified from the molten state back to the solid state. When the warpage of the BGA carrier board and circuit board slowly returns to the shape before deformation, the solder balls and solder paste that have returned to a solid state come into contact with each other again, forming a shape similar to a head resting on a pillow. The welding shape of false welding or false welding.

 

 

 

 

 

How to detect poor HIP (Head-In-Pillow) welding

 

According to the above theory, most of the pillow effect (HIP) should occur at the edges of BGA parts, especially at the four corners, because the warpage is the most severe there. If this is the case, you can try to use a microscope or optical fiber. Use an endoscope to observe, but usually you can only see the outer two rows of solder balls, and it is difficult to identify them further inside. Moreover, when observing the BGA solder balls in this way, you must make sure that there are no tall parts next to them to block the view, so now The high-density design of the circuit board has many limitations in implementation.

 

 

 

 

 

 

 

 

In addition, the pillow effect (HIP) is generally difficult to detect with current 2DX-Ray inspection machines, because most X-Ray can only inspect from top to bottom, and cannot see the position of the BGA solder ball breakage. The rotation angle should be visible in X-Ray. Sometimes it may be detected through in-board testing (ICT, In Circuit Test) and functional testing (FVT, FunctionVerification Test), because this type of machine usually uses a needle bed operation method and needs to add additional external pressure to the circuit board. This gives the solder balls and solder paste that were originally next to each other a chance to separate, but many defective products will still flow to the market. Usually, such defective products will be quickly discovered by customers to have functional problems and be returned. Therefore, how to prevent and control the pillow effect is actually an important issue for SMT.

 

 

 

In addition, you can also consider using the Burn/In method to screen out boards with HIP (if a single board is burned in, you need to increase the temperature), because the temperature of the board will increase when burning, and the temperature will If the board is deformed, empty/fake solder joints may emerge. Therefore, when burning the machine, a program must be added for self-diagnostic testing. If the position of the HIP is not on the circuit tested by the program, it cannot be detected. .

 

 

 

Currently, the more reliable methods for analyzing HIP adverse phenomena are the Red Dye Penetration and Cross Section analysis. However, these two methods are destructive tests, so they are not recommended unless necessary.

 

 

 

The recent [3DX-Ray CT] technology has made a breakthrough, which can effectively detect the shortcomings of this type of HIP or NWO (Non-Wet-Open) welding, and it has gradually become popular, but the cost of the machine is still not cheap enough. That’s it.

 

 

 

 

 

 

 

Possible causes and mechanisms of HIP (Head-In-Pillow)

 

 

Although the pillow effect occurs during reflow soldering, the real cause of the pillow effect can be traced back to poor materials. At the circuit board assembly factory, it can be traced back to the printing of solder paste and the accuracy of the patch/satch. temperature and reflow oven temperature setting...etc.

 

 

 

Here are a few possible causes of the pillow effect (HIP) disadvantage:

 

BGA package (Package)

 

If the same BGA package has solder balls of different sizes, smaller solder balls are prone to the disadvantage of pillow effect.

In addition, when the temperature resistance of the carrier board of the BGA package is insufficient, it is easy for the carrier board to warp and deform during reflow soldering, thereby forming a pillow effect.

 

(warpage of substrate, inconsistent bump size)

 

HIP-Solder balls vary in size

 

Solder paste printing

 

The amount of solder paste printed on the solder pads varies, or there are so-called vias-in-pad on the circuit board, which may cause the solder paste to be unable to contact the solder balls. And create a pillow effect.

In addition, if the solder paste printing is too far away from the soldering pads of the circuit board and misaligned, which usually occurs when multiple boards are assembled, when the solder paste melts, it will not be able to provide enough solder to form a bridge, which may cause a pillow effect.

 

(insufficient solder paste volume, printing misalignment)

 

 

 

The accuracy of the placement machine is insufficient (Pick&Place)

 

If the accuracy of the placement machine is insufficient or the XY position and angle are not adjusted properly when placing the components, misalignment of the BGA's solder balls and pads will also occur.

In addition, when placing IC components on the circuit board, the placement machine will press down slightly by a certain Z-axis distance to ensure that the solder balls of the BGA are in effective contact with the solder paste on the circuit board pads. This way, the BGA soldering can be ensured during reflow soldering. The ball is perfectly soldered to the soldering pad of the circuit board. If the Z-axis pressing force or formation is insufficient, some solder balls may not be able to contact the solder paste, resulting in the possibility of HIP.

(Inaccurate XYplacement, insufficient placement force)

 

 

 

 

Reflow soldering temperature (Reflow profile)

 

When the reflow temperature or heating rate is not set properly, problems such as lack of melted tin or bending or warping of the circuit board and BGA carrier board may occur, all of which will cause HIP. You can refer to the article Possible causes of simultaneous air soldering and short circuit of BGA to learn about BGA air soldering caused by board bending and warpage due to the large difference in CTE between the BGA carrier board and the circuit board and the excessive length of TAL (Time Above Liquids). and short circuit analysis.

In addition, it should be noted that if the temperature in the preheating zone rises too fast, it will easily drive the flux to evaporate prematurely, which will easily cause solder oxidation and cause poor wetting. Secondly, it is best not to adjust the maximum temperature (Peak Temperature) too high or for too long. It is recommended to refer to the temperature and time recommendations of the parts.

(inadequate reflow profile that results in component & PCB warpage, Liftingof BGA bumps due to wetting force, Excessive Peak Temperature, too much TAL)

 

 

 

 

Solder ball Oxidization

After the BGA is completed in the IC packaging factory, probes will be used to contact the solder balls for functional testing. If the cleanliness of the probes is not handled well, there is a chance that contaminants will contaminate the BGA solder balls and cause poor welding. Secondly, if the BGA package is not properly stored in a temperature and humidity controlled environment, there is a high chance that the solder balls will oxidize and affect the solder jointability.

 

 

 

 

How to improve and prevent poor HIP (Head-In-Pillow) welding

 

 

 

Now that it is known that the main cause of HIP is the high-temperature deformation of the FR-4 of the circuit board and the IC carrier board, there are two directions to prevent or avoid the occurrence of HIP.

 

 

 

§ Method one is to improve the rigidity of circuit board materials and IC carrier boards. Generally, materials with higher Tg ≥ 170℃ are used, but the cost will also increase accordingly. Generally, the material of Tg board for lead-free process circuits uses medium Tg (Tg≥150℃).

 

§ The second method is to increase the amount of solder paste to fill the gap caused by high temperature warping of the circuit board and IC carrier. That is, keep the BGA solder balls in contact with the solder paste printed on the circuit board during all reflow processes. However, be careful that if the amount of solder paste is increased too much, it will cause a soldering short circuit. Do not be careless.