Essential Elements in Reflow Soldering Technology for BGA Components

BGA, short for Ball Grid Array, takes advantage of solder balls that play a role as pins at the back of base. BGA is a type of package accepted by SMT (surface mount technology) assembly, applied by multi-pin LSI (large scale integration).

BGA Package Categories

Up to now, BGA package can be classified into three categories in accordance with base types: PBGA (plastic ball grid array), CBGA (ceramic ball grid array), TBGA (tape ball grid array).


• PBGA


PBGA places solder balls on the base with the following properties:
a. Features excellent hot pressing matching with epoxy resin;
b. Solder balls contribute to the generation of solder joints, leading to flexible coplanarity that is approximately 250μm;
c. Features low cost;
d. Performs electrically excellently;
e. Can be accurately aligned with PCB pad through package edge.


• CBGA


Solder balls belonging to CBGA are manufactured by high-temperature solder, which are then connected with ceramic base through the application of eutectic solder with a low melting point (usually 63Sn/Pb) that is then leveraged to make solder balls connected with PCB (printed circuit board). CBGA features the following characteristics:
a. Feature higher reliability;
b. Features good coplanarity that is approximately 100μm and solder joints are easily generated;
c. Not sensitive to humidity;
d. Feature a high package density;
e. Due to different coefficients of thermal expansion, CBGA features bad hot pressing matching with PCB boards applying epoxy resin as substrate so that solder joint fatigue has become the primary failure type of CBGA;
f. It's difficult for alignment between package edge and PCB to take place, leading to a high cost of packaging.


• TBGA


TBGA is a type of package taking advantage of tape interconnection for connection achievement between chip, solder balls and PCB. Characteristics of TBGA package contain:
a. Brings forward good hot pressing matching with PCBs using epoxy resin as substrate material;
b. Capable of aligning with PCB pad through package edge;
c. Features the lowest cost;
d. Sensitive to both humidity and heat, perhaps leading to relatively low reliability;


Based on the brief introduction of different categories of BGA packages, features of BGA package components can be summarized into the following:
a. Lead to a low failure rate;
b. Dramatically improve component pins while decreasing package size, reducing the application area of base;
c. Evidently defeat coplanarity issue and greatly reduce coplanar damage;
d. Feature solid pins, which is different from pin deformation occurring to QFP (quad flat package);
e. Include short pins that signal paths are thereafter short with lead inductance and capacitance reduced and electrical performance improved;
f. Beneficial to thermal dissipation;
g. Compatible with packaging requirement of MCM (multi-chip module), leading to high density and high performance achievement of MCM.

Reflow Soldering Technology for BGA

Basically, BGA package assembly is compatible with SMT assembly procedure. First, solder paste is printed on pad array on PCB through the application of stencil or flux is coated onto pad. Second, pick and place machine is used to make BGA components placed onto PCB pad array with full alignment. Then, BGA components will go through reflow soldering in reflow soldering oven. Due to particularity of BGA package components, this article will discuss reflow soldering technology with PBGA as an example.


• Preheating Phase


Preheating phase is usually composed by 2 to 4 heating zones with temperature constantly going up to 150°C within 2 minutes so that volatile substances in solder paste can be volatilized out. As a result, those substances won't lead solder to be splashed or base to be over-heated. Meanwhile, the temperature of PCB devices can be sufficiently high for wettability achievement of solder. It's optimal for temperature rise to reach a rate of 1.5°C per second.


• Soaking Phase


The target of soaking phase is to get hot melting sufficiently achieved and temperature of all the solder joints on PCB can be as close to soldering temperature. The extent of hot melting directly determines soldering quality of solder joints. Temperature should be maintained at approximately 170°C within 60 to 120 seconds.


• Soldering Phase


Soldering phase has to witness the temperature of solder joints quickly climbing to the soldering temperature. Duration time should be controlled within the range from 60 seconds to 120 seconds when temperature is over 183°C. It's best to set the highest temperature in soldering phase to be in the range from 200°C to 210°C and peak temperature of components shouldn't exceed 220°C. It's optimal for temperature rise rate to reach 2°C to 3°C per second.


• Cooling Phase


Cooling phase contains two cooling modes: air cooling and natural cooling. It's optimal for cooling rate to reach the range from 1°C to 3°C per second. Moreover, temperature difference between component surface and bottom shouldn't exceed 7°C, or thermal stress aggregation will be caused.


Because components with different packages feature different thermal absorption and thermal dissipation rates, temperature rise rate in soldering phase and temperature fall rate should be distinguishably treated.


Temperature and time duration of each phase during reflow soldering procedure can be summarized into the following table.


Temperature Phase Up Phase Down Phase Set Time Duration (s)
Set Temperature (°C) Practical Temperature (°C) Set Temperature (°C) Practical Temperature (°C)
1 140 140 140 140 35
2 120 120 120 120 45
3 160 160 160 160 50
4 180 179 180 180 45
5 200 200 200 200 55
6 210 210 210 210 55
7 230 230 230 230 45
8 245 244 245 244 50

It has to be noted that this table can never be totally conformed to at all situations. Differences do occur among components, reflow soldering ovens, PCBs, assembly environment, operators' manufacturing experience etc. so more accurate setting parameters depend on practical assembly experience.

BGA Component Inspection

A good soldering is just half done. Solder joints can never be guaranteed to be perfectly achieved unless inspections are implemented. BGA packages hide their components under their bodies so visual inspection can hardly work. Plus, optimal inspection can only make solder joints at the edge exposed, failing to provide complete and accurate inspection result. As a result, BGA solder joints should be inspected through X-ray inspection devices. Two methods are available for X-ray inspection devices: transmitted inspection and cross-section inspection both of which are capable of inspecting bridging between solder joints and mis-alignment. In fact, both inspection methods perform differently in terms of BGA solder joint shape and size inspection capabilities.


• X-ray Transmitted Inspection


X rays transmit all high-density materials in a vertical direction. When it comes to CBGA, solder balls prohibit eutectic solder at soldering station level from being generated and component-level eutectic solder tends to be covered by solder balls. As far as PBGA package is concerned, solder image at a soldering station tends to be stopped at solder joint. As a result, X-ray transmitted inspection fails to correctly the defect of insufficient solder.


• X-ray Cross-Section Inspection


X-ray cross-section inspection can explore solder connection defects and accurately obtain the shape of BGA solder joints and critical dimension of cross section. Circular ring thickness inspection at a solder station level reflects solder reflow procedure or change situation of solder at solder station. Radius inspection at a solder station level indicates the changes of solder volume at a solder station, which is caused by solder paste printing technology or too much reflow solder. Radius inspection of solder balls demonstrates coplanarity from solder joint to solder joint or from board to board.

Miniaturization and high performance are essential development trends for electronic products, leading circuit module assembly density to constantly go up. As a result, high-integrity micro components become diversified with their assembly methods moving forward as well. As modern package technology becomes flourishing, BGA package technology is moving towards μBGA and MCM. As a type of high-density assembly components, different soldering temperatures should be applied compatible with different package requirement. As long as the essential elements are taken into careful consideration during BGA reflow soldering, reliability of BGA components and SMT assembly can be fully guaranteed.


Helpful Resources
Four Steps to Know BGA
An Introduction of BGA Packaging Technology
A Brief Introduction of BGA Package Types
Factors Affecting the Quality of BGA Assembly
Some Engineer-Friendly Methods to Achieve Optimal Solder Joints in BGA SMT Assembly Process

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